Symposium EL03—Multiferroics and Magnetoelectrics

While classical spintronics has traditionally relied on ferromagnetic metals as spin generato Classical spintronics traditionally relied on ferromagnetic metals as spin generators and detectors. A new approach called spin-orbitronics exploits the interplay between charge and spin currents enabled by the spin-orbit coupling in non-magnetic systems. We studied the potential of the bidimensional electron gases (2DEG) that forms at the interface between LaAlO₃ and SrTiO3 [1] or by deposition of Al on SrTiO3 by sputtering [2]. We exploited the sizeable Rashba spin-orbit coupling of the gas to obtain a very large spin-current to charge-current conversion through the inverse Edelstein effect [2, 3]. The efficiency of the conversion, characterized by the Inverse Edelstein effect length, l_IEE, is larger than the one measured for the topological insulator a-Sn or at Bi/Ag interfaces [4]. Moreover, this conversion efficiency can be highly modulated by a gate voltage. We used angle-resolved photoemission spectroscopy and Boltzmann calculations to map this peculiar gate dependence and linked it to the band structure. We will show that the conversion process is amplified by enhanced Rashba-like splitting due to orbital mixing, and in the vicinity of avoided band crossings with topologically non-trivial order [3]. This suggest that oxide interfaces have a strong potential for spin-based information readout in novel memory such as the magneto-electric spin-orbit transistor proposed by Intel[5].

[1] E. Lesne et al., Nat. Commun. 5, 4291 (2014) ; D. C. Vaz et al., Adv. mat. 29, 1700486 (2017).
[2]; D. C. Vaz et al., Nat. Mat. (2019)
[3] E. Lesne et al., Nat. Mat. 15, 1261 (2016)
[4] J.-C. Rojas-Sánchez et al., Nature Com. 4, 2944 (2013) ; PRL 116, 096602 (2016).
[5] S. Manipatruni et al., Nature 565, 35 (2019)

Transition metal dichalcogenides (TMDs) have been extensively investigated as 2D materials last decade. A large amount of transition metals (M) can be intercalated into the van der Waals gaps of a wide range of TMD materials, but a limited recent studies in intercalated TMDs have been reported. The limited examples include Fe_xTaS₂ crystals with x=1/4 and 1/3, which exhibit intriguing configurations of antiphase and/or chiral domains related to the ordering of intercalated M ions with 2a×2a and √3a×√3a superstructures, respectively. In addition, Cr_1/3NbS₂ undergoes helical spin order below 133 K, and shows an interesting soliton-lattice behavior when in-plane magnetic fields are applied in the helical spin state. We have explored a series of chiral M_1/3Ta(Nb)S(Se)₂ to investigate the correlation among crystallographic symmetries, magnetic domain topologies and their physical properties. These results as well as Moire patterns with self-twisted TMDs induced by intercalation will be discussed.

Requirements to "good multiferroics" are tough. They are supposed to have a spontaneous magnetization and polarization, preferably parallel to each other, with a strong magnetoelectric coupling between them. Inevitably, this leads to a multiferroic state that is described by a very complex set of order parameters – complex enough to provide the symmetry degrees of freedom to fulfil so many requirements at once [1]. With the focus on electric-field-controlled magnetic order, it may go unnoticed that these degrees of freedom will permit many functionalities other than bulk magnetoelectric coupling. In my talk, I will describe the quest for such functionalities in our group. This includes inversion of a ferroelectric or a ferromagnetic multi-domain state in a homogeneous electric or magnetic field: In each domain, the direction of the order parameter is reversed but the domain pattern as such is left untouched [2]. In addition, a ferroelectric domain pattern on a uniformly magnetized background is transduced into a ferromagnetic domain pattern on a uniformly polarized background. Finally, a multiferroic bulk state is "condensed" into a multiferroic domain wall in a non-multiferroic environment.

[1] M. Fiebig et al., Nature Rev. Mater 1, 16046 (2016)
[2] N. Leo et al., Nature 560, 466 (2018)

We describe recent progress on materials and devices for the direct conversion of heat to electricity applicable to the small temperature difference regime, 10-200 C. This regime includes abundant natural and waste heat sources, but there is currently no reasonable method to harvest the energy. We are pursuing an idea based on the use of first order phase transformations with either an abrupt change of magnetization or polarization at the transformation. In the ferromagnetic case the electricity is harvested by induction; in the ferroelectric case, by capacitance. It is a “direct” method in the sense that there is no separate electrical generator. We survey the theory of this method, the design of the materials and devices, and the analysis of various cycles. We compare theoretical predictions and the behavior of a prototype under cyclic heating/cooling. These devices provide interesting possible ways to recover the vast amounts of energy stored on earth at small temperature difference. They move heat produced by natural and man-made sources from higher to lower temperature and therefore contribute negatively to global warming. This lecture draws on work with Kanwal Bhatti, Ashley Bucsek, Xian Sherry Chen, Bharat Jalan, Bill Nunn, Yintao Song and Vijay Srivastava.

BiFeO₃ has attracted the increasing scientific attention as the most important room temperature single phase multiferroic material. However, the weak magnetoelectric coupling in BiFeO₃ seriously obstructs its applications. The cycloidal spin structure is actually a hint of the existence of the magnetoelectric coupling. In the present work, the magnetoelectric coupling has been realized by destroying the cycloidal state and switching it to the weak ferromagnetic state through symmetry modulation. Electric field-controlled magnetism is achieved in Bi_1-xR_xFeO₃ solid solutions by tuning the symmetry from polar R3c to polar Pna2₁, where two morphotropic phase boundaries (MPB) are detected together with the greatly enhanced ferroelectric polarization and magnetism. The electric field-controlled magnetism is realized by an electric field induced structural transition from Pna2₁ back to R3c which results in the magnetic transition from weak ferromagnetic state to cycloidal state, and this transition is shown to be reversible with additional thermal treatment.

Multiferroics and metallic conductivity seem mutually exclusive and cannot be obtained in one material.[1] Dielectric property requires the localized charge and the electrical conducting behavior is based on the free charge carriers. The confined electronic structure in one-dimensional (1D) materials might break this long-standing contradiction. Electrons confined to a 1D chain exhibit various instabilities followed by symmetry breaking, such as superconductor,[2] topological insulator[3] and multiferroics[4]. Here we report the discovery of laser shock induced insulator-metal transition in K-TCNQ radical,[5] where the effect of antiferromagnetic spin ordering causes dipole ordering. The manipulation of spin exchange interaction with external laser shock wave, electric field and magnetic field is accompanied by insulator to metal transition, gigantic magnetoelectric and magnetocapacitance effects, respectively. Therefore, 1D systems with strongly coupled multiple degrees of freedom of electrons-spin, charge, and orbital-provide a new area to search for conducting magnetoelectric media. Our study opens the way for electron engineering, symmetry breaking and establish of self-organized electronic order from modular building blocks, permits the rational design of functional electronic materials that exhibit technologically useful behaviour.

Reference
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[2] J.-K. Bao, J.-Y. Liu, C.-W. Ma, Z.-H. Meng, Z.-T. Tang, Y.-L. Sun, H.-F. Zhai, H. Jiang, H. Bai, C.-M. Feng, Phys. Rev. X 2015, 5, 011013.
[3] G. Autès, A. Isaeva, L. Moreschini, J. C. Johannsen, A. Pisoni, R. Mori, W. Zhang, T. G. Filatova, A. N. Kuznetsov, L. Forró, Nat. Mater. 2016, 15, 154.
[4] A. S. Tayi, A. K. Shveyd, A. C.-H. Sue, J. M. Szarko, B. S. Rolczynski, D. Cao, T. J. Kennedy, A. A. Sarjeant, C. L. Stern, W. F. Paxton, Nature 2012, 488, 485.
[5] R. Kumai, Y. Okimoto, Y. Tokura, Science 1999, 284, 1645.

Thin film magnetoelectric antiferromagnets (AF) have potential to revolutionize spintronics due to their inherently magnetic-field stable magnetic order and high-frequency operation. To explore their application potential, it is necessary to understand modifications of the magnetic properties and magnetoelectric responses of AF thin films with respect to their bulk counterparts.
Our approach is based on the electron transport characterization of magnetic responses of thin film antiferromagnets [1-4]. This task is difficult as minute uncompensated surface magnetization of antiferromagnets needs to be detected, which imposes strict requirements to the sensitivity of the method. We will outline our developments of zero-offset anomalous Hall magnetometry [3] applied to study the physics of insulating magnetoelectric Cr2O3 antiferromagnets. To build a reliable description of the material properties, the analysis of the transport data is backed up by structural characterization and real space imaging of AF domain patterns using NV microscopy [1,2]. Considering grainy morphology of thin films, we address questions regarding the change of the intergranular exchange [2], criticality behavior and switching of the order parameter [3].
The fundamental understanding of the magnetic microstructure of magnetoelectric α-Cr2O3 thin films and the possibility to read-out its antiferromagnetic order parameter all-electrically enabled the entirely new recording concept where a magnetoelectric memory cell can be addressed without using a ferromagnet, the purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) [1]. In addition to the outline of the memory concept, we discuss on the physics of the readout signal in α-Cr2O3 interfaced with Pt [4].

[1] T. Kosub et al., Nat. Commun. 8, 13985 (2017).
[2] P. Appel et al., Nano Letters 19, 1682 (2019)
[3] T. Kosub et al., Phys. Rev. Lett. 115, 097201 (2015).
[4] R. Schlitz et al., Appl. Phys. Lett. 112, 132401 (2018).

Spintronic devices take advantage of the nonvolatility of magnetism and electrical control high energy efficiency, and thus offer promise to deliver next-generation systems in memory and computation as well as RF devices and systems. Spintronic devices offer the advantages of non-volatility, low switching energy (< fJ), high speed/frequency (<10 ns), and an almost unlimited endurance (>10¹⁶). The development of this technology is especially crucial in the advent of big data and artificial intelligence, which demand fast access to large amounts of data at high processing speeds. In particular, recent advances on memory (magnetic random-access memory, MRAM) and logic devices have also drawn interest in memory-centric computing paradigms. In this talk, we will first provide an overview of the progress of magnetic materials for high spin orbit coupleing (SOC) and spin orbit torque (SOT) applications. We will discuss SOC and magnetoelectric effects as two routes for energy efficient MRAMs. In SOT systems, a high spin-SOC material is used to create a tranverse spin current to manipulate or switch the magnetic moment of an adjacent magnet through a torque in a magnetic tunneling junction device (MTJ). We will discuss recent advances in SOTs, including the applications of topological insulators surface states for increasing the switching energy efficiency. A major challenge for SOT switching is that it requires an additional inversion symmetry breaking to become deterministic, which is usually provided by applying an in-plane external magnetic field. We discuss the recent progress for realizing field-free deterministic SOT switching through symmetry breaking using various techniques, including antiferromagnetic materials and structural symmetry breaking.
Second, we will highlight progress in synthetic layered materials to achieve magnetoelectric effects for memory and logic devices using the voltage-controlled magnetic anisotropy (VCMA) effect, very much like multiferroic mateirals. In this case, a voltage is applied across the magnetic tunnel junction to reduce the anisotropy energy barrier to allow for the voltage controlled switching of the free layer in an MTJ but with a thicker MgO barrier layer, instead of of passing current through the MTJ. VCMA devices are two-terminal and have a much higher density compared to SOT structures which require three terminals for read/write. The reduction of energy barrier per unit voltage is characterized by the VCMA coefficient; to improve the effectiveness of voltage control and scaling to high densities, it is necessary to improve the VCMA coefficient > 1,000 fJ/Vm (first 1,000 challenge). We discuss the recent progress in this area, with a focus on interface and seed/spacer/insertion engineering. For this and related MTJ devices, the readout uses a tunneling magnetoresistance ratio (TMR). The state of the art TMR is of 200~300%. For improved read operation a large signal-to-noise ratio for readout in large arrays, TMR > 1,000%, is desirable and warrant as the second 1,000 challenge. We show strategies for enhancing TMR based on using materials with high spin polarization, such as Heusler alloys. Another challenge for VCMA devices is their high write-error-rate, which can be mitigated by using circuits-based approaches or new device concepts (such as skyrmion-mediated switching). The similar concepts may be applied to energy efficient RF systems. Lastly, for THz applications, ferrimagnetic and antiferromagnetic materials for enhancing the operation speed/frequency will also be briefly disussed.

Composite materials consisting of magnetostrictive and piezoelectric constituents are known for efficient strain mediated magnetoelectric (ME) coupling. Magnetic field sensors made out of such composites yield high sensitivity and low detection limits in the pT field range. Thin film ME sensors reach sensor performance similar to that of their bulk material counterparts provided that the frequency of magnetic field coincides with a mechanical resonance frequency of the sensor structure. In general this condition is not fulfilled. The measurement of e.g. biomagnetic signals requires extremely low detection limits at frequencies of few Hertz and a minimum bandwidth in the order of 20 Hz.
This presentation considers three different ME sensor concepts to enhance sensitivity to low frequency signals, i.e. magnetic frequency conversion (MFC), surface acoustic wave (SAW) sensors, and piezoelectrically driven sensors based on converse ME effect and inductive read-out by a surrounding pick-up coil. The focus is on designing the magnetostrictive phase, which must be tuned to the sensor concepts differing in underlying effects, utilized substrate, sensor size and thickness, as well as operating magnetic or piezoelectric excitation. In MFC the ME sensor structure is exposed to strong modulating magnetic fields changing the magnetization of the magnetostrictive film periodically with a frequency of about 1 kHz. In contrast to that, the converse ME effect sensors are piezoelectrically driven at comparably high frequencies (515 kHz). The SAW sensors are operated by means of interdigital transducer electrodes at 146 MHz leading to mechanical wave propagation at the device surface and thus through the magnetoelastic phase. Utilizing the ΔE effect, phase change between input and output signal serves as a measure of the applied field. With regard to the various concepts, the interplay between magnetic configuration, associated magnetic noise, and resultant ME sensor performance is investigated.
All sensors are fabricated by bulk micromachining. The sensors for MFC and converse ME effect are of Si cantilever type with 2 µm thick AlN serving as piezoelectric phase. The cantilevers are 350 µm thick and have lateral dimensions of 2.2 mm x 25.2 mm. Enabled through the thin film approach and magnetron sputter deposition, single FeCoSiB and complex multilayers comprising up to 80 individual layers form the magnetostrictive phase. The multilayers consist of repetitions of the stacking order unit Ta 5 / Cu 3 / MnIr 8 / FeCoSiB x [thickness in nm] with varying thickness x of the FeCoSiB layer. The total thickness of the FeCoSiB layers adds up to 1 µm, 2 µm, and 4 µm.
The SAW sensors are based on single crystal Quartz substrates with a 4.5 µm thick silicon oxide layer functioning as guiding layer for horizontal shear waves. Single FeCoSiB layers with a thickness ranging from 25 nm to 750 nm are deposited on the guiding layer and in between the transducers. SAW sensor sensitivity and insertion losses are investigated in dependence on the FeCoSiB layer thickness.
At current status, similar detection limits ranging between 60 pT/Hz^1/2 and 150 pT/Hz^1/2 are observed at 10 Hz frequency for all three kinds of sensor concepts. However, converse ME effect sensors and SAW sensors benefit from wider bandwidth of about 1 kHz and 1 MHz, respectively.
The presented work was conducted within the framework of the Collaborative Research Center CRC 1261, funded by the German Research Foundation (DFG).

We report on the synthesis of coaxial nanowires of ferrimagnetic hexagonal ferrites and ferroelectric lead zirconate titanate (PZT) or barium titanate (BTO), assembly of nanostructures into superstructures with the aid of magnetic and/or electric fields, and studies on the nature of coupling between the magnetic and electric subsystems [1]. Such multiferroic nano-composites are expected to show strong strain mediated magneto-electric (ME) coupling due to very high surface area-to-volume ratio [2]. The efforts were on fibers of Y- or W-type hexagonal ferrite and PZT or BTO and their assemblies. Core-shell fibers with (Ni, Zn) Y-type or (Co, Zn) W-type hexaferrites and ferroelectrics were prepared by electrospinning. The choice of Zn-substituted ferrites with planar or uniaxial anisotropy was aimed at control of magnetic order parameters for ME studies at low frequencies and at resonance modes of electric and magnetic subsystems and over a wide frequency range, from 1Hz to 110 GHz. The fiber synthesis was carried out by dispensing sol gels of ferrites and ferroelectrics through a coaxial syringe with a dual syringe pump. It was possible to control the core and shell diameters by controlling the gel viscosity, strength of the electric field and core and shell diameters of the needle. Electron-, scanning probe-, and scanning microwave microscopy of fibers annealed at 700-900 C showed fibers of shell diameter 200-800 nm and core diameter of 50-200 nm with well defined core-shell interface. Fibers were characterized in terms of magnetic, ferroelectric, and electromagnetic parameters. The magnetization for the fibers were in general agreement with values for bulk ferrites, but the polarization was one to orders of magnitude smaller than expected values. Investigations on ME effects were performed on 2D and 3D fibers assembled in a magnetic field and on discs of the fibers annealed at high temperatures. Low-frequency ME voltage coefficients (MEVC) for the films and discs were in the range 0.3 to 3 mV/cm Oe with the highest values measured for fibers of (Ni, Zn) Y and PZT. Magnetic field assembled films showed a higher MEVC than unassembled films. Polarization P vs electric field E measurements under an applied static magnetic field H on discs indicated a fractional change in the remnant polarization as high as 7% for H = 7 kOe. Magneto-dielectric measurements on films at 12-26 GHz showed 2-3 % fractional change in the dielectric permittivity for H = 7 kOe. The nanofibers studied in this work show strong ME coupling and are of interest for application in sensor and energy harvesting technologies.
The research was supported by a grant from the NSF (DMR-1808892)
1. D. Viehland, J. F. Li, Y. Yang, et.al., J. App. Phys.124, (2018): 061101.
2. V. Petrov, J. Zhang, H. Qu, et.al., J. Phys. D: Appl. Phys. 51, 284004 (2018).

Recent progress in the field of magnetoelectric (ME) materials have led to ultra-compact ME antennas with size of 2-3 order of magnitude smaller than that of conventional state-of-the-art compact antennas [1]. These miniaturized antennas could be suitable for Medical Implant Communications Service (MICS), in particular for wireless brain monitoring. A wireless implantable device should be able to perform multiple tasks in order to effectively monitor an individual’s condition. For instance, a wireless implantable device should harvest their required energy in order to power their circuitry systems. In addition, the implant needs to record the body’s information such as magnetic or electric fields in order to successfully track the changes the body. Finally, the implant should transmit the recorded data to the external antenna or device.

In this work we’re presenting an ultra-compact and smart magnetoelectric antenna that can single-handedly perform all three above-mentioned tasks: energy harvesting, magnetic field sensing, and data communication. This complimentary combination makes ME antennas suitable for brain implantation where magnetic field recording is required to monitor the brain functionality. Fig. 1 shows the optical microscope image of fabricated ME antenna with AlN/FeGaB thin-film structure. AlN acts as a piezo-material and FeGaB as a magneostrictive material. Proposed antenna has three parallel ME element, each with the size of 50×250 µm². Fig. 2a shows the architecture of energy harvesting setup. Fig. 2b shows the ME antenna concept and its functionality in receiving (energy harvesting) mode. Fig. 2c shows the S11 of ME antenna showing the resonant frequency of 2.25GHz. Fig. 2d shows the harvesting voltage from ME antenna, showing an induced voltage of 2.13mV when ME antenna is in presence of a 4.5nT RF magnetic field. Fig. 3a shows the ME antenna’s phase response in magnetic sensing mode. Here, ME antenna is in presence of a 1kHz magnetic field with 6.6µT strength. Fig. 3b is the zoomed-in version of Fig. 3a and shows the 1kHz phase peak which corresponds to the 6.6µT magnetic field under measurement. The limit of detection of this antenna is 917pT.


[1] T. Nan, H. Lin, Y. Gao, A. Matyushov, G. Yu, H. Chen, et al., "Acoustically actuated ultra-compact NEMS magnetoelectric antennas," Nature communications, vol. 8, p. 296, 2017.

Topological structures and defects including ferroelectric and ferromagnetic vortices emerging near spontaneous symmetry-breaking transitions are ubiquitously observed in wide branches of science. They are invariant under continuous deformations, or perturbations, and protected by topology, thus are promising candidates as information carriers for future memory and logic devices. However, their controlled manipulation including creation and annihilation remains an important challenge towards practical applications. In this presentation I will give a few examples. 1) Structural transformation of sixfold ferroelectric vortex domains into crystallographically forbidden two-, four-, and eightfold vortices via a second topological defect in hexagonal manganites. Combining high-resolution electron microscopy and Landau-theory-based numerical simulations, we investigate the remarkable atomic arrangement and the intertwined relationship between polarization and topological defects. 2) Controlled evolution of helical and skyrmion phases in multiferroic oxides as function of doping, temperature, and magnetic field from direct imaging using in-situ Lorentz phase microscopy. We reveal that skyrmion channeling effectively suppresses the second skyrmion phase formation at low temperature, envisaging designing of skyrmion flow circuits based on multiferroic thin films for spintronics. If time allows, other material systems will also be presented.

Collaborations with S. Cheng, J. Li, and MG. Han at BNL and Jan Seidel at UNSW are acknowledged. Work at BNL was supported by DOE/BES-MSD under Contract DESC0012704.

Multiferroic heterostructures comprising strain-coupled ferromagnetic and ferroelectric layers can be exploited as programmable hybrids in magnonics. Attractive properties of multiferroic heterostructures include the imprinting of magnetic domain patterns with a regular modulation of magnetic anisotropy, the creation of abrupt anisotropy boundaries, strong pinning of straight magnetic domain walls, and electric field control of magnetic anisotropy, magnetic switching, and magnetic domain wall motion [1-3]. Here we show that these attributes provide an attractive platform for the emission of short-wavelength spin waves [4], the confinement of spin-wave modes in structurally uniform films [5], and the manipulation of spin-wave transmission [6]. In our studies, the multiferroic heterostructures consist of a ferroelectric BaTiO₃ substrate and a strain-coupled ferromagnetic film with a thickness of 20 – 50 nm. At room temperature, this material system comprises fully correlated stripe domains in the ferroelectric and ferromagnetic subsystems. Utilizing strong pinning of ferromagnetic domain walls onto ferroelectric boundaries, we demonstrate active manipulation of spin wave transmission from ~0% to ~100 % by non-volatile reprogramming of the domain-wall spin structure in a magnetic field [6]. We also present results on electric field control of spin wave transmission using local domain switching via lateral wall motion in perpendicular electric fields. Many of the magnonic concept realized in multiferroic heterostructures are extendable to other material systems with regular modulations of magnetic anisotropy.

[1] T. H. E. Lahtinen, J. O. Tuomi, and S. van Dijken, Adv. Mater. 23, 3187 (2011).
[2] T. H. E. Lahtinen, K. J. A. Franke, and S. van Dijken, Sci. Rep. 2, 258 (2012).
[3] K. J. A. Franke, B. Van de Wiele, Y. Shirahata et al., Phys. Rev. X 5, 011010 (2015).
[4] B. Van de Wiele, S. J. Hämäläinen, P. Baláz, F. Montoncello, and S. van Dijken, Sci. Rep. 6, 21330 (2016).
[5] S. J. Hämäläinen, F. Brandl, K. J. A. Franke, D. Grundler, and S. van Dijken, Phys. Rev. Appl. 8, 014020 (2017).
[6] S. J. Hämäläinen, M. Madami, H. Qin, G. Gubbiotti, and S. van Dijken, Nat. Commun. 9, 4853 (2018).

Rare-earth nickelates perovskites (RNiO₃, with R=rare earth) are of high academic interest as they show sensitive variability in their properties with subtle structural changes. Unlike other RNiO₃ compounds, LaNiO₃ is the only compounds showing no metal-insulator transitions with temperature as the relatively large size of La suppresses the distortive mode stabilizing insulating phase. Instead, LaNiO_3-δ exhibits interesting electronic/magnetic transitions with varying oxygen contents δ. Specifically, the metal-semiconductor-insulator transition occurs concurrently with paramagnetic (PM)-ferromagnetic (FM)-antiferromagnetic (AFM) transition in bulk materials [1]. The origin of these transitions can be associated with ordered oxygen-vacancies in LaNiO_3-δ, which transforms NiO₆ octahedra to NiO₄ square planer units along (110)_p direction. As the square planar units are with different coordination environment and crystal-field splitting of d-orbital states, the assembly of two units results in unique electronic/magnetic structures.

In this work, we investigate the role of ordered-oxygen vacancies in LaNiO_3-δ by using first-principles calculations on LaNiO_3-δ phases with δ=0, 0.25, and 0.5. The LaNiO_2.75 is composed of octahedral slabs separated by square planer plane, while LaNiO_2.5 is with a columnar arrangement of square planes and octahedra. We show that the magnetic properties of LaNiO_3-δ are mainly determined by octahedral units with limited connectivities, as Ni²⁺ on square planer unit is with low-spin configuration and magnetically inactive. We also explain the structural distortions induced by insertion of square planer units, and conclude by connecting our findings to experimental bulk properties.

[1] R.D. Sanchez, M.T. Causa, A. Caneiro, A. B. Metal-Insulator Transition in Oxygen-Deficient LaNiO3-x Perovskites. Phys. Rev. B 1996, 54 (23), 574–578.

Multiferroics materials are scientifically and technologically promising because of their potential applications in multi-state memory for data storage, magnetic recording, spintronics, quantum electromagnets, and sensors. BiFeO₃ (BFO) is one the rare single-phase room temperature multiferroics and shows ferroelectricity up to 820^oC and antiferromagnetism below 370^oC. However, high leakage current, weak magnetoelectric coupling, presence of cycloidal spin spiral, and critical structural stability of BFO are the bottlenecks for practical applications. Rare earths show many Interesting optoelectronic and magnetoptic properties. In order to enhance the magnetoelectric properties, we investigated Yb and Er co-doped BFO [Bi_1-2x(Yb_xEr_x)FeO₃] ( x = 0.05, 0.10, 0.15, 0.2, and 0.25) bulk and thin film samples. We found that samples with x =0.1 and 0.15. Samples with 20 and 30% rare earth show significant improvement in magnetoelectric properties with very high polarization (~ 100 µc/cm²). The enhanced magnetic properties can be explained by the localization of 4f electrons and high effective magnetic moments of rare earth. A structural phase transition has been observed above 20% doping of rare earth. The effect of co-doping of rare earth on enhanced magnetoelectric properties and phase transition will be discussed in detail.

Hydrogenated graphene has been of great interest for the scientific community due to properties like ferromagnetism and piezoelectricity that are not usually present in graphene; requiring subsequent functionalization after synthesis to obtain them. Hereby, we present a novel method for synthesizing hydrogenated graphene via a single-step process. Hydrogenated graphene was characterized primarily by Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction spectroscopy (XRD) and Physical Properties measurement system (PPMS). Our Raman spectra confirms the presence of the 2930 cm^-1peak associated with hydrogen functionalization, while the PPMS reveals the strongest ferromagnetism at room-temperature of a carbon-based material with a curie temperature higher than 350 K. Preliminary research also suggests the presence of ferroelectricity within our material. These results are strong evidence of hydrogenated graphene’s capabilities for its implementation to spintronics & memory applications.

A₂B₂O₇ pyrochlores are of great scientific interest because of their interesting magnetic orderings (spin-liquid, spin-ice state, Neel-order) and crystal structure. Rare earth pyrochlores shows Ising-like spins, frustrated ferroelectricity, and multiferroic behavior. Pyrochlores also have the ability to immobilize actinides, thus could be used for the disposal of nuclear waste. We report on dielectric, electrical conductivity, magnetic behavior of pyrochlore Ho₂Ti_2-xO_{7- d} (x = 0.0 and ± 0.08) down to 5 K . For the better understanding of dynamics of phase transition and spin-phonon coupling, we employed micro-Raman spectroscopic studies of phonon spectra from 82 K- 700 K. Magnetic excitation induced phonon renormalization is evident in the low temperature magnetic phase. The presentation will discuss the results in details.

The discovery of single-phase magnetoelectric materials and their analysis of coupling mechanisms between spin and polarization is important from the point of view of next generation logic and memory devices. Herein, we report fabrication, dielectric, ferroelectric and magnetic measurement of a Pd-doped room-temperature magnetoelectric multiferroic PbPd_0.3Ti_0.7O₃ (PbPdT) thin film. Highly oriented PbPdT thin films were deposited on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 substrates by following laser ablation processes in oxygen atmosphere using pulsed laser deposition technique. Diffraction studies revealed that the film had tetragonal phase with (001) orientation. The surface morphology studies using atomic force and scanning electron microscopic techniques suggest a smooth and homogeneous distribution of grains on the film surface with roughness ~2 nm. The dielectric measurements as a function of temperature were carried out in Pt/PbPdT/LSMO metal-dielectric-metal capacitors that showed diffused behavior over a large range of temperatures at several frequencies, and exhibited a higher dielectric constant ~3200 at room temperature. The measured polarization hysteresis loops at room temperature were attributed to its ferroelectric behavior. A well-saturated magnetization M-H loop with remanent magnetization of 3.5 emu/cm³ was observed at room temperature. The origin of magnetization is argued on the mixed oxidation states of Pd²⁺/Pd⁴⁺ in the PbTiO₃ matrix and complemented by x-rays photoelectron spectroscopic experimental results. These results suggest that our thin films are multiferroic (ferroelectric-ferromagnetic) at room temperature. The details will be presented in the meeting.

Magnetoelectric materials have been of great interest due to their potential for low-power spintronic devices via the electric field switching of magnetization. Antiferromagnet Cr₂O₃ is one of a very few room temperature magnetoelectrics and possesses unique properties such as uncompensated surface spins and perpendicular magnetic anisotropy. [1] Since the first demonstration of the electric field control of exchange bias in bulk single crystal Cr₂O₃ heterostructures [2], intense effort has focused the demonstration of magnetoelectric switching using Cr₂O₃ thin films at room temperature. [3,4] The existence of twin domains in thin films grown on metallic electrodes, however, leads to high leakage current and dielectric breakdown fields that can only be circumvented by growing rather thick films (250-500 nm). [4,5] By using an isostructural epitaxial oxide electrode, V₂O₃, recent studies have shown the reduction and even possible elimination of twin domains in Cr₂O₃ films. [3] Dielectric and magnetoelectric switching studies of 200 nm thick films show bulk like performance, however, for next generation logic and memory the films must be scaled down. [6]
Here we present an investigation of the electrical endurance and magnetic properties of very thin (30-60 nm) single crystal Cr₂O₃ films grown by pulsed laser deposition onto V₂O₃ buffered (0001) oriented Al₂O₃ substrates. Our results show that 60 nm single crystal thin film has bulk-like resistivity ( 10¹² cm) and significantly improved breakdown voltage (150-300 MV/m). Using magnetometry, we investigate exchange bias of thin film Cr₂O₃/ferromagnet heterostructure. The blocking temperature is found to be at 285 K which is higher compared to twinned films with similar or greater thickness in literature. [7] Further, Second Harmonic Generation confirms bulk magnetoelectric order of our single crystal thin film at room temperature. These results indicate the importance of crystallinity to realize bulk like properties in very thin films at room temperature.
[1] X. He, Y. Wang, N. Wu, A. N. Caruso, E. Vescovo, K. D. Belashchenko, P. A. Dowben, and C. Binek, Nat. Mater. 9, 579 (2010).
[2] P. Borisov, A. Hochstrat, X. Chen, W. Kleemann, and C. Binek, Phys. Rev. Lett. 94, 117203 (2005).
[3] T. Kosub, M. Kopte, R. Hühne, P. Appel, B. Shields, P. Maletinsky, R. Hübner, M. O. Liedke, J. Fassbender, O. G. Schmidt, and D. Makarov, Nat. Commun. 8, 13985 (2017).
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The complex perovskite Na_0.5Bi_0.5TiO₃ (NBT) is a promising lead-free piezoelectric with interesting ferroelectric (T_c = 670 K) property and phase transition. The unusual A-site positional disorder and relexor ferroelectric behavior in NBT makes it an important material for practical applications. Rare earth doped perovskites show interesting magneto-optic and opto-electronic properties. In order to develop a room temperature magneto-electric multiferroic material, we explored Eu-doped NBT Na_0.5Bi_0.5TiO₃ (NBT:Eu) with different concentrations. In this paper we report on optical, dielectric, ferroelectric, magnetodielectric, and magnetic properties NBT:Eu prepared by solid-state reaction. The as-synthesized NBT:Eu materials have Eu concentration between 1 to 20 at.% using A-site substitution in the general ABO₃ perovskite oxide are single phase, polycrystaline materials as determined by powder X-ray diffraction study [1]. The luminescence intensity monitored at 617 nm increases linearly with the Eu concentration increase up to 20 at.% without indication of typically observed concentration quenching effect. The photoluminescence (PL) and cathodoluminescence (CL) spectra are dominated by sharp characteristic emission lines corresponding to Eu³⁺ intra-4fⁿ shell transitions centered at 593 nm (⁵D₀→⁷F₁), 617 nm (⁵D₀→⁷F₂) and 700 nm (⁵D₀→⁷F₄), respectively. The PL excitation spectroscopy reveal that optically active Eu³⁺ ion centers can be effectively excited with photons between 325 nm to 550 nm via energy migration or direct resonant excitation processes. The electron paramagnetic resonance shows that characteristic electron spin resonance signal of Eu²⁺ ions are not observed in these material. The crystal field calculations and temperature dependent PL and CL studies have shown that the luminescence of Eu³⁺ ions occupying centro-symmetric and non-centro-symmetric sites; however corresponding luminescence temperature thermal quenching mechanisms are different for photon- and electron-excited spectra. Recently studied dielectric permittivity, dielectric loss, impedance, and phase angle of NBT:Eu as function of frequency [2] were correlated with corresponding magnetic properties of the NBT:Eu subjected to up to 1 Tesla. It was found that material shows a novel room temperature ferromagnetic like behavior, which is unusual, but can be explained by the local structural disordering or phase coexistence. Finally, we will discuss the defects in NBT:Eu and their effect on observed optical, electric and magnetic properties of this novel multifferroic material.
[1] R. Prusty, P. Kuruva, U. Ramamurty, T. Thomas, Sol. State Commun., 173 (2013) 38-41.
[2] A. Kalaskar, B. Narayana Rao, T. Thomas, and R. Ranjan, J. Appl. Phys. 117 (2015) 244106.

To date, physical coupling of complex-oxide materials has been predominantly demonstrated through epitaxial means, which limits the breath of possible combinations of materials and restricted by lattice mismatch conditions between the epitaxial layer and substrate. Therefore, there is an increasing trend across all functional single-crystalline materials to lift-off the epitaxial layer from the substrate and transfer the membrane onto foreign substrates. This not only opens up immense amount of freedom to integrate various functional materials on one platform, it also potentially allows reuse of expensive and exotic substrates, expediting commercialization. To this end, we have developed a method to generate freestanding complex-oxide membranes by growing on top of graphene as well as precise cleaving at an epitaxial interface. This allows us to transfer and stack arbitrary complex-oxide membranes which was not possible by epitaxial means, allowing us to study new oxide interfaces and coupling phenomena.

We exfoliated several single-crystalline complex oxide films (e.g. SrTiO₃, CoFe₂O₄, Y₃Fe₅O₁₂ etc.) by growing them on top of graphene coated substrates. The graphene layer acts as a slippery exfoliation layer which allows crystal growth but also epitaxial lift-off^1,2. Additionally, we have found that it is possible to exfoliate single crystalline PMN-PT from STO surface with atomic precision. PMN-PT has one of the highest piezoelectric coefficients, and also shows exotic properties such as high pyroelectricity which has only started to be investigated³. Not only were we able to exfoliate the films, we were able to reuse the substrate several times to continuously produce freestanding oxide membranes using a single substrate.

Using these films, we were able to fabricate a freestanding multiferroic bilayer laminate (CFO/PMN-PT), where CFO is a magnetostrictive material and PMN-PT is a piezoelectric material. We compared the magnetoelectric coupling of our freestanding device to a device not exfoliated from the substrate. An order of magnitude larger signal could be measured for the freestanding device compared to the device still clamped to the substrate, which demonstrate the applicability of our method.

Phase transitions in transition metal oxides, such as metal-Insulator transition, ferromagnetic, ferroelectric materials, are useful phenomena to realize novel electrical/magnetic switching and sensing devices. Among them, the strongly correlated electron system of VO₂ exhibits an ultrafast metal-insulator transition (MIT) under external stimuli. If the MIT in VO₂ can be controlled by a voltage, a field effect transistor (FET) could be demonstrated with ultrafast and ultralow power consumption switching properties. Here, to enhance the resistance modulation in VO₂ FETs, we employ, as a gate insulator, a layered material of hexagonal boron nitride (hBN) which is atomically flat and has much fewer electron trap sites than the other gate oxides have. The VO₂ thin film with a thickness of 10 nm was grown on TiO₂ by the pulsed laser deposition method and, then, was etched down to 5 micrometer in width. The thickness of hBN was 30nm as top gate geometry. This VO₂ thin channel undergoes a metal-insulator transition at around 325 K in the heating process. The transfer curve and gate leakage current of the VO₂ FET was measured at 310 K (the drain bias V d is 1.0 V). The drain current increased clearly when applying the positive gate voltage (electron doping), while the current decreased under the application of the negative gate voltage (hole doping). We find no hysteresis and memory behavior in the VO₂ FET under gating, indicating no chemical reactions and trapping at the interface. This gate-response speed was observed to be faster than the measurement limit of 400 ms. Namely, a slow response caused by the carrier trap at the interface was not observed. We also investgated the stability of the hBN gate insulator under gating. We can find that the drain current rarely changes during the application of the gate voltage for 3000sec. These observations show that hBN is a chemically and electrically stable material for the gate insulator application. The resistance of VO₂ was modulated by up-to 1.2% at a gate voltage of 20 V, which is 4 times larger than in previously reported VO₂ FETs with oxide gate insulators such as SiOx. This is likely because with hBN the interface carrier scattering is strongly suppressed. Our results indicate the advantage of use of h-BN as 2D material in order to demonstrate voltage control of transition metal oxides via electrostatic modulation.

There is currently quite a bit of interest in multiferroics and magnetoelectrics for low power electronics. While there have been many new multiferroics added to the well-studied BiFeO3 system, most of them have transition temperatures below room temperature. Thus, discovering new multiferroic and magnetoelectrics is desirable. Doped magnetic ferroelectrics (DMFE) are attracting attention in this regard since many of them have to potential to multiferroic/magnetoelectric property at room temperature. One such model system is BaTiO3 doped with magnetic impurities. Our strategy is to take well known ferroelectrics, such as Hf0.5Zr0.5O2 (HZO) and introduce magnetic impurities, such as cobalt into the lattice. Prior work on Co-doping of TiO2 has revealed interesting magnetic effects and that forms the backdrop for our work. HZO is a simple fluorite showing robust ferroelectricity at the nanoscale and good compatibility with silicon and hence is a favored candidate for the next generation memory and logic devices. HZO has many phases, among which orthorhombic and rhombohedral phases possess ferroelectricity. Using epitaxy as the underlying driver, we have been able to demonstrate high quality HZO thin films in the 1-20nm thickness range. Doping with 5% Co does show the existence of a magnetic moment. We are presently studying the effects of Co concentration on the magnetic and ferroelectric stability. We demonstrated the existence of ferroelectricity by PFM and P-E loop measurements and tested the magnetic hysteresis loops. We will present results of this investigation on the role of HZO crystallinity and Co-doping on the magnetoelectric properties.

Spin capacitors have the potential to store both the electronic charge and magnetic spin that can produce conventional electric current and spin polarized current. The time evolution of spin polarized electrons injected into the piezoelectric material can be used for accurate sensing of magnetoelectric fields. To further study the application of multiferroic spin capacitors for future use in memory applications, Ferromagnetic/Ferroelectric/Ferromagnetic tri-layer artificial multiferroelectric structures in spin capacitor configuration were fabricated. Previous experiments were done by sputtering Iron (Fe) and Nickel (Ni) electrodes on lead zirconate titanate (PZT). In this paper, we report dielectric, magnetic, and magnetoelectric coupling in the tri-layer composite with Cobalt (Co) as the magnetic electrode sputtered onto PZT polycrystalline of different thickness. Various dielectric measurements were carried out, including: capacitance, impedance, loss coefficient, dielectric permittivity, and phase angle measurements done by a wide range of frequencies (100 Hz – 5 MHz) and magnetic fields (0 Tesla – 2.0 Tesla) at room temperature, and their interaction with the presence of the applied magnetic field (magnetocapacitance and magnetoimpedance) were analyzed. We also compared the magnetodielectric measurements of the Co/PZT/Co spin capacitor with Co/PZT/Ag and Ag/PZT/Ag tri-layers structures with their respective their behaviors. The magnetodielectric studies of different PZT layer thickness will be discussed in details.

Multiferroics materials have attracted a lot of attention due to their potential technological applications. GdCrO₃ that belongs to the family of rare-earth chromites (RCrO₃) has recently been reported to be a single phase magnetoelectric multiferroic. However, the origin or existence of ferroelectricity in GdCrO₃ remains ambiguous. We have, therefore, examined the properties of bulk and thin film samples to explore their multiferroic properties. Magnetic measurements have revealed that it shows an antiferromagnetic transition with weak ferromagnetism below the Néel temperature T_N 169 K as well as spin reorientation, and giant magnetocaloric effect (MCE) at lower temperatures. In this work, structure, particle size, substitution effect, magnetic, and dielectric properties of this family of material will be presented.

Polar iron oxides, which exhibit both ferroelectricity and (anti)ferromagnetism, are multiferroic materials, which promises potential application for new type memory devices. Hexagonal ReFeO₃ (h-ReFeO₃) (Re = Rare earth element) with YMnO₃(YMO)-type structure exhibits ferroelectricity and weak-ferromagnetism. First principle calculation revealed that h-ReFeO₃ exhibits improper ferroelectricity (geometric ferroelectricity). Due to the strong magnetic interactions between Fe³⁺ cations, the higher magnetic order temperature in h-ReFeO₃ compared with hexagonal manganites is expected. A magnetic phase transition temperature of h-ReFeO₃ increases with decreasing ionic radius of Re cation. h-ReFeO₃ which consists of small ionic Re cation is a candidate for a high temperature multiferroic material.
Metastable h-ReFeO₃ were obtained by containerless melt crystallization process and crystallization from amorphous phase. Masui et. al. prepared Sc substituted LuFeO₃ by containerless process, and h-Lu_1-xSc_xFeO₃ was obtained in the range of x = 0 - 0.8. h-ReFeO₃ (Re = Tb-Lu, Y ) were stabilized on substrates such as YSZ(111) and Pt(111)/Al₂O₃(0001) in film form by pulsed laser deposition (PLD) and sputter techniques. Previously, we stabilized h-ScFeO₃ film on an Al₂O₃(0001) substrate using PLD technique. However, preparation of h-ScFeO₃ have not been reported yet. In this study, we attempted to prepare h-ScFeO₃ film on electrodes and investigated their physical properties and interface structure.
h-ScFeO₃ was deposited on various electrodes such as ITO, ZnO, SrRuO₃, and (La,Sr)MnO₃ by PLD technique. Single phase h-ScFeO₃ was obtained on La_0.8Sr_0.2MnO₃//SrTiO₃(111). Crystal structure of film was characterized by X-ray diffraction (XRD) and scanning transmission electron microscope (STEM). XRD measurements and HAADF-STEM observation revealed that h-ScFeO₃ with YMO-type structure was epitaxially grown with the relationships of ScFeO₃(0001)//La_0.8Sr_0.2MnO₃(111)//SrTiO₃(111).
The YMO-type ScFeO₃ film showed a ferroelectric P-E hysteresis loop with Pr ~ 4.9 μC/cm² and weak ferromagnetism with T_N = 195 K. The T_N value is the highest in hexagonal ReFeO₃ and ReMnO₃. Interface studies revealed that the monolayer consisted of two paired Sc cations connected to the YMO-type and perovskite structures. One of two paired Sc cations was displaced upward because of Coulomb repulsion from the Mn cations of La_0.8Sr_0.2MnO_3. We deposited ScFeO₃ film on various perovskite films or substrates. Single phase h-ScFeO₃ film was obtained on perovskite films or substrates with low valence B site cations. This results indicated that the valence of B cations in perovskite played an important role in the formation of the interface monolayer.

The B2-orderd intermetallic magnetic compound FeRh exhibits a thermodynamically first-order phase transition in vicinity of room temperature that makes it a highly intriguing subject for both fundamental and applied study. The structure, magnetic, and magnetocaloric properties of Pd doped in FeRh alloy have been investigated. Experiments revealed that there are same structures type FeRh (CsCl-type) with different lattice parameters and Fe_15.7Rh (bcc-W type) phases are coexist in FeRh_0.95Pd_0.05 sample. The wide phase transition temperature range is about 250 K, which is from 150 K to 400 K for this alloy. The maximum magnetic entropy change (-ΔS_M) is 16.6 J/kg K for 0 - 30 kOe field changes and refrigerant capacity (RC) of 1347 J/kg was obtained due to the contribution of large transition temperature range. The excellent magnetocaloric performance indicates the applicability of FeRh_0.95Pd_0.05 as an appropriate candidate for magnetic refrigerant.

Magnetic properties of cobalt nickel spinel oxides Co_3-XNi_XO₄(CNO) have been extensively studied over decades [1,2]. In the Co-Ni-O system, CNO is known to be metastable under atmospheric conditions as the rock-salt phase (Co_1-YNi_YO) is stable when T > 1173 K while NiO and Co₃O₄ phase are stable when T ≤ 1173 K [3]. Due to CNO’s metastability, it is believed to be difficult to chemically synthesize CNO for X > 1 through conventional process such as thermal decomposition and co-precipitation [2,4]. CNO thin films grown by physical vapor deposition for X > 1[5,6], reportedly yield relatively low Curie temperatures (T_C = ~100-200 K for X = 1) in contrast to higher T_C for powder CNO (T_C = ~400 K) [1]. Recently, it was reported that CNO samples for X > 1 can be synthesized by thermal decomposition of organic metal frameworks (MOF) for super capacitor electrodes applications [7]. In this case, CNO for X > 1 can be synthesized from Co-Ni MOFs where Co and Ni ions are uniformly mixed at an atomic scale. In this study, we report successful synthesis of CNO particles for X = 0-1.28 from Co_1-YNi_Y(OH)₂ precursors in which Co and Ni ions are uniformly mixed at an atomic scale, and the magnetic properties of the CNO at room temperature.
[Experimental] The Co_1-YNi_Y(OH)₂ precursors were prepared from a hydrochloric aqueous solution containing Co²⁺ and Ni²⁺ by a uniform precipitation process. They were the annealed at 573-873 K for 3 hours in the air to transform into CNO particles. Their magnetic properties were investigated by vibrating sample magnetometry (VSM), electromagnetic resonance (EMR) spectrometry, and superconducting quantum interference device (SQUID) magnetometry at temperature from 5 K to room temperature (RT).
[Results and Discussion] X-ray diffraction spectrometry (XRD) of the samples have shown that CNO particles are successfully synthesized for X ≤ 1.28 without producing any other phases. The lattice parameter of CNO increases linearly with increasing X, yielding with Vegard’s law. The mean particle size of CNO estimated from XRD data using Scherrer’s equation is 8~33 nm. Magnetic measurements by VSM show that CNO particles for X ≥ 0.74 possess remanent magnetization and coercivity at RT, it reveals that the CNO particles are ferrimagnetic at RT. Microwave absorption peaks are observed in the ENR spectra of CNO particles for X ≥ 0.74. The EMR spectra of the CNO samples exhibit absorption peaks best fitted only by the Gaussian function. The peaks can generally be fitted by the Gaussian when it contains a ferromagnetic (ferrimagnetic) component and/or the Lorentzian when it has a paramagnetic one in the sample. Hence, it is suggested that the ferromagnetic CNO samples possess a fairly high crystallinity with few defects breaking magnetic interaction in the crystal. SQUID measurements of the samples shows that their saturation magnetization increases with increasing X for X < 0.74 and decreases for X ≥ 0.74.
[References]
[1] S. Chikazumi, Physics of Magnetism (Shokabo, Tokyo, 2001) 18th ed.,Vol. 1, p. 225 [in Japanese]. [2] F. K. Lotgering., Philips Res. Rep. 11, 337 (1956). [3] R. J. Moore, et al., J. Mater. Sci. 9, 1393, (1974). [4] S. Kuboon, et al., Ind. Eng. Chem. Res. 50, 2015, (2011). [5] J. S. McCloy, et al., J. Phys. Chem. C. 119, 22465, (2015). [6] Y. Bitla, et al., Sci. Rep. 5, 15201, (2015). [7] S. Chen, et al., J. Mater. Chem. A. 3, 20145, (2015).

Over the past few decades, phase change materials (PCM) has received a lot of attention for its application such as novel non-volatile memory,[1-4] switches[5,6] and sensors.[7] Typically, PCMs exist in an amorphous and one or more crystalline phases, in which significant optical and electrical contrast can be observed. The change in state can be rapidly and repeatedly switched by inducing optical pulses or electrical (Joule) heating.[8,9] In 2011, F. Xiong et al. proposed a low power switching using GST with programming current of 1 to 8 µA and programming voltage of below 1V, indicating an energy loss of femtojoules scale.[5] This design not only brought a promising prospect to ultra-low power electronics and memory development but also proposed inspiration in voltage control of ferromagnetic resonance (FMR).
Here, a systematic study of the coupling effect in Si/Ge₂Sb₂Te₅/FeCoB heterostructures is presented. We investigated the FMR change in FeCoB film induced by crystalline phase change in GST film. A large shifting in FMR field of 15 mT in FeCoB film can be observed when GST is highly crystallized after annealing at 300 °C. And the uniaxial magnetic anisotropy field is also strengthened when Ge₂Sb₂Te₅ is highly crystallized in hexagonal phase via interfacial coupling effect. With all these results, a novel structure using PCMs for voltage control of FMR is proved to be applicable. This structure can be predicted to have very low operation voltage as well as ultra-low energy consumption, showing an extraordinary advantage in the development novel fast response, energy efficient tunable microwave devices and non-volatile magnetic memory.

Oxide materials with unexpected transport and magnetic properties at interfaces have generated great interest in multifunctional heterostructure design. One of the best-known systems is the interface between LaAlO₃ and SrTiO₃[1], exhibiting a two-dimensional electron gas in a strongly asymmetric quantum well due to possible switching or spintronic device application[2]. There is growing experimental evidence that the carrier accumulation in a quantum well at the interface leads to magnetic order that may be tunable or switchable if sufficient carrier density control can be achieved. However, except for limited tunability by electrostatic back-gating, it has proven to be difficult to control the carrier density in the electron gas.
In this work, we have chosen to use a similar electronic system consisting of a thin LaTiO₃ film, with a thickness of a few unit cells, embedded in SrTiO₃. While metallic conductivity at the LaAlO₃/SrTiO₃ interface appears abruptly when the LaAlO₃ film thickness exceeds 4 unit cells, the total carrier number in the LaTiO₃/SrTiO₃ system is continuously tunable over a wide range by changing the number of La atoms in the LaTiO₃ layer. Since the presence of a single LaO layer in the perovskite lattice is sufficient to induce metallic conductivity, a polar discontinuity does not arise, but carriers are still confined in a quasi-2D quantum well and form a high-mobility layer, similar to the LaAlO₃/SrTiO₃ interface. When the heterostructure is grown on a single crystal SrTiO₃ substrate and capped with a SrTiO₃ film that is grown at low temperature, the difference in the dielectric permittivities of the substrate and the cap layer leads to an asymmetric quantum well, similar to the LaAlO₃/SrTiO₃ heterostructures.
By using this specially designed heterostructure, we discuss how to determine the depth distribution of carriers while changing the LaTiO₃ or SrTiO₃ cap layer thickness and look at the effect of the carrier distribution on the magnetotransport characteristics. Our experiments show that a large negative in-plane magnetoresistance effect occurs at low temperature when the magnetic field is applied parallel to the current, in a qualitatively similar fashion to LaAlO₃/SrTiO₃ interfaces[3]. We also discuss the effects of back-gating on the magnetotransport characteristics and interpret the results in terms of carrier depth distribution changes in the heterostructure under electrostatic bias. We attempt to distinguish whether the observed negative in-plane magnetoresistance is related to carriers confined at the interface or those that distribute over a larger depth in the substrate-side tail of the quantum well and how such carrier redistribution affects the carrier density in the quantum well.

References:
[1] A. Ohtomo, and H. Y. Hwang, Nature, 427, 423 (2004).
[2] N. Reyren, M. Bibes, E. Lesne, J.-M. George, C. Deranlot, S. Collin, A. Barthelemy, and H. Jaffres, Phys. Rev. Lett. 108, 186802 (2012).
[3] M. Diez, A. M. R. V. L. Monteiro, G. Mattoni, E. Cobanera, T. Hyart, E. Mulazimoglu, N. Bovenzi, C. W. J. Beenakker, and A. D. Caviglia, Phys. Rev. Lett. 115, 016803 (2015).

Magnetoelectric (ME) sensors currently have attracted great interest due to their capability to detect magnetic fields in the picotesla regime at room temperature, showing a high potential in the biomagnetic field detection. Based on the magnetoelectric effect of ME sensors, the applied magnetic field induces a change in the strain of magnetotrictive layer which will be transferred to the piezoelectric material via the mechanical coupling. An electrical voltage across the piezoelectric layer is thus generated and can be measured directly. However, for the application of non-invasive medical imaging like magneto-encephalography or -cardiography (MEG, MCG), a magnetic field sensor with a limit of detection (LOD) in the range of picotesla is required. To reach this goal, the LOD of ME sensors still needs to be further improved.

There are many approaches to improve LOD of ME sensors, either by increasing the sensitivity, or by decreasing the intrinsic noise level. A simple solution for improving the sensitivity is to optimize the geometry parameters of the ME sensor through a parameter study. Recent studies have investigated the influence of the geometry parameters of the individual layers and the substrate layer on the sensitivity of cantilever ME sensors using finite element model [1][2]. Here, an experimental model is built up for determining the basic parameters of the cantilever ME sensor.

In this study, we present cantilever ME sensors based on magnetoelectric composites with various geometry parameters. The cantilever contains poly-silicon layer, piezoelectric material (AlN) and magnetoelectric material (FeCoSiB) in sequence. The performance of the presented sensors is studied based on the sensitivity, voltage noise density, quality factor and LOD. Three different parameters including the cantilever length, the substrate thickness and the thickness ratio between the magnetostrictive and piezoelectric layers are analyzed to understand their effect on sensors’ performance. Since the devices were fabricated in 8 inch silicon technology, the availability of a relavant number of devices allows a statistical analysis. The results are used to identify the optimal geometry parameters for cantilever ME sensors and can be applied to develop next generation devices.

Funding by the DFG via the Collaborative Research Center SFB 1261 is gratefully acknowledged.

[1] M. C. Krantz, J. L. Gugat and M. Gerken, AIP ADVANCES, 3, 062135 (2013)
[2] J. L. Gugat, M. C. Krantz and M. Gerken, IEEE Transactions, 49, 5287 (2013)

Lead free environment friendly multiferroic perovskites thin films are of research interest due to its reduction in dimensionality leading to high energy storage and magneto electric behavior and hence are suitable for several electronic device applications. Herein, we report the tetragonal multiferroic heterostructure thin films fabricated by pulse laser deposition technique consisting alternate of Ba(Zr_0.30Ti_0.70) _0.99 Fe_0.01 O₃ (BZTF) and Ba(Zr_0.30Ti_0.70) O₃ (BZT) layeres grown upon LSMO coated on MgO (100) substrate. X-ray diffractometry data analysis suggests phase purity of thin films oriented along (100) direction. Micrograph structural analysis using atomic force microscopy revealed a homogeneous distribution of grains with roughness ~ 3- 5 nm. Dielectric measurements on Au/BZT/BZTF/LSMO and Au/BZTF/BZT/LSMO metal insulator metal capacitors using a impedance analyzer as a function of temperature (100-650 K) and frequency (10²-10⁶ Hz) suggest their relaxor behavior. The slim P-E loop also corroborate relaxor behavior. The temperature dependent magnetic measurements using a physical property measurement system suggest the existence of magnetic nano clustures dispersed in a paramagnetic matrix and attributed to its super para magnetic behavior. A large energy storage density is estimated from our P-E loop data. These results suggest the present heterostructured thin films could be promising multiferroic materials for high energy density applications.

The environmentally benign nature of Sn²⁺ would make SnTiO₃ (SnTO) derived compounds an attractive alternative to lead-free dielectric material. Several theoretical studies suggested that the spontaneous polarization of SnTO is larger than that of classical ferroelectric PbTiO₃. Pure phase of SnTO is one of the challenging jobs due to often mixed oxidation states of Sn²⁺ and Sn⁴⁺ in perovskite titanates. The material was synthesized using a chemically route followed by a solid-state reaction method and were annealed it in a mixture ratio of 1:1 methane:hydrogen environment. The phonon spectra of tetragonal ferroelectric SnTO compound were measured as a function of temperatures in the range 82-1250 K using a micro-Raman spectrometer equipped with a CCD detector. Seven phonons are identified in the frequency range 100-1000 cm^-1 at the room temperature and are assigned as per the C_4v point group symmetry. The reduced temperature dependent spectra were analyzed using the damped harmonic oscillator model to obtain the thermal evolution of the mode frequencies and their intensities. Upon increasing temperature, the optical phonon modes were found to be soften and their intensities decreases, as normally expected, due to involved multi-phonon scattering processes. At 823 K, the A₁(TO) phonon located at 441 cm^-1, exhibits anomaly and begins to harden upon further increase in temperature; in addition, the E(LO) mode at 827 cm^-1 disappear around 823 K, suggesting a tetragonal ferroelectric to a high symmetry paraelectric cubic phase transition. The nature of transition is found to be first order displacive type. The appearance of symmetry forbidden broad Raman bands in the high temperature cubic phase is attributed to contribution from phonon density of states. These studies on phonon behaviors on this putative green ferroelectric could provide insight about its structure-property relations and its thermodynamics properties as well. The detail results will be presented at the meeting.

Potassium niobate (KNbO₃) is a lead-free piezoelectric material, respectful of the environment and technologically very important. This is because it has excellent electromechanical properties and diverse applications in electronic and photonic devices. The electro-physical properties, such as the crystalline phase transition, the optical band gap, the dielectric constant, the Curie temperature, the electrical conductivity and the ferroelectric behavior of KNbO₃ can be tuned by suitable dopant, such as isovalent Ba in it’s A-site, and heterovalent Ni on B-site results in a single phase ceramic material with stoichiometric formula [KNbO₃]_1-x [(BaNb_0.5Ni_0.5O_3-δ)]_x for x = 0.1 and δ = 0.25 (KBNNO). The structural, optical, dielectric, charge transport phenomena and ferroelectric properties of this solid solution were systematically investigated. The ceramic material was prepared by the solid-state reaction method and calcined at 1198 K, using starting precursors as K₂CO₃ (99.5%), BaCO₃ (99.95%), Ni₂O₃ (99.9%) and Nb₂O₅ (99.9%). The orthorhombic ferroelectric phase formation of the synthesized samples is confirmed by using X-ray diffractometry and Raman spectroscopic analysis. Surface morphology study using scanning electron microscopy revealed well-sintered nature of the sample with grains are interconnected and densified. Temperature-dependent in-situ x-rays diffraction studies (300-873 K) revealed the phase transitions from room temperature orthorhombic to tetragonal to a cubic phase at elevated temperatures. The temperature dependent Raman spectra studies in the T-range 82-1000 K suggest that several bending and stretching phonon mode frequencies and their intensities exhibit anomalous changes across the phase transitions temperatures. These transition temperatures were corroborated with the DSC measurements. A direct optical band gap of 3.16 eV is estimated from the analysis of the diffuse reflectance spectra using Kubelka-Munk analysis. The dielectric properties of KBNNO were studied as a function of temperature between 80-500 K in a frequency range from 100 Hz to 1 MHz in a metal-ferroelectric-metal capacitor nanostructure. The dielectric constant and the loss tangent at 100 kHz were 280 and 0.01, respectively. The frequency dependence of the AC conductivity showed typical characteristics of the universal dynamic response. Our studies reveal the structure-property relation, fundamental physics and materials science of KBNNO electroceramics, and establishing its potential for ferroelectric device applications.

The one-dimensional (1D) Rashba effect has become much important due to its key role in basic science to realize exotic electronic phenomena, such as Majorana bound states. Similar to the 2D or 3D systems, the modulation of Rashba effect in 1D matrix is the kernel of spintronics for manipulating electron spin. Herein, by investigating the effects of transverse and vertical external electric field (EEF) on the Rashba spin splitting and magnetic anisotropy energy (MAE) in a new type of highly flexible 1D system (Gd-adsorbed zigzag graphene nanoribbons) from first principles, we found that the Rashba spin splitting in such 1D system can be effectively regulated by the transverse EEF. Moreover, perpendicular magnetic anisotropy holds with either transverse or vertical EEF applied, despite obvious modulation of the MAE contributions in k-space as well as the Rashba spin splitting. Based on the analysis of the orbital-decomposed band structures and in-plane spin density under EEF, it is revealed that the MAE contributions along k-line as well as the Rashba spin splitting can be modulated by E_y due to the regulated coupling of Gd-5dx²-y² and d_xy orbitals near E_f, which is driven by the edge states. The modulation of Gd-5dx²-y², d_xy by C-p_z orbitals of edge states is the key to manipulating the magnetic anisotropy, which even plays a decisive role on modifying the Rashba spin splitting in such 1D nanoribbon system. The MAE and 1D Rashba spin splitting are expected to be controlled through modifying the edge states in such systems via EEF or other means. Our study introduces a new strategy to manipulate Rashba spin splitting by edge states and provides new insight into the magnetic anisotropy in 1D Rashba system, which would revitalize further research in ZGNR-based systems within the spintronics and exotic electronic phenomena.

Anatase titanium dioxide (TiO₂) is an important dilute magnetic semiconductor (DMS) as it exhibits room temperature ferromagnetism. However, even after a decade of research, the origin of ferromagnetism in this material system has not reached consensus. In this study, the magnetic properties of two systems, Ta-doped anatase-TiO₂ and undoped-anatase-TiO₂ had been studied in detail. To avoid the effects from a substrate and the interface we have fabricated free-standing membranes using water-soluble Sr₃Al₂O₆ sacrificial layer. A strong correlation between the observed ferromagnetism and Ta-doping has been observed. The magnetic centers and the exchange mechanism in the undoped-anatase TiO₂ have identified. This study proves the intrinsic nature of these dilute magnetic semiconductors.

In recent years, the multifunctional electric systems have a strong tendency toward flexibility because of the flourishing development of wearable and portable devices. As the candidate for non-volatile memory of next generation, Bismuth ferrite (BiFeO₃), one of multiferroics, draws our attention due to its large ferroelecrtric polarization and antiferromagnetism with high transition temperature. This work presents a direct way to combine BiFeO₃ with muscovite in order to make it meet the requirement of the wearable device. In this study, we successfully fabricate high-quality BiFeO₃ thin film on muscovite through van der Waals heteroepitaxy via pulsed laser deposition. In analyses of X-ray diffraction, BiFeO₃ thin film has great crystallinity with the out-of-plane (111), which exactly is the direction of the strongest polarization. Through the PFM and the CV measurements, the results exhibit excellent ferroelectricity. Furthermore, the ferroelectricity does not show observable difference under bending test, which means its great stability against external stress. In the oncoming future, this innovative BiFeO₃/muscovite system will be the most powerful elements in the application of electric technology.

Rashba spin texture is the property of inversion asymmetric systems to maintain a momentum-dependent spin splitting in the reciprocal space. This property has been predicted to support an electric-field control of spin degree of freedom promising for spintronic and magnetoelectric applications. Till now, its tunability is not so satisfactory. Here, we predicted that substantial modulation of Rashba splitting strength can be realized in the ferroelectric (FE) perovskite trilayers by reversing the FE polarization. Using first-principles calculations, we demonstrate that PbTiO3/BiMO3/PbTiO3 (M=Al, Ga, In, Sc) heterostructures along [001] direction are promising candidates accessible experimentally. The middle layer supports large Rashba splitting at the CBM and the change of Rashba spin-orbit strength by FE polarization exceeds 80%. The splitting bands are locked at the band edge by carefully choosing the middle layer and are adjustable due to oxygen network. The designed structure improves the feasibility of utilizing Rashba effect in spintronic devices.

The magnetoelectric (ME) composite has recently attracted considerable attention both for ME effect and for its potential applications[1-2]. S.X. Dong et al. reported that the ME composite can detect a small DC magnetic field less than 10⁻⁷ Tesla[2]. Especially, the ME effect based on magnetic sensor can be used to measure both DC and AC magnetic fields.
In this report, we study the extracting equivalent circuit model and electrical impedance and capacitance dependence on dc magnetic field of magnetoelectric composite resonators. Our research exhibits that the equivalent circuit parameters are strong function of external DC magnetic field. The properties of magnetostrictive materials dependence on external dc magnetic field lead to magneto-impedance and magneto-capacitance effects. Furthermore, the magneto-impedance and magneto-capacitance effects for FeGa/PZT, Terfenol-d/PZT, FeSiB/PZT and FeNiCoSiB/PZT composite resonators are presented.The experimental results show that the magneto-impedance and magneto-capacitance effects for the magnetoelectric composite resonator strongly depends on DC magnetic field (H_dc). 240% and 170% of the magneto-impedance can be achieved at anti-resonance frequency (f_a) of the FeGa/PZT and FeNiCoSiB/PZT resonator, respectively. The variation of magneto-capacitance for FeNiCoSiB/PZT composite resonator exceeds ~112% at anti-resonance frequency. This find represents another principle for design dc magnetic field sensors, MI devices, or tunable electronic devices.

[1]T.X. Nan, Y. Hui, M. Rinaldi, N.X. Sun, Self-biased 215 MHz magnetoelectric NEMS resonator for ultra-sensitive DC magnetic field detection, Sci. Rep.(2013)
[2]S.X. Dong, J.Y. Zhai, J.F. Li, D. Viehland, Small dc magnetic field response of magnetoelectric laminate composites, Appl. Phys. Lett. 88 (2006) 082907.

Multiferroic composite materials with strong magnetoelectric (ME) coupling have led to many novel devices, such as information storage, sensors, energy harvester and tunable magnetic inductors. The capability of tuning magnetic properties by electric field makes such multiferroic composites have great potential for tunable magnetic devices application.
Magnetic coupled resonance wireless power transmission whose energy is delivered from the transmitter (Tx) coil to the receiver (Rx) coil by mutual coupling has received considerable attention because of its use in applications in recent years. The mutual inductance is distance dependent when the coils have been designed, therefore the transfer efficiency of the magnetic resonance coupled wireless power transfer system decreases rapidly once the transfer distance is extened to the undercoupled region or the distance is shortened to the overcoupled region. High permeability materials, such as ferrite cores, are utilized to enhance the mutual inductance in the undercoupled region to achieve high transfer efficiency. In contrary, different coils configurations are presented with lower mutual inductance in overcoupled region to suppress frequency splitting to increase the efficiency. Mutual inductance should be enhanced by using magnetic core in undercoupled region and be reduced by using complicated loop structure in overcoupled region to achieve high transfer efficiency in. However, the mutual inductance value is usually determined in a previously built prototype, the transfer efficiency is decreased when the transfer distance is changed. The mutual inductance determined by coils configuration limits the transfer efficiency in the overcoupled region and undercoupled region for magnetic resonance coupled wireless power transfer system.
The tunable mutual inductance by electric field using multiferroic composite core has a potential application for efficient magnetic resonance wireless power transfer. Electrostatically tunable mutual inductance with multiferroic composite core consisting of two layers of Nanocrystalline ribbonsand one slab have been demonstrated to tune the mutual inductance
The multiferroic composite coil core consists of two layers of Nanocrystalline ribbons Metglas 1K107(30um thick each) and one PZT (0.5 mm thick) piezoelectric slab that was poled along its thickness direction were bonded together with ethyl cyanoacrylate glue. A control voltage ranging from 0 to 600 V, which results in an electric field from 0 to 12 kV/cm in the PZT slab, was applied across the thickness direction of the PZT slab.
The magnetic hysteresis loops of ME core is tested by VSM(Vibration Sample Magnetometer) , the magnetic hysteresis loops is changed when the control voltage is applied on the PZT slab. Since the magnetic anisotropy is different under different electric field, the relative permeability of the matglas ribbons μ_eff =4πM_s/H_eff +1 can be changed, where H_eff is the total effective anisotropy fields in the magnetic ribbons, thus a tunable mutual inductance is expected.
Electrostatic tunable mutual inductance based on multiferroic composite core has been made and tested. According to the experiments results, the mutual inductance tunability, which is mediated by electric field controlled magnetic anisotropy and permeability, achieved a high value of 63.2% with minimum negligible power consumptions, which can be assumed to be a passive devices. The change of the magnetic anisotropy can also been observed in magnetic hysteresis loops. The mutual inductance tunability achieved in such coils based magnetoelectric core has a great potential for efficiency optimization of wireless power transfer in a large region of distance.

Double perovskite (DP) compounds with general formula A₂BB’O₆ (where A = alkaline-earth or rare-earth ions and B = transition metal elements), belong to a family of materials that have attracted much attention due to their numerous fundamental physical properties. In these materials, the d electrons at BB’ sites give rise to strongly correlated electronic and magnetic properties, such as metal-insulating transition, half-metallicity, large magnetoresistance, multiferroicity, long/short-range and frustrated magnetic ordering and many others. In this research, we obtained new DP compounds by traditional solid-state reaction method at 1300 K in air. Rietveld refinement of Nd₂CoFeO₆ and Pr₂CoFeO₆ indicates that both compounds crystallize in the orthorhombic Pnma space group with similar lattice parameters, and with the BO₆ octahedral half occupied by Co and Fe ions at 4b Wyckoff site. Electrical transport measurements have shown a huge decreasing in the electrical resistivity from 10⁷ Ω.mm to 1 Ω.mm as a function of temperature. DC magnetization of both compounds indicates a critical temperature around 220 K. Above this critical temperature and up to 380 K, the magnetic susceptibility does not follow the conventional Curie-Weiss law in the paramagnetic state. At higher temperature interval, from 380 K up to 700 K, we found a lower second slope in χ^-1 vs. T suggesting an unexpected increase in effective magnetic moment which are not predicted by taking into account the possible scenarios where one considers different valences and spin states for Nd/Pr, Fe and Co ions. Interesting, another peak in dM/dT curve is observed at low temperature. In this context, we will discuss these results correlating the complex magnetic, electrical transport, and structural properties of these disordered double perovskite compounds.

Emergent phenomenon is observed on the heterostructure interface, consisting of complex coupling of spin, orbital, lattice and charge. As a room-temperature stabilized G-type antiferromagnetic materials, the NiO is a favourable physic stage for investigating the interfacial magnetic coupling. Co/NiO interface has been investigated continually for its magnetic coupling and domain structure in (100) and (111) orientation, but few researchers focus on the in-plane orbital structure and related magnetic anisotropy in (110)-oriented Co/NiO interface. We find in-plane magnetic uniaxial anisotropy of polycrystalline Co films grown on the (110)-NiO, contrast to its isotropic crystallology nature. Depending on the X-ray linear dichroism spectrum(XLD) and X-ray circular dichroism spectrum(XMCD) shows, we found this uniaxial anisotropy emergent from the charge transfer of Co to Ni in the [-110] direction, and the hybrization of d-orbitals of Ni and Co. This work promotes the comprehension of the complex coupling of spin, orbital and charge, and provides possibilities for the NiO-based spintronics.

The changes in crystal and magnetic structure of BiFeO₃ produce by partial replacement of Bi ions by Ho ions has been evaluated with powder X-ray diffraction, neutron powder diffraction, dielectric and magnetization techniques. The present work demonstrates that Bi_1-xHo_xFeO₃ (x = 0.05, 0.10, 0.15, & 0.2) multiferroics shows change in crystal structure at x > 0.05. The sample with x = 0.05 exhibits rhombohedral structure (space group R3c), while the other three samples (x > 0.05) exhibit mixed phase with coexisting rhombohedral (R3c) and Orthorhombic (Pnma) structure. This change in the crystal structure is attributed to the distortion of FeO₆ octahedra via substitution of Ho at phase boundaries. The magnetization studies indicate that doping of Ho in pristine BiFeO₃ leads to enhancement in the ferromagnetic moment. We find that doping of Ho breaks the spin cycloid phase of BiFeO₃ and creates a canted G-type antiferromagnetic structure in the hexagonal phase whereas the orthorhombic phase exhibits a collinear G-type AFM structure. The canting angle increases with increase in doping with Ho, leading to an enhancement in the ferromagnetic component in magnetization. Dielectric constant (ε′) and loss factor (tanδ) are measured in frequency range 1 kHz to 7 MHz and ε′ and tanδ show dispersion behaviour at low frequencies. The significant improvement in magnetization and dielectric properties is achieved by Ho substitution which in turn enhances the potential of BiFeO₃ for multiferroics applications.

Multiferroics having composition Bi_0.80-xBa_0.20Ho_xFeO₃ (BBFO, BBHFO-5, BBHFO-10, BBHFO-15 and BBHFO-20 for x = 0, 0.05, 0.10, 0.15 and 0.20 respectively) were synthesized by method of solid state reaction. The crystal structure has been studied using X-ray diffraction technique. The X-ray patterns show enormous transform in crystal structure at concentration x=0.20. The Rietveld refinement of XRD patterns indicates that at concentration x=0 sample have rhombohedral structure with R3c space group while for the concentration x= 0.05, 0.10, 0.15 and 0.20, the mixed phase including rhombohedral R3c and triclinic P1 space groups were obtained with best fitting. This phase transformation in crystal structure is observed due to mismatching of ionic radii of doped ions and parent ions. Magnetic properties of all samples were carried out by using vibrating sample magnetometry. M-H hysteresis loops shows that with doping of Ba and Ho antiferromagnetic BiFeO₃ (BFO) transforms into ferromagnetic. The dielectric properties were carried out by impedance spectroscopy which shows that dielectric properties are enhanced with co-doping in comparison of the pristine BFO due to structure deformation and decrease in oxygen vacancies with higher concentration of Ho.

Multiferroics are materials possessing different ‘ferroic’ orders such as ferro-magnetism, ferro-electricity, ferro-elasticity etc. Many rare earth ortho-chromates and ferrites were reported to exhibit multiferroic properties and magneto-electric coupling[1]. It was reported that in many of the rare earth chromates, the onset of polar ordering appears at temperatures slightly higher than their antiferromagnetic ordering[2]. Here we investigate the temperature evolution of structural and dielectric properties of Holmium based ortho-chromate, HoCrO₃. Polycrystalline HoCrO₃ was prepared by conventional solid-state synthesis route. Reitveld refinement of the synchrotron x-ray diffractogram at room temperature confirms the perovskite orthorhombic structure crystallizing in the Pbnm space group. Temperature evolution of synchrotron x-ray diffraction points to the presence of variation in unit cell volume around 242 K and 100 K. Magnetic measurement confirms the antiferromagnetic transition at 140 K. The compound exhibits a dielectric constant of 19.46 at 1.784 kHz at room temperature. Also, temperature variation of the real part of the dielectric constant reveals a typical relaxor-like behaviour with low temperature and high temperature plateaus along with two anomalies at 240 K and 140 K. Analysis of peaks in the loss tangent curves confirms thermally activated dielectric relaxation. The dielectric anomaly at 140 K indicates a possibility for magneto-electric coupling in the material. However, the peak at 242 K corroborates the volume change observed in synchrotron analysis, which is associated with a possible structural transition (Pbnm to Pna2₁), usually observed in similar rare earth based ortho-chromates having ferroelectric ordering. Two feeble features around 100 K and 242 K were observed in the temperature dependence of specific heat curve and these corresponds to the structural transitions observed in the unit cell volume derivative curve with respect to temperature. In the present work synchrotron x-ray diffraction technique was used for the first time to confirm the structural transition in HoCrO₃.

Dielectric capacitors with high electrostatic energy density, long-term stability, and environmental friendliness are strongly demanded in modern electrical devices. Here we fabricated Bi_4-xLa_xTi₃O₁₂ (x = 0.75, 1.0 and 1.2) thin films by pulse laser deposition. It is revealed that the substitution of Bi by La leads to a continuous transition from ferroelectric to relaxor-ferroelectric and then to paraelectric with concurrently improved energy density and efficiency. Based on the transition, high energy densities of ~60 J cm⁻³, together with high efficiency(>80 %) as well as excellent cycling(~10⁸ cycles) and thermal stability(-100~130^oC), can be achieved in the Bi_2.8La_1.2Ti₃O₁₂ films. These excellent performances should be ascribed to the paraelectric characteristics and suppressed leakage current of the film. The results prove the great potential of the Bi_4-xLa_xTi₃O₁₂ films for electrostatic energy storage. More importantly, our findings show the advantage of paraelectrics as high performance energy storage dielectric materials.

Materials that exhibit simultaneous order in their electric and magnetic ground states hold tremendous promise for use in next-generation, low-power memory and logic devices in which electric fields control magnetism. Such materials are, however, rare as a consequence of the competing requirements for ferroelectricity and magnetism, and until recently BiFeO₃ was the only material with this functionality at room temperature. Interface materials are a way to overcome these competing requirements, as was recently demonstrated for (LuFeO₃)_m/(LuFe₂O₄)₁ superlattices [J.A. Mundy et al. Nature 537 (2016) 523–527.]. The rumpling imposed by the geometric ferroelectric hexagonal LuFeO₃ imposes a local distortion on the neighboring LuFe₂O₄—a distortion that removes the mirror symmetry that the LuFe₂O₄ layers would otherwise have. This breaking of symmetry enables the LuFe₂O₄ to become simultaneously ferrimagnetic and ferroelectric. This rumpling is distinct from strain engineering because no macroscopic strain is involved. In this presentation we extend this atomically engineered design methodology to LuFeO₃/CoFe₂O₄ superlattices producing a robust ground state that is simultaneously ferroelectric and ferrimagnetic at temperatures well above room temperature.

There is currently a lot of R&D activity worldwide on two broad, systems level topics, both within the field of Microelectronics. One of them has to do with the emergence of the “Internet of Things”, which is the interconnectivity of microelectronics based sensing, communicating and information processing systems. The second has to do with the re-emergence of Artificial Intelligence/ Machine Learning, which is currently experiencing a worldwide explosion. Thus, the worldwide market for microelectronics is likely to go up significantly. A key side consequence of this is the total energy consumed in such applications, which could sky-rocket towards 20-25% of primary energy and thus of concern. Indeed, in the field of AI, a “Computing Wall” driven by a “Memory Wall” has emerged as a key bottleneck in systems level applications, such as in “Connected Autonomous Vehicles” or driver-less cars. At the same time, the key driver for the field of Microelectronics, namely Moore’s Law, is reaching the end of size scaling and power scaling. As a consequence, there is a large amount of research activity focused on the new Moore’s Law that is instead focused on Energy Scaling. Thus, looking for a new generation of ultra-low power memories and switches is an area of significant current research. Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration with an eye towards such applications. Over the past decade the oxide community has been exploring the science of such materials as crystals and in thin film form by creating epitaxial heterostructures and nanostructures. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics, particularly, the coexistence of ferroelectricity and some form of ordered magnetism (typically antiferromagnetism). The scientific community has been able to demonstrate electric field control of both antiferromagnetism and ferromagnetism at room temperature. Current work in our collaboration is focused on ultralow energy (1 attoJoule/operation) electric field manipulation of magnetism as the backbone for the next generation of ultralow power electronics. We are exploring many pathways to get to this goal. In this talk, I will describe our progress to date on this exciting possibility. The talk will conclude with a summary of where the future research is going.

In linear magnetoelectric materials, an electric (magnetic) field induces a magnetization (electric polarization) with a magnitude that is linearly proportional to the applied field strength. Here we use the prototypical magnetoelectric material, Cr₂O₃, to show that magnetoelectric materials with a diagonal magnetoelectric response host a hidden hedgehog-like magnetic order known as the magnetoelectric monopolization. We describe several consequences of this hidden order: First, we show that the magnetoelectric monopolization leads to a surface magnetization, independently of the choice of surface cut, that in turn should influence the exchange bias coupling in a heterostructure. Second, we show that a charge on the surface of such a magnetoelectric induces a divergent magnetic field above the surface, and describe an experimental search for this monopolar field using muon spin resonance spectroscopy [1]. Finally, we discuss the implications of an isotropic diagonal magnetoelectric response, and propose candidate materials for such behavior.

[1] Search for the Magnetic Monopole at a Magnetoelectric Surface, Q. N. Meier et al., Phys. Rev. X 9, 011011 (2019)

Two-dimensional (2D) materials such as graphene and transitionmetal dichalcogenides reveal the electronic phases that emerge when a bulk crystal is reduced to a monolayer. Transition-metal oxide perovskites host a variety of correlated electronic phases, so similar behavior in monolayer materials based on transition metal oxide perovskites would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored. Here we report the fabrication of freestanding perovskite films with high crystalline quality almost down to a single unit cell. Using a method based on water-soluble Sr₃Al₂O₆as the sacrificial buffer layer1, recently developed at Stanford University, we synthesize freestanding SrTiO₃and BiFeO₃ultrathin films by reactive molecular beam epitaxy and transfer them to diverse substrates, in particular crystalline silicon wafers and holey carbon films. We find that freestanding BiFeO₃films exhibit unexpected and giant tetragonality and polarization when approaching the 2D limit. Our results demonstrate the absence of a critical thickness for stabilizing the crystalline order in the freestanding ultrathin oxide films. Furthermore, the systematic characterization of the cross-sectional samples by atomic resolution electron microscopy reveals that the crystal symmetry and physical properties such as local electrical polarization are strongly dependent on the local strain and boundary conditions of the film. The ability to synthesize and transfer crystalline freestanding perovskite films without any thickness limitation onto any desired substrate creates opportunities for research into 2D correlated phases and interfacial phenomena that have not previously been technically possible.

The nanoscale transistor is perhaps the most numerous man made object, exceeding 10¹⁸ transistors used in ubiquitous computing and communications devices. However, as we reach the hyper scaled dimensions for electronic transistors (smallest transistors reaching sub 10 nanometer dimension)[i], a search for identifying the next scalable transistor is ongoing. For use in ubiquitous computing, such a “transistor” not only needs able to operate at room temperature but also possess features commensurate with beneficial miniaturization (Moore’s law). We have studied >25 device topologies for new transistor technologies and have considered the use of nearly all the breakthroughs in materials science from the last 3 decades[ii]. We have come to the understanding that new materials and phenomenon enabled by quantum materials form the path to the next “transistor” that is suitable for consumer electronics and cloud computing.
The Technology Need: We describe the imminent need for a revolutionary new transistor for classical Boolean computing based on a) The thermal power limit of the computers reaching the historic trigger point of 10 W/cm² (Figure 1), this has earlier led the conversion of computing from Bipolar to CMOS in 1980s. Another such trigger is imminent b) The exponential growth in energy demands due to proliferation of compute devices c) The continued need for classic computations that are described by a large variety of computational algorithms (e.g Geometric, combinatorial problems, graphical processing, computational fluid dynamics, prediction of complex physical/biological phenomenon).
A Quantum Materials pathway for new computing[iii]: We will describe a fundamentally new scaling path for computing where we bridge the breakthroughs in quantum materials for creating the next general purpose, room temperature computing technology. The term Quantum Materials is used to describe a broad class of materials that exhibit strong interactions amongst the charge, spin, orbital and lattice degrees of freedom. Over the past 2-3 decades, since the advent of high temperature superconductivity in cuprates, there have been several classes of quantum materials that include i) the CMR manganites where new states of matter and exquisite tunability of such states with external stimuli are driven by the strong electron-electron correlations and the consequent mesoscale phase coexistence, and ii) topological insulators that are driven by strong spin-orbit coupling leading to unusual band topology and Dirac-like fermion behavior, as key examples.
Magneto-electrics and Topological Conversion Path way for Computing[iv] : I will describe the concept device (named Magneto-electric Spin Orbit Logic) to drive high functionality materials in magneto-electrics and topological materials. In particular, the path to computing at aJ/bit (~30X more energy efficient than advanced CMOS) with 10-20X relaxation of the interconnect requirements will be described.


[i] Kuhn, K.J., ….Manipatruni, S., Nikonov, D., Pawashe, C. and Radosavljevic, M., 2012, December. The ultimate CMOS device and beyond. In 2012 International Electron Devices Meeting (pp. 8-1). IEEE.

[ii] Nikonov, D.E. and Young, I.A., 2015. Benchmarking of beyond-CMOS exploratory devices for logic integrated circuits. IEEE Journal on Exploratory Solid-State Computational Devices and Circuits, 1,.

[iii] Manipatruni, S., Nikonov, D.E. and Young, I.A., 2018. Beyond CMOS computing with spin and polarization. Nature Physics, 14(4), p.338.

[iv] Manipatruni, S., Nikonov, D.E., Lin, C.C., Gosavi, T.A., Liu, H., Prasad, B., Huang, Y.L., Bonturim, E., Ramesh, R. and Young, I.A., 2019. Scalable energy-efficient magnetoelectric spin–orbit logic. Nature, 565(7737), p.35.

BiFeO₃ (BFO) is one of the most studied multiferroic materials because of the coexistence of magnetic and polar orders at room temperature. Here we show that previously unidentified structural phases of BFO, with functional properties different from those of the rhombohedral ferroelectric bulk structure, can be stabilized by exploiting the electrostatic boundary conditions in thin film heterostructures. This provides a complementary approach to the earlier successes in stabilizing some of the many low-energy phases that have been identified computationally, by using coherent heteroepitaxial strain [1,2]. Using density functional theory, we identify a new antipolar phase of BFO and find that the energy barrier to convert it into a ferroelectric phase is small enough for it to be a candidate antiferroelectric. According to our strain calculations and a simple electrostatic model, this antipolar phase can be stabilized with insulating non-polar electrodes for a certain thickness range of the BFO layer on a substrate imposing a small amount of compressive strain; the rhombohedral phase can be recovered by increasing the thickness. These predictions are consistent with a new antipolar phase recently observed experimentally in La_1-xBi_xFeO₃/BiFeO₃ superlattices, which reveal a double hysteresis loop, characteristic of an antiferroelectric material [3]. Finally, we identify other new low-energy phases for BFO and propose possible ways to stabilize them experimentally.

References
[1] O. Diéguez et al., Physical Review B, 83, 094105 (2011)
[2] H. Béa et al, Physical Review Letters, 102, 217603 (2009)
[3] J. Mundy et al. arXiv:1812.09615

The key to integrating the concepts of spintronics into conventional nanoelectronics lies with the ability to control the magnetic order in nanoscale devices. With the continuous shrinkage of integrated circuits, the energy efficiency required to control these tiny magnets as power dissipation becomes a determining factor below 7 nm node. Over the past decades, the oxide community has been exploring the materials that can provide opportunities to control magnetism. Among the large investigated materials, multiferroics might be one of the most promising material family. Multiferroics are defined as the materials which possess at least two order of parameters, particularly, the coexistence of ferroelectricity (P) and magnetism (M), and exhibit coupling from one to another. In this talk, I will discuss the magnetoelectric switching mechanism in lanthanum-doped BiFeO₃ thin films. BiFeO₃ is by far the best-studied multiferroic, which shows strong ferroelectricity (~100 ��C/cm²) and G-type antiferromagnetism, above room temperature. It also shows weak ferromagnetism (M_C) induced by the canted spin configuration described by the Dzyaloshinskii–Moriya interaction (DMI). Moreover, these ferroic orderings, P and M_C, are strongly coupled, thus one can switch the magnetism by an electric field. However, the strong spontaneous polarization in BiFeO₃ needs a large voltage to switch. Here, we introduce the chemical doping to modify the order parameters in BiFeO₃ and achieve an ultralow-voltage (< 500 mV) and non-volatile manipulation of ferromagnetism at room temperature. Moreover, while the lanthanum-doping increased, we observed a very different magnetoelectric switching pathway and magnetic anisotropy compare to pure BFO. This discovery leads to the enhancement of perpendicular magnetic anisotropy (PMA) on multiferroics thin film, which will be very attractive to the practical applications. Finally, I will conclude this talk with a summary of current challenges and future direction of multiferroics, especially BFO, toward the low-power electronics.

Self-assembled two phase epitaxial oxide nanocomposites, in which a magnetic phase and a ferroelectric phase are coupled via strain transfer at their interfaces, can exhibit multiferroic and magnetoelectric responses. Such nanocomposites hold promise for making new types of electrically-switchable magnetic devices. The most commonly studied nanocomposite consists of a perovskite matrix (e.g. BiFeO₃ (BFO)) surrounding vertical pillars of a ferrimagnetic spinel (e.g. CoFe₂O₄ (CFO)). An electric field applied to the BFO changes its strain state and hence the strain state and magnetic anisotropy of the CFO, promoting changes in magnetization and even reversal. We will first describe BFO/CFO nanocomposites including strategies for templating the pillar positions and the behavior of (110)-oriented nanocomposites with fin-shaped CFO exhibiting in-plane anisotropy, and compare the structure and properties of nanocomposites grown by sputtering vs. pulsed laser deposition. We then describe nanocomposites containing YFeO₃ (YFO) as the ferroelectric phase. Bulk YFO is not ferroelectric, but thin films of YFO grown on certain substrates exhibit a polarization that can exceed 100 µC/cm². Based on first principles calculations the polarization is attributed to epitaxial mismatch strain which promotes off-centering of the cations. YFO/CFO nanocomposites grown on SrTiO₃ (001) substrates show a magnetoelectric coupling in which the ferroelectric hysteresis is sensitive to an in-plane magnetic field. We also demonstrate the control of the magnetic properties of nanocomposites containing SrFe_1-xCo_xO₃ (SFCO) using ionic liquid gating. Single phase films of SFCO are non-magnetic as grown and form either a perovskite or brownmillerite phase depending on deposition conditions and Fe:Co ratio. However, negative gating inserts oxygen and leads to a magnetization of up to 100 emu cm^-3 and a structural change from brownmillerite to perovskite with an associated substantial volume change. This magnetic switching is reversible over multiple cycles and is also observed in nanocomposites of SrFeCoO₃ codeposited with a spinel phase including Co₃O₄ and CFO. The inclusion of new materials, SFCO or YFO, in nanocomposites therefore expands the palette of possible structures, properties and control strategies, enhancing the functionality of these magnetoelectric heterostructures.

The role of magnetic domain formation and reorientation processes reveals fascinating physics and is of great relevance for technological applications. Especially the class of magnetic sensors that depend on magnetic thin film technology, rely on magnetic domain control for proper operation. Recent advances in thin film magnetoelectric (ME) composites offer a promising route for sensing ultra-low magnetic signals. Yet, one obstacle of achieving very low limit of detection is caused by magnetic domain activity, which is a well-known noise source in various magnetic field sensing applications. Irreversible and hysteretic magnetization changes from domain nucleation and other hysteretic domain effects impact the sensor’s performance. Moreover, the characteristics of magnetic domains reflect stress induced spatial alterations in the magnetic anisotropy distribution. Overall, due to the formation of magnetic domains in the piezomagnetic phase, the magnetization reversal in magnetic layers is complex.
The role and relevance of magnetic domains and domain walls for the ME response will be discussed in detail. Domain effects in different types of ME devices, including magnetically and electrically modulated thin film devices as well as surface acoustic wave (SAW) devices will be discussed. Domain activity in operating devices is studied from low frequencies, in the hundred kHz range, and at 150 MHz. Domain investigations at the operational frequencies of modulated and SAW sensors are perfomred by time-resolved magneto-optical Kerr effect microscopy.
A direct connection between specific magnetic domain activities and the exhibited noise characteristics is obtained. Even minimal domain activity is restricting magnetic sensor performance. Magnetic noise density and not sensitivity is the main figure-of-merit for optimizing sensor performance. Controlling magnetic domain behavior is a key to improved sensor performance.
Support through the DFG through the Collaborative Research Centre SFB 1261 is highly acknowledged.

1. N. O. Urs, V. Röbisch, S. Toxværd, S. Deldar, R. Knöchel, M. Höft, E.Quandt, D. Meyners, J. McCord, Direct linking specific magnetic domain activities and magnetic noise in modulated magnetoelectric sensors (submitted)
2. M. Jovičević Klug, L. Thormählen, V. Röbisch, S. Salzer, M. Höft, E. Quandt D. Meyners, J. McCord, Applied Physics Letters 114, 192410 (2019)
3. R.B. Holländer, C. Müller, J. Schmalz, M. Gerken, J. McCord, Scientific Reports 8, 13871 (2018)
4. A. Kittmann, P. Durdaut, S. Zabel, J. Reermann, J. Schmalz, B. Spetzler, D. Meyners, N.X. Sun, J. McCord, M. Gerken, G. Schmidt, M. Höft, R. Knöchel, F. Faupel, E. Quandt, Scientific Reports 8, 278 (2018)
5. S. Salzer, V. Röbisch, M. Klug, P. Durdaut, J. McCord, D. Meyners, J. Reermann, M. Höft, R. Knöchel, IEEE Sensors Journal, PP, 99 (2017)
6. V. Röbisch, S. Salzer, N. Urs, J. Reermann, E. Yarar, A. Piorra, C. Kirchhof, E. Lage, M. Höft, G. Schmidt, R. Knöchel, J. McCord, E. Quandt, D. Meyners, Journal of Materials Research 1-11 (2017)
7. N.O. Urs, B. Mozooni, P. Mazalski, M. Kustov, P. Hayes, S. Deldar, E. Quandt, J. McCord, AIP Advances 6, 055605 (2016)
8. J. McCord, Journal of Physics D: Applied Physics 48, 333001 (2015)
9. N. O. Urs, I. Teliban, A. Piorra, R. Knöchel, E. Quandt, J. McCord, Applied Physics Letters 105, 202406 (2014)
10. E. Lage, N. O. Urs, V. Röbisch, I. Teliban, R. Knöchel, D. Meyners, J. McCord, E. Quandt, Applied Physics Letters 104, 132405 (2014)

The field of nanoscale strain mediated multiferroics has grown considerably during the last decade due to the promise offered for efficiently controlling magnetism in the small scale. Strain mediated multiferroics consist of piezoelectrics mechanically coupled to magnetoelastic materials. An applied voltage to the piezoelectric induces a strain in the magnetoelastic material reorienting the magnetic spin moments. This approach is contrasted with electrical current approaches to control magnetic moment such as to reorient small magnetic bits with spin transfer torque STT. For informational purposes the energy required to reorient a 100 nm magnetic memory bit using STT is on the order of 100 fJ with the promise that strain mediated multiferroic is less than 0.1 fJ, i.e. > 4 orders of magnitude improvement. While strain mediated multiferroic approaches promise substantial efficiency improvements, many research effoerts focus on magnetic materials that are on moderately or weakly magnetoelastic due to fabrication related issues. In this presentation a review of progress on fabricating and testing Terfenol-D (Tb_0.3Dy_0.7 Fe_1.92) small scale structures, i.e. one of the highest room temperature magnetoelastic materials, is described to help advance the multiferroic field. This work includes discussions of fabrication, magnetic spin assessment, thermal effects, corrosion properties, and application demonstrated work.

Polycrystalline Terfenol-D thin films (<100 nm) are produced using a DC magnetron sputtering process on Sapphire, Si and PMN-PT substrates. The crystallized Terfenol-D films are micropatterned with photolithographic methods using Ar etching. The continuous and micropatterned (<20 microns) structures are characterized with XRD, SQUID, MOKE, MFM, PEEM, and XMCD to evaluate magnetoelastic properties. For micro-patterned structures as large as 20 microns in diameter, magnetic single domain structures are observed (MFM and PEEM) which is unusually large for this soft magnetic material. The large single domain behavior is attributed to the residual stresses produced during fabrication to further stabilize the magnetic spin states along preferred magnetic easy axis. Furthermore, an absence of oxidation is observed in these small-scale structures. While there is a capping layer on the Terfenol-D films, one would expect stress corrosion cracking to arise along the micropatterned side walls but this is absent. We attribute the corrosion absence to both the capping layer as well as the residual stresses preventing stress propagation and arresting oxidation. XMCD results on elemental spin states in the Terfenol-D reveal that the magnetic moments predominantly arise from the Tb and Dy spin/orital contributions with Fe magnetic spin pointing in the opposite direction, i.e. ferrimagnetic material. However, the main contribution for the Dy element arises from orbital moment rather than spin which is slightly unusual. Furthermore, the Dy elements spin orbital moment changes prominently as a function of temperature. We believe that the spin orbit moment coupling arising from Dy significantly contributes to the changes observed in the magnetocrystalline anisotropy as a function of temperature. Finally, demonstrations of a multiferroic PMN-PT/Terfenol-D (20 micron size microstructure) to capture and release magnetic particles attached to cells are presented for showing efficient control of magnetism in the small scale.

Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) is a type of emerging memory which holds the promise of high speed, high endurance, non-volatility, and good scalability. Since the theoretical prediction of the STT switching mechanism in 1996, significant progress has been made in the field, largely through materials innovations. In this talk, I will review the key materials discoveries that enabled the advancement of STT-MRAM technology. This includes the theoretical prediction and experimental realization of large tunneling magneto-resistance (TMR) with MgO tunnel barrier and the discovery of CoFeB based materials with interfacial perpendicular magnetic anisotropy (iPMA). This talk will also discuss our recent results at IBM on methods to lower the switching current of Spin-Transfer-Torque MRAM and achieve low write-error-rate by using optimized magnetic materials.

Smart infrastructure functionalized with Internet of Things (IoT) technology is driving need of the wireless sensor network system and its sustainable power source. Magneto-mechano-electric (MME) energy conversion is the most promising technology to supply a sustainable power into the IoT sensor devices as it efficiently converts low frequency stray magnetic field (50/60 Hz), which exists everywhere in modern infrastructure, into the electricity. However, currently reported MME generators produce relatively low power under low magnetic field (≤300 μT), which commonly exist in infrastructure. In this work, we demonstrate magnetoelectric (ME) coupled MME generator to produce high output power from low amplitude magnetic field. Comprehensive energy conversion mechanisms of ME coupled MME generator, including structural factors and contribution of ME coupling on power output, are investigated in order to achieve high energy conversion efficiency in low amplitude magnetic field. Building upon the mechanism study, an optimum MME generator, producing milliwatt power below 300 μT magnetic field, is realized utilizing outstanding magnetoelectric composite cantilever. Exploiting the harvested power near a home appliance, sustainable powering integrated sensors and wireless communication system is demonstrated. The fundamental study performed in this work provides a direction to achieve the efficient magnetic field energy harvesting and this will enable practical implantation of the IoT devices into the smart infrastructure.

Magnetoelectric (ME) thin film composites consisting of a sputtered piezoelectric (PE) and a magnetostrictive (MS) layer may be employed for measurements of magnetic fields passively, i.e. an AC magnetic field generates an ME voltage by mechanical coupling of the MS deformation to the PE phase, thus exploiting the direct ME effect. In order to achieve high field sensitivities a magnetic bias field, necessary to operate at the maximum piezomagnetic coefficient of the MS phase is used, using mechanical resonances further enhances this direct ME effect size. Despite being able to detect very small field amplitudes passively, by exploiting mechanical resonances, implies a limitation to available signal bandwidth, because of rather high Q factors. The requirement for a magnetic bias field along with the inability to detect DC magnetic fields makes practical implementation troublesome.
In the presented work the PE phase of such thin film ME composites is actively excited, thus exploiting the converse ME effect [1], remedying shortcomings of the direct ME effect. The experimental work makes use of surface micromachined thin film cantilever composites of mesoscopic scale (25mm x 2.5mm x 0.35 mm) of which a high frequency mechanical resonance at about 500 kHz, showing a mechanical quality factor of about Q~1000 is analyzed. This mechanical oscillation, being rigidly coupled to the magnetostrictive material, leads to a voltage induced in a pickup coil surrounding the sensor composite, giving rise to magnetoelastic interactions. Major signal components of bio magnetism lie in the range from DC to 500 Hz and amplitudes are of vanishingly small amplitude < ~ 50 pT, this regime is targeted by this research. This entirely passive readout, combined with low spatial and power requirements makes the presented ME sensor system suitable for integration into arrays as anticipated for biomagnetic imaging. The converse ME voltage response with respect to small external fields shows sensitivities up to 40 kV/T, offering linearity up to a field magnitude of several μT, no external magnetic driving field is required. The sensor sensitivity scales nearly linearly with drive amplitude supplied to the PE phase, unfortunately the measured noise floor in the frequency regime of interest shows an abrupt increase as soon as an excitation threshold of about 200 mV is reached. Massive magnetization activity in the MS phase is presumably the dominant source of noise. High speed vibrometry measurements give insight concerning the high frequency mechanical oscillation mode, magneto optical imaging reveals magnetoelastic modulations of the MS phase.
Furthermore, we demonstrate the performance of electrically modulated thin film ME composites, using magnetron sputtered AlN and amorphous FeCoSiB films. This approach enables composite magnetoelectrics to detect low DC (~ 250 pT/sqrt(Hz)) as well as AC (~ 75 pT/sqrt(Hz) @ 10 Hz) magnetic fields without the need of an external magnetic AC or DC bias field. Finally, lower system noise is achieved using specially tailored antiparallel exchange biased MS layers thus inhibiting domain wall activity and even further improve the detection limit.
Funding via DFG, SFB1261 “Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics” is gratefully acknowledged.
References
[1] Hayes, P., V. Schell, S. Salzer, D. Burdin, E. Yarar, A. Piorra, R. Knöchel, Y. K. Fetisov, and E. Quandt. “Electrically modulated magnetoelectric AlN/FeCoSiB film composites for DC magnetic field sensing.” Journal of Physics D: Applied Physics 51, no. 35 (2018): 354002. doi:10.1088/1361-6463/aad456.

Magnetostriction makes it possible to drive a magnet into resonance using purely a voltage signal. In a piezoelectric-ferromagnetic bi-layer, an alternating voltage applied to the piezoelectric transforms into a pseudo alternating magnetic field for the magnet. Combined with an internal static field that comes from the magnetic anisotropy, this alternating magnetic field can put the magnet into resonance. We shall describe our recent experimental work that demonstrates this effect in a LiNbO3/Ni heterostructure. It was observed that a highly efficient transduction (>99.9%) is possible in this process. A magnet put into resonance in this way also shows potential for sensing very small magnetic fields (~pT/sqrt(Hz)). In addition, they can be used for locally driving two level quantum systems without needing an optical excitation. We shall discuss our recent progress regarding these applications.

Iron containing perovskites have been intensively studied as potential functional materials with applications ranging from Solid Oxide Fuel Cell (SOFC) cathodes[1] and magnetoelectric multiferroics[2] to half-metallic magnetoresistive[3] materials. On the other hand, high temperature superconducting cuprates are another example of correlated materials that are placed in the frontier between localized and itinerant electronic behaviour[4], with superconductivity emerging from a Mott insulating type material upon doping.
The title material combines two functionalities: magnetism and superconductivity, associated to the presence of separated Fe-O2 and Cu-O2 layers orderly stacked along the c-axis
The formal presence of FeV cations resulted from ozone oxidation, leads to a long-range magnetic ordering, coexisting with the superconducting interactions (TN= 110 K > Tc= 70 K). The somewhat unexpected A-type AFM structure, with a μ(Fe) approximately 2 μB magnetic saturation moment, associated to the hypervalent iron sublattice, suggests an unusual low spin state for the iron cations; on the othe hand, the low dimensionality of the magnetic structure results in a soft switching towards ferromagnetism under small external magnetic fields.
It is also interesting to mention the relatively high superconducting Tc in a highly overdoped cuprate [5]
The role of the crystal structure and the high charge concentration in the stabilization of this unusual electronic configuration for the iron cations is discussed.

[1] X. Martínez de Irujo-Labalde, D. Muñoz-Gil, E. Urones-Garrote, D. Ávila-Brande, S. García-Martín, J. Mater. Chem. A 2016, 4, 10241– 10247.
[2] N. A. Spaldin, R. Ramesh, Nat. Mater. 2019, 18, 203–212.
[3] T. K. Mandal, M. Greenblatt, in Funct. Oxides, John Wiley & Sons, Ltd, Chichester, UK, 2010, pp. 257–293.
[4] M. Imada, A. Fujimori, Y. Tokura, Rev. Mod. Phys. 1998, 70, 1039– 1263
[5] A. Gauzzi, Y. Klein, M. Nisula, M. Karppinen, P. K. Biswas, H. Saadaoui, E. Morenzoni, P. Manuel, D. Khalyavin, M. Marezio, and T. H. Geballe

Perovskite rare earth nickelates (RNiO₃), where R is a rare earth cation are well-known for the sharp metal-to-insulator transition (MIT) observed when decreasing temperature, at T = T_MI (R different than La) (1-3). Many studies have focused on manipulating the MIT for instance through strain (4), growth direction (5) and superlattice heterostructure (6). In this work, we use experimental and theoretical methods to design and study superlattices of two distinct rare earth nickelate oxides SmNiO₃ (SNO) and NdNiO₃ (NNO) that in bulk form show a MIT at two very different temperatures (400K and 200K, respectively). We find that these new complex oxide superlattices display different behavior than that of either bulk material, showing either a single MIT at an intermediate temperature, or two transition temperatures that converge asymptotically towards the strained bulk values of each material. This system allows the origin of the coupling at such interfaces to be studied. We show that the length scale of the MIT transition in the superlattices is set not by the length scale of the propagation of structural motifs across the two materials, which ab-initio calculations and TEM analysis suggest is minimal, but rather by the balance between the energy cost of having metallic-insulating domain walls (the cost of bending the order parameter across the metallic-insulating interface) and the energy gain of the bulk phases.
References:
1. M. L. Medarde, Structural, magnetic and electronic properties of RNiO₃ perovskites (R = rare earth), Journal of Physics Condensed Matter, 9, 1679 (1997).
2. G. Catalan, Progress in perovskite nickelate research, Phase Transitions 81 729-749 (2008).
3. S. Catalano, et al., Rare-Earth Nickelates RNiO₃: Thin films and heterostructures, Rep. Prog. Phys. (2017).
4. S. Catalano, et al., Electronic transitions in strained SmNiO₃ thin films, APL Materials 2 116110 (2014).
5. S. Catalano, et al., Tailoring the electronic transitions of NdNiO₃ films through (111)pc oriented interfaces, APL Materials 3 062506 (2015).
6. Z. Liao, et al., Metal-insulator-transition engineering by modulation tilt-control in perovskite nickelates for room temperature optical switching, Proc. Natl. Acad. Sci. 115 (2018).

The metal-insulator transition (MIT) of transition metal oxides is often associated with a simultaneous lattice and electronic symmetry breaking; however the relative roles of the electronic and lattice degrees of freedom is still an unresolved question. Further, the length-scale of the metal-insulator transition and what sets it is not well understood. Through the study of heterostructures of rare-earth nickelates (RNiO₃), we get insight into these questions. First, using a combination of first-principles, many-body computational methods and experiment [1,2,3,4], we study the effect of heterostructuring NdNiO₃ with the band insulator NdAlO_3. We find two competing effects: the effective lattice cost to disproportionate the NiO₆ octahedra increases due to resistance from the interfacial Al-O bonds. Separately, electronic confinement favors an insulating state by lowering the electronic kinetic energy. These effects combined lead to a higher MIT temperature than the bulk, accompanied by a lower signature of bulk disproportionation on the X-Ray spectra. To understand the length scales involved in the MIT we study superlattices of two nickelate materials that have the same type of transition but two different transition temperatures (NdNiO₃~150K and SmNiO₃~400K). We find that for thin enough layers of each material, a single transition at an intermediate temperature exists, while at higher layer thickness two MITs appear at temperatures that asymptotically tend to the bulk MIT temperatures with increasing layer thickness. We find that this behavior is set not by the gradual propagation of structural motifs, but by the competition between the domain wall energy cost at the interface between the metallic and insulating materials and the bulk free energy cost of the materials not being in their favored state. Our work thus opens new paths in the systematic study of complex oxide materials.

[1] A. B. Georgescu, O. E. Peil, A. S. Disa, A. Georges and A. J. Millis, in press at PNAS, arXiv:1810.00480
[2] O. E. Peil et al, in press at PRB, arXiv:1809:03720
[3] Q. Han and A. J. Millis PRL 121, 067601 (2018), arXiv:1801.06215
[4] A. Disa, A.B. Georgescu et al PRM 1, 024410 (2017), arXiv:1809.07810

HfO₂-based ferroelectrics exhibits two regimes of “wake-up” and “fatigue” in which remnant polarization increases and decreases during electric field cycling, respectively. These two phenomena are considered as the effect of defect generation and diffusion including redistribution of oxygen vacancies. The structural transition in the wake-up stage has been focused in ferroelectric HfO₂ thin film. However, in order to operate a reliable device, it is necessary to understand the ferroelectric characteristics and dielectric properties in wake-up and fatigue processes.
Grimley et al. proposed that the variation of bulk defect concentration which constrains domain wall motion by defect pinning is the origin of fatigue behavior. However, Lou et al. suggested that the structure decomposition caused by charge carrier injection was the main reason for fatigue endurance. The mechanism of fatigue behavior of HfO₂ thin films is still unclear. So, understanding of the ferroelectric characteristics with respect to the fatigue behavior is required for practical device operation.
We report on the ferroelectric switching dynamics of fatigued 4.2% Si-doped HfO₂ thin film. Prior to the fatigue behavior, Si-doped HfO₂ thin films exhibited the wake-up behavior where the remnant polarization value increased with the repeat of external bias cycling. After the wake-up behavior, Si-doped HfO₂ film exhibited the degradation of the remnant polarization value alongside the sweep of external bias, i.e., fatigue behavior. The investigation of ferroelectric switching dynamics revealed the retardation and recovery of the characteristic switching time for ferroelectric nucleation coincident with the wake-up behavior and fatigue behavior, respectively. We analyzed the interface trap density with the capacitance-voltage characteristics and attributed the fatigue behavior to the increase of oxygen vacancies with fatigue behavior.

Ferroelectricity with partial polarization switching is considered as one of a feasible candidate for an analog device in the form of the ferroelectric tunnel junction and ferroelectric transistor. However, deterministic control of ferroelectric polarization states with conventional ferroelectric materials have been met with accessibility problems due to the complex nature of ferroelectric switching mechanism and/or defect mediated uncertainty. Ferroelectric HfO₂ thin film has been investigated intensively as an alternative to perovskite materials in ferroelectric random access memory due to the advantages for nonvolatile memory applications, such as good scalability, compatibility with conventional complementary metal-oxide-semiconductor (CMOS) process technology.
Here, we report unprecedented stability of the sub-loop switching behavior in ferroelectric HfO₂. We suggest that the stable accessibility with robust stability of multiple polarization states in ferroelectric HfO₂ can be attributed to the large activation field for ferroelectric switching with small critical volume for the ferroelectric nucleation of HfO₂. The switching dynamics indicated a large activation field for ferroelectric switching. The temperature dependence of hysteresis and piezoresponse force microscopy measurements indicated a small critical volume for ferroelectric nucleation. The theoretical calculation demonstrated the stable switching energy landscape for single dipole flip in ferroelectric HfO₂. Monte-Carlo simulation demonstrated that the small critical volume for nucleation induces stable accessibility of multiple polarization states in ferroelectric HfO₂.

The unique properties displayed by canonical relaxor systems, such as the electric field induced transformation from a pseudo-cubic relaxor state to a polar ferroelectric state[1], the associated huge strain [2] and the phase transition mediated polarization reversal mechanism [3], make these materials highly interesting objects for fundamental studies as well as versatile components for industrial applications.
Especially, the achievement of high strains has drawn significant attention, the origin of which is based on the reversibility of the phase transformation between the ferroelectric and the relaxor state [4]. Many relaxor-ferroelectric systems exhibiting high actuating performance are located in the morphotropic phase region. They often exhibit multiple crystallographic phases arranged as core and shell within single grains and their ability to adapt the phase ratio depending on the external stimuli [5], is another important factor contributing to their exceptional electromechanical performance.

Here, we report on the development of interfacial stresses between different crystallographic phases in a multiphase (Bi_1/2Na_1/2)TiO₃-BaTiO₃ relaxor ceramic as induced by thermal treatment. Upon electric field application, this system exhibits an “intrinsic core-shell structure” of a polar minority phase embedded into a polar majority phase. While the majority phase stays stable with increasing temperature up to the transition into the relaxor state, a gradual de-texturization of the minority phase is observed over the whole temperature range. The surface domain structure was found to decay already at significantly lower temperatures than expected from bulk observations. Development of interfacial stresses due to thermal expansion mismatch between majority and minority phases as well as differences in local stress state between surface and bulk are discussed as driving factors of the phase transition dynamics.
Tailoring of interfacial stresses through adaption of phase fractions opens up a pathway to optimize the strain performance of actuator materials and can become a useful tool for the stabilization of usually metastable crystallographic phases as well as for property tuning e.g. in piezotronics.

[1] Z.-G. Ye, Key Eng. Mater. 1998, 155–156, 81.
[2] S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, J. Rödel, Appl. Phys. Lett. 2007, 91, 112906.
[3] J. Glaum, H. Simons, J. Hudspeth, M. Acosta, J. E. Daniels, Appl. Phys. Lett. 2015, 107, 232906.
[4] M. Hinterstein, M. Knapp, M. Hölzel, W. Jo, A. Cervellino, H. Ehrenberg, H. Fuess, J. Appl. Crystallogr. 2010, 43, 1314.
[5] M. Hinterstein, L. A. Schmitt, M. Hölzel, W. Jo, J. Rödel, H.-J. Kleebe, M. Hoffman, Appl. Phys. Lett. 2015, 106, 222904.

The need for efficient energy utilization is driving research into ways to harvest waste-heat which is ubiquitous, abundant and free. Thermal harvesting is a promising method for capturing freely available heat and converting it to a more usable form, such as electrical energy. Thermal harvesting for low power electronic devices using ferroelectric materials is one of the emerging areas of research because these ferroelectric materials possess spontaneous polarization and exhibit excellent piezoelectric as well as pyroelectric coefficients. These materials are unique as they only sense time-dependent temperature change to generate electric power. We have grown lead-free BaZr_0.2Ti_0.8O₃ (BZT)/ Ba_0.7Ca_0.3TiO₃ (BCT) multilayer heterostructures and studied their structural, dielectric, ferroelectric, and pyroelectric properties. The BZT/BCT multilayer epitaxial heterostructures were grown on SrRuO₃ (SRO) buffered SrTiO₃ (STO) single crystal substrate by optimized pulsed laser deposition technique. The large angle x-ray scans showed only diffraction peaks from the substrate and pseudocubic reflections (l00) from the multilayer heterostructure, confirming that these films are phase pure, highly crystalline, and epitaxial in nature. The atomic force microscopy (AFM) studies indicate that the surface roughness is low and that film growth is of high quality. The ferroelectric phase transitions have been probed above room temperature with a relaxor behavior. The polarization versus electric field (P-E) measurement shows that the multilayer heterostructure exhibits slim and well-saturated hysteresis loop with high saturation and low remnant polarization of 100 and 20 µC/cm², respectively at 1.7 MV/cm. Solid-state, thin-film devices, that convert low-grade heat into electrical energy, are demonstrated from temperature dependent P-E loops using pyroelectric Ericsson cycles with energy conversion density of 1.61 J/cm³. Our findings suggest that the BZT/BCT multilayer ferroelectric thin film may be competitive with other thermoelectric materials for low-grade thermal harvesting.

While ferroelectric-field-controlled Rashba-Dresselhaus effect has long been proposed as a method to manipulate the valley and spin degrees of freedom in ferroelectrics with strong spin-orbit coupling, the experimental realization of such device concepts remains limited. Although ferroelectric two-dimensional (2D) materials with strong spin-orbit coupling have been widely investigated, most chalcogenide layered 2D or quasi 2D materials considered suffer from low band gap, low Curie temperature or high electrical leakage, rendering the electrical control difficult at room temperature, while the insulating nature of most ferroelectric (quasi-) 2D oxides with heavy elements limits their optical activity. In this work, we report the first experimental observation of ferroelectricity in a quasi-2D 10~100 nm thick nanosheet Sillen-Aurivillius semiconducting split-ion oxyhalide perovskite at room temperature. Optoelectronic, transport and temperature-dependent time-resolved photoluminescence studies show that Bi₄NbO₈Cl single crystal exhibits a direct band gap of 2.4 eV, decent optical quantum efficiency and carrier dynamics, making it promising candidate for photon-valley coupling. Electrical transport shows up to 100× I-V rectification in Bi₄NbO₈Cl single crystal nanosheet-based planar devices with symmetric graphite as electrodes. By electrical poling, the diode polarity becomes switchable, rendering repeatable rectified current. Such diode effect in Bi₄NbO₈Cl is attributed to the asymmetric Schottky barriers at Bi₄NbO₈Cl–graphite interfaces that could be adjusted by switchable ferroelectric dipoles. In the oxyhalide Bi₄NbO₈Cl, the combination of appropriate band gap from ion splitting, considerable visible light optical quantum efficiency, strong polarization and the possible strong spin-orbit coupling from Bi provides a novel design platform for realizing emergent spin-orbitronics.

The last few decades have seen significant scientific discoveries in single crystal complex oxide materials including colossal magnetic resistance, superconductivity, voltage control of magnetism and many different ferroic orders, many of which could lead to completely new applications. However, integration of such oxides using direct synthesis on microelectronics-platforms such as Si, amorphous SiO₂, GaN, and unconventional flexible substrates has faced significant challenge due to various growth-related issues and the interface chemistry at high temperature. Therefore, the functionality and performance that are otherwise available for epitaxial, single-crystal, perovskite materials have remained unachievable in microelectronic devices for a long time. The recent invention of complex oxide layer transfer techniques (LTT) has opened a promising route to the monolithic integration of these materials with arbitrary substrates ^[1],[2],[3],– thus offering a possible replacement to the rarely-successful direct growth techniques.^[4]
In LTT, complex oxides epitaxially grown on the lattice-matched substrate are made free-standing and then interfaced with arbitrary material on-demand. Although proof-of-concept devices such as field effect transistor and memory devices have already been demonstrated using such transferred complex oxide ferroelectric (FE) materials,^[1,2] little is known about their ferroic properties, particularly at the mesoscopic length scale. In this work, we have studied free-standing, single-crystal PbZr_0.2Ti_0.8O₃ using piezoforce (PFM) and electrostatic force microscopy (EFM) and synchrotron X-ray nanodiffraction techniques. We find that the epitaxial strain imposed on the ferroelectric layers by the substrate during growth by pulsed laser deposition technique is alleviated when the layers are made free-standing. The reduction of elastic strain slightly widens the domain walls, reduces the spontaneous polarization and increases the piezoelectric response. We have studied ferroelectric properties of both the top layers and interfacial layers (which were in contact with the substrate) and found a negligible difference between them. This hints that the intrinsic dead-layer issues may not be the limiting factors for utilizing transferred epitaxial layers in electrostatically controlled devices.

[1] S. R. Bakaul, C. R. Serrao, O. Lee, Z. Lu, A. Yadav, C. Carraro, R. Maboudian, R. Ramesh, S. Salahuddin, Adv. Mater. 2017, 29, 1605699.
[2] S. R. Bakaul, C. R. Serrao, M. Lee, C. W. Yeung, A. Sarker, S. L. Hsu, A. K. Yadav, L. Dedon, L. You, A. I. Khan, J. D. Clarkson, C. Hu, R. Ramesh, S. Salahuddin, Nat. Commun. 2016, 7, 10547.
[3] D. Lu, D. J. Baek, S. S. Hong, L. F. Kourkoutis, Y. Hikita, H. Y. Hwang, Nat. Mater. 2016, 15, 1255.
[4] L. Shen, L. Wu, Q. Sheng, C. Ma, Y. Zhang, L. Lu, J. Ma, J. Ma, J. Bian, Y. Yang, A. Chen, X. Lu, M. Liu, H. Wang, C. L. Jia, Adv. Mater. 2017, 29, 1702411.

Epitaxial strain is a powerful tool to generate ferroelectricity owing to polarization-strain coupling. Local lattice degrees-of-freedom such as rotations of metal-oxygen octahedral also couple to strain, and can be used to tune a material’s oxygen rotational-related properties such as metal-insulator transitions and magnetic reconstruction by strain. Here, we first use electronic structure calculations to investigate the strain effects on (001) thin films of the hybrid-improper ferroelectric (HIF) A₃B₂O₇ compounds. Surprisingly, other than the bulk polar Cmc2₁ phase, we find a new nonpolar phase becomes the ground state under both experimentally accessible biaxial compressive and tensile strains, which is beyond the people’s believe about the rule of the polarization-strain coupling. Furthermore, the generality of the polar-to-nonpolar (N-NP) transition in HIFs (not only in A₃B₂O₇ compounds) leads us to find a novel route to tune the physical properties that are classified as mechanical, optical and magnetic responses, which we also propose could be electric-field tunable, near the P-NP phase transition boundary. Our results may offer a route to search for new functionalities in hybrid-improper ferroelectrics.

Lead Zirconate Titanate (PbZr_0.52Ti_0.48O₃, PZT) alloy has been extensibly studied to be used for piezoelectric nanogenerators to harvest energy from mechanical motions. In this study, PZT nanofiber-based nanogenerators were fabricated to test its true piezoelectric performance without triboelectric effect. Aligned PZT nanofibers were fabricated by sol-gel electrospinning process. The thickness, area, and orientation of the PZT textile made by electrospinning the PZT solution onto multi-pair metal wires or metal mesh were controlled to form a composite textile. After the calcination, the PZT textile mixed with polydimethylsiloxane (PDMS) was placed between two flexible ITO-PEN substrates. The performance parameters of the nanogenerators were investigated under the bending motion, which excludes the triboelectric effect. The assembled nanogenerator of an area of 8 cm² and a thickness of 80 mm could generate an electrical output voltage of 1.1 V and a current of 1.4 mA under the bending strain. The piezoelectric voltage depended on the thickness of the PZT textile, while the piezoelectric current depended on both the thickness and the area of the PZT textile. The electrical performance of the device was significantly affected by the orientation of the PZT fiber and the bending direction. The output voltage and the output current were strain-dependent, while the total integrated charge was independent of the strain rate. The properties of the flexible nanogenerator could be quantified to verify the pure piezoelectric performance of the device.

Hexagonal barium titanate (h-BaTiO₃, P6₃/mmc), which is structurally non-perovskite and energetically stable above 1460 °C, can be an excellent platform for designing an exotic phase which does not exist in nature and for achieving new room-temperature multiferroics with high performance. Owing to the high synthetic temperature, a phase transition to the h-BaTiO₃ and the related physical properties have not been studied in detail. In this work, we experimentally demonstrate a structural phase transition from perovskite BaTiO₃ with tetragonal symmetry to non-perovskite hexagonal polymorph in Ni-doped BaTiO₃ ceramics. Using various experimental techniques, we monitored the evolution of the structural, ferroelectric/dielectric, and electronic properties in our Ni-doped BaTiO₃. We found that the reduction of Ti oxidation state by Ni doping plays a key role in the observed structural transition to h-BaTiO₃. More details of our experimental results will be presented in conjunction with a discussion about the underlying mechanism of this polymorphic phase transition in BaTiO₃.

Octahedral tilts are the most ubiquitous distortions in perovskite-related structures that can dramatically influence ferroelectric, magnetic, and electronic properties; yet the paradigm of tilt epitaxy in thin films is barely explored. Non-destructively characterizing such epitaxy in three-dimensions for low symmetry complex tilt systems composed of light anions is a formidable challenge. Here we demonstrate that the interfacial tilt epitaxy can transform ultrathin calcium titanate, a non-polar earth-abundant mineral, into high-temperature polar oxides that last above 900 K. The comprehensive picture of octahedral tilts and polar distortions is revealed by reconstructing the three-dimensional electron density maps across film-substrate interfaces with atomic resolution using coherent Bragg rod analysis. The results are in excellent agreement with density functional theory. The study could serve as a broader template for non-destructive, three-dimensional atomic resolution probing of complex low symmetry functional interfaces.

(1) Yuan Y, Lu Y, Stone G, et al. Nat. Commun. 2018, 9 (1).
(2) Haislmaier R, Lu Y, Lapano J, et al. APL Materials, 2019, 7 (5), DOI: 10.1063/1.5090798

There is currently significant interest in ultralow power microelectronics for memory and logic functions. Recent reports have identified multiferroic systems as possible candidates for such applications, with the ultimate goal of reducing the switching voltage to below one hundred millivolts (translating to approximately one attojoule per unit operation). Thus, measuring the ferroelectric and piezoelectric responses of ultrathin ferroelectric layers at such low voltage scales is essential. Piezoelectric measurements are, in principle, easier to execute and analyze since the primary response arises solely from the ferroelectric state (unlike polarization based measurements that are susceptible to leakage and nonlinear dielectric effects). To understand switching at voltages on the order of one hundred millivolts, we are carrying out a careful study of the piezoelectric switching of five, ten, fifteen, and twenty nanometer thick lead zirconate titanate films as a model system. We specifically focused on characterizing the piezoresponse of such nanoscale thin films using very low alternating current voltages (down to ten millivolt excitations). Our research indicates that statistically credible piezoelectric switching information can be obtained at these voltages. We will report the results of these scaling studies using piezoresponse force microscopy (PFM) as well as pulsed polarization measurements.

New Lead-Free Materials on the Base of Sodium-Potassium Niobate and Sodium-Bismuth Titanate Perovskites
E.D. Politova¹, G.M. Kaleva¹, A.V. Mosunov¹, N.V. Sadovskaya¹, S. Yu. Stefanovich^1,2
D.A. Kiselev³, A.M. Kislyuk³, T.S. Ilina³,
¹L.Ya.Karpov Institute of Physical Chemistry, Vorontsovo pole str. 10, Moscow 105064 Russia,
²Lomonosov Moscow State University, Leninskie gory 1, Moscow 119992 Russia,
³National University of Science and Technology “MISiS”, Leninskii pr. 4, Moscow 119991 Russia,

E-mail: [email protected]

Lead-free materials are being intensively studied in order to replace widely used Pb-based ones. We study influence of cation substitutions and preparation conditions on structure parameters, microstructure, dielectric, relaxor ferroelectric, and piezoelectric properties of solid solutions in the systems based on (Na_0..5Bi_0.5)TiO₃ (NBT) and (K_0.5Na_0.5)NbO₃ (KNN) perovskites.
Ceramic samples in systems (Na_0.5Bi_0.5)TiO₃ - BaTiO₃ (NBT-BT) and (K_0.5Na_0.5)NbO₃ – BaTiO₃ (KNN-BT) with compositions close to Morphotropic Phase Boundaries (MPB) were prepared by the two-step solid-state reaction method at temperatures of 900 – 1500 K. To modify properties of the samples, in A- and B-sites of perovskite lattice Li⁺, La³⁺, Mn³⁺, Ni³⁺, Fe³⁺, Nb⁵⁺ , Sb⁵⁺ and W⁶⁺ cations were added. To improve density of ceramics overstoichiometric KCl and LiF additives were used.
The samples were characterized using the X-ray Diffraction, Scanning Electron Microscopy, Second Harmonic Generation (SHG), Dielectric Spectroscopy (DS), and Atomic Force Microscopy in Piezorespone Force mode (PFM) methods.
The unit cell volume changes were observed in modified KNN- and NBT-based ceramics depending on ionic radii of substituting cations. Ferroelectric phase transitions at ~ 400-500 and 600 – 700 K were confirmed using the DS and SHG methods. Phase transitions near ~ 400 K in NBT-based compositions revealed typical relaxor behavior due to the presence of polar nanoregions in a nonpolar matrix. At high temperatures > 700 K effects of dielectric relaxation were observed in ceramics studied caused by formation of oxygen vacancies in compositions with aliovalent substitutions. At the room temperature, non monotonous changes of the dielectric parameters and increase in the spontaneous polarization value was proved for modified ceramics. Using the PFM method ferroelectric polarization switching at nanoscale was observed, and in some KNN-based ceramics high values of effective d₃₃ piezoelectric coefficient up to 300 pm/V were observed. The results obtained confirmed prospects of new lead-free materials development on the base of modified KNN- and NBT-based compositions.

Acknowledgment. The work was supported by the Russian Foundation for Basic Research (Project 18-03-00372).

Generally, the chemical or physical properties strongly depend on the change of lattice. The control of lattice strain, therefore, much affects the chemical or physical properties of functional materials, which has been widely used in superconductivity, giant magnetoresistance, multiferroics, catalysis and etc. Ferroelectrics are an important functional material, which has been widely used in the field of ferroelectric memories, tunable microwave devices, large-capacity capacitors, piezoelectric sensor devices, etc. The intriguing properties of ferroelectric materials utilize the basic functional primitive parameter of polarization. This team has realized this method, interphase strain, by creating a single-lattice-parameter epitaxial composite film on SrTiO₃ substrate from two tetragonal materials but with different lattice parameters, PbTiO₃ ferroelectrics and PbO non-ferroelectrics. The results show that the method improves the lattice distortion of PbTiO₃ to c/a = 1.238, compared to 1.065 in bulk. The remanent polarization is as high as 236.3 μC/cm², which is near twice the highest value of the known ferroelectrics. This composite ferroelectric thin film is very stable, and the super-tetragonal ferroelectric phase is stable up to 725 °C, compared to the bulk transition temperature of 490 °C.
The proposed “interphase strain” is a new concept for strain engineering to regulate lattice strain of ferroelectrics, and successfully achieved giant polarization in the super tetragonal PbTiO₃/PbO based ferroelectric thin films. The idea of "interphase strain" is as follows: if two kinds of materials with similar crystal structures, but different lattice parameters, are growing into a single-lattice-parameter epitaxial film, the material of the small lattice is inevitably subjected to the tensile stress from the large lattice material, thereby introducing a large lattice strain. The regulation of lattice strain can cause significant changes in the physical and chemical properties of the material. This new approach of interphase strain for strain engineering can be utilized to enhance the physical and chemical properties of other functional materials, such as superconductivity, giant magnetoresistance, multiferroic, and catalysis.

[1] Zhang L, Chen J, Fan L, et al. Giant polarization in super-tetragonal thin films through interphase strain[J]. Science, 2018, 361(6401): 494-497.

Conventional ferroelectric perovskite materials carry a debatable critical thickness below which the depolarization field is large enough to destabilize ferroelectricity. 2D ferroelectric materials are discovered to hold robust in-plane polarization down to a single unit cell, contending the continuous miniaturization of ferroelectric devices. However, due to the nonuniformity of the electrostatic field from quasi-2D metal electrodes, the in-plane intrinsic dipoles would experience a mismatched screening (screening frustration) making the in-plane polarization switching dynamics complex. Until now there is rare direct experimental proof of the in-plane polarization switching in a 2D crystal. In this talk, for the first time, we will present the in-plane polarization switching in a Dion-Jacobson quasi-2D layered oxide. The coupling between the in-plane ferroelectricity and other physical properties, e.g. the switchable photo diode effect and the photon-induced domain switching, in layered materials may enable versatile applications down to the atomic scale.

Topological objects or field textures, such as skyrmions, merons, and vortices, are intriguing features found in ordered systems with spontaneously broken symmetry. A plenty of topological field textures have been discovered, especially in magnetic and ordered soft matter systems, due to the existence of chiral interactions, and this has provided a fruitful platform for unearthing additional groundbreaking functionalities. However, despite one of the most important classes of ordered systems, ferroelectrics scarcely form topological polarization structures due to lack of such chiral interactions. Here, we show that a rich assortment of nontrivial topological polarization textures, including hedgehogs, antivortices, multidirectional vortices, and vortex arrays, can be spontaneously formed in three-dimensional nanoporous ferroelectric structures using the phase-field modelling based on the Ginzburg–Landau theory. We demonstrate that confining ferroelectrics to trivial geometries that are incompatible with the orientation symmetry may impose extrinsic frustration to the polarization field through the enhancement of depolarization fields at free porous surfaces. This frustration yields symmetry breaking, resulting in the formation of nontrivial topological polarization field textures. We also characterize the topological feature of polarization structures according to the topological theory of defects and homotopy theory. The results indicate that the nanoporous structures possess topological objects composed of two or more elementary topological polarization structures. This study therefore offers an intriguing playground for exploring novel physical phenomena in ferroelectric systems as well as a novel nanoelectronics characterization platform for future topology-based nanotechnologies.

Within Landau theory, magnetism and polarity are homotopic, displaying a one-to-one correspondence between most physical characteristics. However, despite widely reported noncollinear magnetism, spontaneous noncollinear electric dipole order as ground state is rare. Here a dioxydihalides MO₂X₂ family (layered structure, where M= Mo and W; X= Cl and Br) is predicted to display noncollinear ferrielectricity, induced by competing ferroelectric and antiferroelectric soft modes. This intrinsic noncollinearity of dipoles generates unique physical properties, such as Z₂×Z₂ topological domains, atomic-scale dipole vortices, and negative piezoelectricity.

Systematic studies on sintering temperature and Li:Al stoichiometry of Li and Al co-doped ZnO ceramic shows that the Zn_0.99(Li_0.1, Al_0.2)_0.033O sample exhibits a colossal dielectric constant (CDC) phenomenon with low dielectric loss ( and =0.159 at the frequency of 1 kHz) at the room-temperature. The CDC and low dielectric loss have good frequency stability. SEM and CL measurements show the grain boundary of the as-grown LAZ sample is accumulated with high density of V_O, and the spatial non-uniformity of V_O concentration is removed after the O₂ annealing. With the impedance analysis, it is shown that the R_GD (resistance of grain boundaries) is only ~16 times larger than R_G (resistance of grain), and the CPE_GD is even smaller than CPE_G, implying that internal barrier layer capacitance (IBLC) effect is not likely the dominant source for the observed CDC of the LAZ samples. Dielectric spectrum analysis shows that the CDC is associated with two relaxation processes P1 and P2. Annealing the sample at 900 ^oC in oxygen leads to the drop of to 2250 but maintaining a low dielectric loss of ~0.10, while P1 vanishes and P2 persists. Comparing the synchrotron based hyperfine Al 2p X-ray photoelectron (XPS) spectra of the purely Al-doped ZnO sample and the Li+Al co-doped ZnO sample shows that the electronic environment around the Al_Zn in these two samples are different, indicating that defect complex consisting of Al_Zn and Li_Zn is probably presence in the Li+Al co-doped sample. Furthermore, after the Raman comparisons among the co-doped, singly Li- or Al- doped ZnO, it is shown the defect complex has altered the Zn-O bond’s local environment, which lead to the shift of A₁ stretching mode. This defect complex is possibly the electron pinning defect dipole responsible for the CDC associated with the P2 relaxation process. With the correlated barrier hopping model fitting, it is found that two energy level E_C-0.34 eV and E_C-0.87 eV are existed in as grown and O₂ annealed LAZ sample, causing the P1 and P2 relaxation, respectively. With two potential well model, we theoretically calculate the dielectric constant and loss against frequency for P1 and P2 relaxation, which are around ~7173 and ~2817.

Magnetoelectric multiferroics offer the possibility of controlling the electric (magnetic) polarization by applying a magnetic (electric) field and have attracted significant interest in view of the interesting physics of coupling between magnetic and ferroelectric order parameters and potential technological applications in several multifunctional devices. BiFeO₃ is unique amongst various magnetoelectric multiferroics, as its ferroelectric and magnetic transition temperatures (T_C ~1103 K, T_N ~643 K) are well above the room temperature. In recent years, the (Bi_1-xBa_x)(Fe_1-xTi_x)O₃ (BF-xBT) system have received considerable attention due to large ferroelectric polarization, large remnant magnetization, linear magnetoelectric coupling and highest depolarization temperature for piezoelectric applications and considered to be alternative to toxic lead (Pb) containing piezoelectric ceramics like Pb(Zr_xTi_1-x)O₃ and (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃. In the present investigation, we have carried out a comprehensive study on 0.3 wt% Mn-doped BF-xBT samples in the composition range 0≤ x ≤1.0. Our synchrotron x-ray diffraction results reveal two structural phase boundaries, one from rhombohedral (R3c space group) to cubic (Pm-3m space group) at x_c1~0.33 and other from cubic (Pm-3m space group) to tetragonal (P4mm space group) at x_c2~0.80. The composition dependence of the polarization and dielectric constant at room temperature exhibits a peak around x=0.33. This confirms the existence of morphotropic phase boundary at this composition due to a rhombohedral (R3c) to cubic (Pm-3m) phase transition across the morphotropic phase boundary. We also present a magnetic phase diagram of BF-xBT.

Metal-oxide based Resistive Random-Access-Memory (RRAM) which exhibits multiple resistive/memory states by the formation/dissolution of a conductive filament (CF), has potential to be the next generation memory technology. The performance of RRAM relies on the fast switching speed, large current on/off ratio, and minimized device variability, which are critically dependent on the ion (such as oxygen vacancies) migration driven by the concentration gradient, electrical bias, hear generation and dissipation. In this work we report that the mechanical stress induced by the growth and retraction of the conductive filament is equally important to the resistive switching dynamics. The stress inhibits the ion diffusion and segregation along the electric field direction and reduces the current on/off ratio and the switching time. To further understand the role of this mechanical stress in the resistive switching process, we developed a comprehensive phase-field model taking into account the electrical, thermal, and mechanical energy, and studied their relative effects on the RRAM resistive switching dynamics. We choose hafnium oxide as an example, and compare the simulation results with existing experimental results to validate the model. Based on this, high-throughput phase-field simulations for different oxides of different properties are performed to help selecting the potential candidates as the oxide switching layer materials with optimized switching speed, current on/off ratio and device uniformity. Our work thus provides fundamental insights into the resistive switching mechanism, and establishes a theoretical framework of materials selection and design for metal-oxide based RRAMs.

Bismuth ferrite (BiFeO₃ or BFO) is the most studied multiferroic due to its robust ferroelectric state coexisting at room temperature with an antiferromagnetic order. Such coexistence and the possible cross coupling between both ferroic orders pave the way to so-called MagnetoElectric RAM combining advantages of the ferroelectric and the antiferromagnetic state. Similarly to the relaxor-ferroelectric systems a morphotropic phase boundary has been observed in La doped BFO (Bi,La)FeO₃ solid solution with peculiar nanoscale mixture. Emergence of such MPB is believed to arise from the competition between antiferrodistortive and ferroelectric instabilities. Our approach to investigate the structural interaction between BFO and LFO is based on superlattices that are ideal platforms for exploring antagonistic interactions at the origin of many exotic systems. For instance Cuprates combined with Manganites in superlattices were investigated to better understand the competition between magnetic and superconducting orders. Similarly to this strategy we grew series of SLs made of BiFeO₃ and LaFeO₃. Structural characterizations and Raman spectroscopy indicate an anti-polar structure in the BFO layers that is strongly dependent on the BFO thickness and temperature [1,2]. This antiferroelectric like structure very similar to the PbZrO₃ system cannot be explained by the nature of the induced strain but by the symmetry mismatch at the interfaces of the SLs. Compatibility of the octahedral tilt system seems to be the main driving force for this induced anti-polar state. Thickness-temperature phase diagram is constructed.
We have highlighted a switchable PV effect in these artificial structures at room temperature. By changing the thickness of the BFO layer we have evidenced a change of PV characteristics from a positive Voc and a negative Jsc to an inverse effect. These results, compared to our results obtained on BFO single films [3], indicate different origin of the mechanisms of photo-carries separation in PV devices. Our results are promising and indicate the possibility to employing the vertical geometry of measurements for tuning the PV effect in multiferroic SLs.
[1] B. Carcan et al., Adv. Mater. Interfaces, 4, 1601036 (2017)
[2] B. Carcan et al., Journal of Applied Physics 124, 044105 (2018)
[3] J. Belhadi et al. Journal of Physics: Condensed Matter, 31, 275701 (2019)

The demand for miniaturized and high performing broadband communication systems in the GHz frequency range is steadily growing due to severely crowded and rapidly changing modern commercial and military spectral environments. Hexaferrite composites have proven to be one of the few key enablers in providing miniaturized form factors due to their high permeability and permittivity as well as their capability to reach high resonance frequencies up to 100 GHz. In this work, a series of Z-type barium hexaferrites were prepared in which dopants of heavy transition metals, i.e., Mo⁴⁺ and Hf⁴⁺, were employed to modify structure and microwave properties. Through systematic measurements it was revealed that each dopant affected the hexaferrites’ properties differently by both shifting the resonant frequency and altering magnetic properties without strongly varying the hexaferrite crystallographic structure. Additionally, ε' = μ' was realized without any additives such as Bi₂O₂, which had been previously been used for tuning permeability and permittivity. Polycrystalline Co₂Z ferrites, having a nominal composition of Ba₃Co_2+XHf_XFe_24-2XO₄₁ and Ba₃Co_2+XMo_XFe_24-2XO₄₁, where x=0 to 0.05, were prepared by a solid-state process. BaCo₃, HfO₂, MoO₂, Co₃O₄, and Fe₂O₃ of high purity were mixed and calcined in oxygen for 5-8 hours at 800-1050 °C, which then were ball milled for 18-24 hours to reduce the grain size to under 1 µm. After ball-milling, ISOBAM was added as a binder and mixed for several minutes. Samples were then pressed to form toroids with an inner diameter of 3 mm and an outer diameter of 7 mm. The toroids were pressed by a cold isostatic press in order to maximize compaction before sintering in oxygen for 4-6 hours at 1000-1250 °C. Finally, samples were measured using the Agilent E864A PNA and a 7 mm precision airline, showing permeability and frequency changes. Results show that a nine-time reduction in size can be achieved while maintaining an impedance of 360 Ω close to that of free space over a broadband.

Ferromagnetic-ferroelectric (FM-FE) heterostructures have been studied as a route to obtain strong room-temperature magneto-electric interactions not commonly found in single-phase materials.¹ In such composites it is critical to understand the role of charge- and strain- mediated interactions at interfaces of the constituent phases, and spatially resolved probes such as x-ray photoemission electron microscopy (XPEEM) at the Advanced Light Source PEEM-3 beamline can observe such interface interactions at the scale of a single FE or FM domain. Polarization-dependent soft x-ray spectro-microscopy as a function of temperature and applied electric and magnetic fields can offer unique insights into coupling at heterointerfaces with sub-100 nm resolution by exploiting the x-ray linear dichroism (XLD) and magnetic circular dichroism (XMCD) effects. I will present a detailed analysis of the magneto-electric coupling in model multiferroic systems (e.g. FM layers grown on FE single crystals, or single phase multiferroic epitaxial thin films) using XLD- and XMCD-PEEM imaging. While the 3D vector orientation of ferromagnetic domains can be mapped with XMCD imaging at different azimuthal orientations, anisotropic strain interactions between FE and FM phases can be mapped by XLD imaging in an analogous fashion.^2,3 The anisotropic nature of the strain imprinted by the FE induces a local variation of the magnetic anisotropy of the FM phase due to large magnetoelastic anisotropy, and by varying applied electric fields or changes in temperature we can observe both the FM domain structure and imprinted strain state evolve due to such pertubations.² Furthermore, we can compare spatially averaged electric-field dependent magnetometry to spatially resolved XMCD images in applied magnetic fields, and directly correlate a giant anisotropy rotation and change of anisotropy symmetry to the local tuning of magnetic anisotropy at the scale of single FE and FM domains.³ Similarly, PEEM-based mapping of multiple ferroic orders can offer insight into optical switching mechanicsms of single-phase multiferroic materials.⁴ Thus, XLD- and XMCD-PEEM imaging offer a facile method to visualize magnetoelectric coupling at the sub-micron scale by probing FE, FM, or antiferromagnetic domain evolution as a function of stimuli such as in situ applied electric or magnetic fields.

1. W. Eerenstein and N. Mathur, Nature 442, 759 (2006).
2. R. V. Chopdekar et al, Phys. Rev. B 86, 014408 (2012).
3. R. V. Chopdekar et al, Sci. Rep. 6 27501 (2016).
4. Liou et al, Nat. Mater. 18 580 (2019).

Recently, the power consumption density has increased compared to the transistor which has increased exponentially per unit chip, making it difficult to store and process a large amount of data. Therefore, the development of low-power semiconductor technology is inevitable. In order to solve this issue on power consumption, research on a negative capacitance field-effect transistor (NCFET) is being actively conducted. In case of metal oxide semiconductor field-effect transistor (MOSFET), which is a conventional CMOS device, the subthreshold slope (S.S) value cannot fall below 60 mV/decade. NCFET, the next generation low-power device to replace MOSFET that encounters this limitation, can dramatically reduce power consumption density. Ferroelectric is negative capacitor in the gate structure of the NCFET, allowing channel surface potential to be amplified more than the gate voltage, enabling drive at less than 60 mV/dec S.S at room temperature. In this study, the ferroelectric Hf_0.5Zr_0.5O₂ (HZO) material in the gate structure was grown through pulsed laser deposition (PLD) process, and MoS₂ was used as a channel material to develop low power, high mobility 2D-NCFET. Conventionally, the ferroelectric HZO grown by atomic layer deposition (ALD), has been widely studied in electronic device development. In addition, research has been carried out focusing on HZO epitaxial growth at various substrates by PLD process. For the first time, we have introduced a new process to operate MoS₂ NCFET based on HZO, grown on Si substrate by PLD. The process conditions such as oxygen partial pressure were controlled during the experiment, and the Al₂O₃ (k~9) as bottom dielectric layer scaling resulted in a five-fold improvement in the subthreshold slope (S.S), operating the device at low power, from approximately 500 mV/dec to 100 mV/dec. To develop low power devices obtaining less than 60 mV/dec, the PLD process will further be optimized for the enhancement of ferroelectricity to obtain larger NC effect.

Gadolinium oxide (Gd₂O₃) is expected to exhibit strong interactions with external magnetic fields due to the high magnetic moment of Gd (7.94 µ_B). Applications range from a positive contrast agent in magnetic resonance imaging (MRI) to use as a cooling agent in magnetocaloric effect (MCE) based refrigerators. In this work, we report on the preparation of Gd₂O₃ nanospindles by a hydrothermal method, followed by measurement of its mass magnetization. Powdered Gd₂O₃ source (CAS: 12064-62-9, 99.9% trace metals basis, Sigma-Aldrich) was subjected to aqueous colloidal hydrothermal synthesis and subsequently annealed at 500^oC to produce cubic phase (JCPDS: 882165) Gd₂O₃, inferred from powder X-ray diffraction (PXRD, Advance Bruker D8, Ni-filtered Cu Kα radiation (λ = 1.5418 Å)). The morphology of nano-spindles (length ~100 ± 10 nm) was studied by field-emission scanning electron microscopy (FESEM-FEI QUANTA 3D FEG). Raman spectra confirmed the cubic phase of Gd₂O₃ with a highly intense and characteristic peak at 361 cm^-1. A small quantity of the product (1.23 mg) was wrapped in Teflon tape and exposed to magnetic fields in the range [-2,2] kOe in an Alternating Gradient Magnetometer (Micromag 2900 PMC) at room temperature. Measurements of magnetic moment (emu), converted into mass magnetization (emu/g), exhibit a quasi-saturated hysteresis behavior at limits of magnetic field excitation. The saturation magnetization (M_s), retentivity (M_r) and coercivity (H_c) are found to be 540.809 ± 4.381 m emu/g, 23.348 ± 0.186 m emu/g, and 21.218 ± 1.024 Oe, respectively over multiple measurements. The absence of saturation at the applied field in the magnetic hysteresis (M-H) curve suggests a mix of paramagnetic (PM) and ferromagnetic (FM) phases at room temperature. We are performing further materials analysis and investigating this magnetization result at different temperatures to validate it, and to understand the precise mechanism, in light of known defect-induced magnetization in various materials, albeit at lower temperatures. Potential applications include possible low-cost solution-processable magnetic devices and sensors.

Topological insulators (TI) have been known to possess topologically protected surface conducting states [1] and Rashba 2D electron gas (2DEG) due to band bending at interfaces [8]. A TI placed in contact with a ferromagnetic (FM) metal generates spin-orbit torques (SOT) on the magnetization in FM [2,3]. A signature of SOT in TI/FM bilayers is an increase in Gilbert damping in FM. SOT in TI/FM (ferromagnet) films have been reported previously with TI films epitaxially grown with molecular beam epitaxy [2, 3]. Here we report enhanced Gilbert damping in sputter deposited TI/FM, Bi_xTe_(1-x)/CoFeB (BT/CFB) films (x = ~0.4). Samples of 20nm BT film were grown on a Si/SiO₂ substrate at 250C and at 30C followed by 20nm CFB deposition at 30C (room temperature) using RF and DC magnetron sputtering respectively. X-ray diffraction (XRD) measurements reveal a crystalline 250C BT film compared to an amorphous 30C BT film. Room temperature broadband ferromagnetic resonance (FMR) measurements were conducted for 250C and 30C heterostructures and a control sample where the TI was substituted with 3nm Al. A full width at half maximum (FWHM) resonance linewidth was extracted as a function of frequency to calculate Gilbert damping. The control and 30C samples show almost identical linewidth, but the 250C sample linewidth is much broader. The Gilbert damping coefficient, α, for 250C sample is α = 0.3, compared to α = 0.03 for 30°C and α = 0.02 for control. The calculated spin-mixing conductance for 250C sample is g = 4e21 m-2 which is in an order of magnitude higher than reported value for CFB/Pt films [4]. Moreover, the presence the crystalline TI film adjacent to FM induces static effects on the magnetization. M-H loop measurements reveal emergence of out of plane magnetic anisotropy (OPMA) for the 250C sample at lower thicknesses (<5nm) of CFB films. This change in the magnetic anisotropy and spin pumping in the BT/CFB heterostructure is attributed to presence Rashba 2DEG because of strong spin-orbit coupling [5-8] at the interface, which is enhanced for the crystalline phase of BT. Further, spin-torque FMR studies on our samples is being carried out to characterize the effects on magnetization dynamics due to sputtered TI/FM interfaces.

References:
1.Y. L. Chen, J. G Anaytis and J. -H. Chu et al., Science Vol. 325, p. 5937 (2009).
2. A. R. Mellnik, J. S. Lee and A. Richardella et al., Nature Vol. 511, p. 449 (2014).
3. J. Han, A. Richardella and S. A. Siddiqui et al., Phys. Rev. Lett. Vol. 119, p. 077702 (2017).
4. M Belmeguenai, M. S. Gabor and F. Xigheim et al., J. Phys. D: Appl. Phys. Vol. 51 p. 045002 (2018).
5. A. Manchon, H. C. Koo and J. Nitta et al., Nature Materials Vol. 14, p. 871 (2015).
6. F. Khatmis, V. Lauter and F.S Nogueira et al., Nature, Vol 533, 513 (2016).
7. J. Kim, K, W. Kim and R. Wu et al., Phys. Rev. Lett. 119, 027201 (2017).
8. M. S. Bahramy, P.D.C. King and A. de la Torre et al., Nature Comm Vol 3, 1159 (2012).

Ferromagnetism in two-dimensional (2D) Van der Waals materials promises to revolutionize and develop spintronic devices and has received increased attention due to the electrical stability and sensitive tunability of an external field. To achieve high performance in spintronic devices, the effect of device substrates on the ferromagnetism of 2D materials also plays a critical role in practical device design and fabrication but has not yet been examined. Here, we systematically demonstrated the substrate-modulated ferromagnetism of 2D Fe₃GeTe₂ based on different substrates. The out-of-plane ferromagnetism of Fe₃GeTe₂ thin films prepared on Al, SiO₂ and Au substrates was probed using Magneto-optical Kerr effect techniques. Based on the experimental data, Fe₃GeTe₂ thin films with the same thickness were modulated to show different T_C on different substrates, with the highest T_C on Au substrate, especially in a thin layer. The underlying reason was explained by the first-principle calculation and mean field approximation, revealing that the charge redistribution and lattice distortion play an important role on magnetic properties changes. Our work provides a practical way to modulate the magnetic properties of ferromagnetic materials permanently without applying any additional treatment processes, and a feasible theoretical method to study the fundamental physical properties of ferromagnetic materials.

Spin-orbit coupling is known to be responsible for non-trivial spin configurations and a number of emergent physical phenomena. Ferroelectric materials are especially interesting in this regard due to broken space inversion symmetry, which enforces the emergence of the bulk Rashba and Dresselhaus spin-orbit coupling, and reversible spontaneous polarization, which allows for a non-volatile electrical control of the spin degrees of freedom. This talk addresses two classes of ferroelectric oxide materials exhibiting dissimilar spin textures due to different space-group symmetries. First, we consider a technologically relevant oxide material, HfO₂, which is known to be ferroelectric in a non-centrosymmetric orthorhombic phase. We show that HfO₂ exhibits Dresselhaus-type spin texture driven by spin-orbit coupling,¹ and argue that this material can be used as a tunnel barrier to produce tunnelling anomalous and spin Hall effects that are reversible by ferroelectric polarization.² Another example involves a class of materials capable to maintain a persistent spin texture. This property has been predicted to support an extraordinarily long spin lifetime of carriers promising for spintronics applications. We show that the persistent spin texture can be enforced by non-symmorphic space-group symmetry of the crystal, which makes it a robust intrinsic property of the bulk material.³ Using symmetry analyses and theoretical modelling based on density-functional theory, we demonstrate this property for a handful of oxide materials, which can be synthesized in laboratory. Among them BiInO₃, a wide band gap ferroelectric semiconductor, is the most promising candidate which sustains persistent spin texture around the conduction band minimum. The reversible spontaneous polarization of BiInO₃, allows for an electrical control of the spin polarization, which can be selectively induced by circular polarized optical pumping.

1. L. L. Tao, T. R. Paudel, A. A. Kovalev, and E. Y. Tsymbal, Reversible spin texture in ferroelectric HfO₂, Physical Review B 95, 245141 (2017).
2. M. Y. Zhuravlev, A. Alexandrov, L. L. Tao, and E. Y. Tsymbal, Tunneling anomalous Hall effect in a ferroelectric tunnel junction, Applied Physics Letters 113, 172405 (2018).
3. L. L. Tao and E. Y. Tsymbal, Persistent spin texture enforced by symmetry, Nature Communications 9, 2763 (2018).

Over the past two decades, new discoveries and major advances have been made to enhance controlled synthesis of epitaxial ferromagnetic oxide films and to gain fundamental understanding of their physical properties. Tunable magetotransport properties, in particular, have been achieved through interfacing ferromagnetic materials with binary oxides at the nanoscale by forming nanocomposite films, where both the ferromagnetic and binary oxides are of nanoscale dimensions. In this talk, we will overview our strategies in synthesis and characterization of epitaxial magnetic nanocomposite films. Using controlled synthesis, advanced probing, and theoretical modeling, we are able to understand the effect of interface strain on the magnetotransport properties of ferromagnetic materials such as La_0.7Sr_0.3MnO₃ films.

Strain-coupled magnetoelectric (ME) phenomena in piezoelectric / ferromagnetic thin-film bilayers are a promising paradigm for sensors and information storage devices, where strain manipulates the magnetization of the ferromagnetic film. In-plane magnetization rotation with an electric field across the film thickness has been challenging due to the virtual elimination of in-plane piezoelectric strain by substrate clamping, and in two-terminal devices, the requirement of anisotropic in-plane strain. We have overcome both of these limitations with lithographically patterned devices with a piezoelectric membrane on a soft substrate platform, in which in-plane strain is freely generated, and a patterned edge constraint that transforms the nominally isotropic piezoelectric strain into the required uniaxial strain. We fabricated 500 nm thick, (001) oriented [Pb(Mg_1/3Nb_2/3)O₃]_0.7-[PbTiO₃]_0.3 (PMN-PT) unclamped piezoelectric membranes with ferromagnetic Ni overlayers. Guided by analytical and numerical continuum elastic calculations, we designed and fabricated two-terminal devices exhibiting Ni magnetization rotation in response to an electric field across the PMN-PT. Similar membrane heterostructures could be used to apply designed strain patterns to many other materials systems to control properties such as superconductivity, band topology, conductivity, and optical response.

This work has been done in collaboration with J. Irwin, S. Lindemann, W. Maeng, J. J. Wang, V. Vaithyanathan, J.M. Hu, L.Q. Chen, D.G. Schlom, C.B. Eom, M.S. Rzchowski.
This work was supported by the Army Research Office through grant W911NF-17-1-0462.

Applications in advanced, beyond Moore’s law computing are driving researchers and industry alike to explore a broader set of materials than ever before. While industry has maintained, in some form, their ability to continually meet the demands of Moore’s law by increasing the number of transistors on chip, largely in part from efforts in scaling and size reduction of the transistors themselves and the associated circuit area, the same cannot be said for energy scaling. In the last fifteen years, scaling has deviated from Dennard’s trend which states that the power density of circuits stays roughly the same as transistors get smaller, assuming, concurrent voltage reductions are made. Herein lies the challenge – while we have reduced transistor and circuit size, voltages of operation have not been scaled at the same rate. As a result, power and energy dissipation now stands as one of the most pressing challenges for advanced nanoelectronics. This has motivated renewed attention to a wide variety of routes to reduce the voltage of operation of next-generation logic. Such approaches have thrust materials – including multiferroics and magnetoelectrics – back into the mix as candidates for beyond CMOS computing.

Despite considerable research on multiferroic and magnetoelectric materials in the last decade and advances in our ability to synthesize, control, characterize, and fabricate these materials, the requirements of beyond Moore’s law computing are already pushing these materials to their limits. In this talk, we will explore recent efforts to reduce the operating voltages of multiferroic and magnetoelectric devices including exploring both new and old materials in ever decreasing sizes. In particular, we will explore efforts to push the size limits of these materials whereby reducing the thickness of films provides a direct pathway to reduced operation voltages. Special attention will be given to challenges in achieving ultra-thin-film operation in multiferroics such as BiFeO₃ and Bi_1-xLa_xFeO₃ as well as in magnetoelectric composite structures based on piezoelectric/ferroelectric materials such as BaTiO₃ and (1-x)PbMg_1/3Nb_2/3O₃-(x)PbTiO₃ (PMN-PT). For example, while considerable work has been done in PMN-PT as bulk ceramics, single crystals, and thick films, essentially no work on ultra-thin films has been considered. We will explore the evolution of relaxor order and actuation potential as we reduce relaxor film thicknesses to the range of 5-100 nm. Studies suggest surprising enhancement of relaxor character with reducing thickness and the presence of a complex, thickness evolution to structure and properties. We will explore other approaches including orientation dependence and chemical routes to reduce polarization or manipulate structure in such as fashion that it reduces the costs of switching polarization. For example, in PbZr_0.2Ti_0.8O₃ thin films, it has been shown that merely changing film orientation from (001)- to (111)-oriented structures can dramatically change coercive field scaling (thus avoiding classical Janovec-Kay-Dunn scaling trends) as a result of changes in materials symmetry which produce low-energy switching pathways. Finally, we will explore the effects of these approaches on the realization of low-voltage magnetoelectric coupling in heterostructures such as BiFeO₃-based spin valves and composite ferromagnet/piezoelectric stacks. To end, we will discus where these ideas could lead us next and what metrics can be met using these materials and methods.

Voltage-controlled magnetization switching at interfaces or boundaries enables dissipationless control of remnant magnetic states. It paves the way towards ultra-low power and non-volatile spintronics. We exploit substitutional Boron doping of the magnetoelectric antiferromagnet Cr₂O₃ to fabricate voltage switchable all antiferromagnetic memory devices which operate between room temperature and 400K, i.e., significantly above the antiferromagnetic ordering temperature T_N = 307 K of pure Cr₂O₃. T_N-enhanced and anisotropy reduced magnetoelectric films are grown via pulsed laser deposition in the presence of borane background gases. Boundary magnetization associated with two degenerate orientations of the Néel vector serves as state variable which can be isothermally written and read out. Writing is achieved through voltage-controlled switching of the Néel vector. Read out takes place by sensing an anomalous Hall-like signal originating from an electric current driven through a lithographically patterned Pt Hall bar deposited on top of the B-doped Cr₂O₃ film. In device architectures based on voltage-controlled exchange bias where remnant magnetization of a ferromagnetic film serves as state variable, B-doping does not translate into improved performance. Although B-doping can increase the blocking temperature, exchange coupling is not accompanied by effective pinning and thus voltage-control of the ferromagnet. The detrimental effect of B-doping on perpendicular exchange bias is attributed to reduced magnetic anisotropy and canting of the interface magnetization relative to the surface normal. It takes place despite perseverance of antiferromagnetic order and magnetoelectric functionality above 307 K as evidenced by magnetometry and photoemission spectroscopy. In contrast to pure chromia, where order parameter switching is controlled by the competition between magnetoelectric and anisotropy energy, the switching mechanism in B-doped chromia seems to be additionally affected by a sizable electric field dependence of the magnetic anisotropy. Our results demonstrate that T_N-enhancement and anisotropy reduction through B-doping of Cr₂O₃ is successfully utilized in voltage-controlled spintronic devices with reduced complexity. Our Hall bar structures based on B-doped chromia show voltage-controlled switching up to 100 K above room temperature paving the way to CMOS compatible ultra-low power magnetoelectric MRAM and logic devices.

We acknowledge support by SRC and NSF through ECCS 1740136, MRSEC DMR-1420645, and NNF under Award NNCI: 1542182 and support by ARO through MURI W(11NF-16-1-0472.

Domain walls often show a novel functionality very different from that of bulk states. Despite their importance, the domain wall physics are not well understood compared to the bulk cases because of the lack of experimental techniques to investigate the confined walls. In the present study, we conducted a SHG imaging measurement which allows us to probe the domain walls in MnWO₄. We found multiferroic domain walls in a non-multiferroic environment.
MnWO₄ crystalizes in a centrosymmetric space-group P2/c, and shows successive magnetic phase transitions below the Néel temperature T_N = 13.5 K. In the AF2 phase, the magnetic order breaks the space-inversion symmetry and ferroelectric polarization appears along the b-axis [1], while the other phases have non-polar magnetic structures. Correspondingly, SHG is allowed only in the AF2 phase, while it is forbidden in the AF1 and AF3 phases. However, it was found that non-zero SHG signal exists even in the SHG-forbidden AF1 phase [2]. We revealed that the SHG signal in the AF1 phase comes from wall-like structures. Polarization anisotropy and spectroscopy measurements of the SHG signal suggest that the wall in the AF1 phase should have a magnetic structure similar to that of the multiferroic AF2 phase. The results indicate the existence of domain walls breaking space-inversion symmetry, although the bulk phase is centrosymmetric. We propose the emergence of multiferroic domain walls in a non-multiferroic environment.

[1] K. Taniguchi et al., Phys. Rev. Lett. 97, 097203 (2006).
[2] D. Meier et al., Phys. Rev. B 80, 224420 (2009).

The domain walls that separate different orientations of electric polarization in ferroelectric materials have long been of interest because their motion governs the process of ferroelectric switching in an electric field [1]. Recently, a range of unexpected behaviors have been discovered at domain walls that do not occur in the bulk of the domains, such as ferrielectricity, magnetoelectricity, and electrical conductivity or even superconductivity, suggesting additional interest in domain walls as functional entities in their own right [2].

At the same time, the magnetization caused by the usual motion of electric charges has been revisited over the last years in the context of time-varying ferroelectric polarizations. This newly described dynamical multiferroicity, associates a magnetization M of the form M ~ P × ∂_t P with a ferroelectric polarization P. A range of existing coupled electric-magnetic phenomena fall within the dynamical multiferroicity framework, and new behaviors, including a phonon Zeeman effect, exotic quantum criticality and phonon orbital magnetism have been proposed [3-5].

Here we establish the link between these two concepts -- dynamical multiferroicity and ferroelectric domain wall functionality -- by showing theoretically that the motion of ferroelectric domain walls can be accompanied by a dynamical magnetic field. We extend the formalism of dynamical multiferroicity to the case of domain wall motion and present numerical results for the prototypical ferroelectric barium titanate (BaTiO₃), based on a combination of density functional theory calculations and phenomenological modeling. We propose two experimental setups that possibly allow the detection of the dynamical magnetic field using nitrogen-vacancy center magnetometry, and we show that the magnitude of the effect lies well above the present detection limit.

[1] Paruch, Giamarchi, and Triscone, Topics Appl. Phys. 105, 339 (2007)
[2] Salje, Phase Transit. 86, 2 (2013)
[3] Juraschek, Fechner, Balatsky, and Spaldin, Phys. Rev. Mat. 1, 014401 (2017)
[4] Dunnett, Zhu, Spaldin, Juricic, and Balatsky, Phys. Rev. Lett. 122, 057208 (2019)
[5] Juraschek and Spaldin, Phys. Rev. Mat. 3, 064405 (2019)

Magnetic field sensors based on the delta-E effect utilize the resonance shift of a high frequency mechanical resonator in a magnetic field, due to the change in Young’s modulus of a magnetostrictive material. Delta-E effect sensors are not affected by 1/f amplifier noise and allow broadband magnetic field measurements at low frequencies down to DC with very high dynamic range. Moreover, they are robust against microphony effects and mechanical noise. Fully integrable magnetic field sensors were achieved via replacement of magnetic by electric excitation. Since our first publication of this sensor concept in 2011 [B. Gojdka et al., Appl. Phys. Lett. 99, 223502 (2011); Nature 480, 155 (2011)], much progress has been made in understanding the complex interplay of magnetic, mechanical and electrical properties. This holds for different designs of cantilever sensors, but also for surface acoustic wave devices. Here we present a comprehensive magneto-electromechanical model that considers the interaction of magnetic, mechanical and electrical properties and show current experimental results. We discuss the delta-E effect in general and its application in diverse types of electrically excited sensors, including the most common types such as bending or bulk resonators, but also shear resonators and surface acoustic wave sensors. The model provides detailed understanding of the general limits of the sensitivity, which arise from using the delta-E effect as the sensing principle. Realistic detection limits are predicted in combination with a noise equivalent model. The simulations are validated with experimental data using different sensor designs and magnetic layers. We also discuss simultaneous operation of delta-E sensors in the delta-E mode and the direct magnetoelectric mode, e.g. for localization of the sensor.

Magnetoelectric (ME) resonators are of significant interest for next generation magnetic field sensors, as the direct coupling of magnetostrictive and piezoelectric phases enables high magnetic field sensitivity with exceptionally low operational power requirements. In this work, we present silicon based ME thin film resonators with clamped-clamped boundary conditions, that are fully suspended to achieve a string mode resonance. The heterostructure is comprised of a magnetoelastic FeCo or FeCoV layer, which is coupled through strain to an AlN piezoelectric layer with high e_31,fpiezoelectric coefficients. A negative or positive shift in the resonant frequency was observed for fields applied parallel and perpendicular to the length of the beam, respectively, consistent with the magnetostriction measurements and the expected magnetoelastic strain developing in the beams due clamped-clamped boundary conditions. A linear behavior in the field dependent resonant frequency when the field is perpendicular to the length is most likely due to a well-defined hard axis in this direction resulting from high shape and stress-induced anisotropy energies. Together, these results suggest that through modification of the magnetic anisotropy, the frequency shift and angular dependence can be tuned, producing highly directional structures for magnetic field sensors. Finally, the sensor performance is evaluated and benchmarked against conventional magnetic field sensors.

Phase-field method has been applied to modeling both direct and converse magnetoelectric effects in multiferroic heterostructures including the coupled evolution of magnetic and ferroelectric domain under electric fields. This presentation will discuss our recent applications of the phase-field method to understanding the strain-mediated voltage control of magnetic skyrmions in nanoscale multiferroic heterostructures. It is shown that a nanoscale skyrmion in a magnetic disk can be repeatedly created and deleted by strain imparted from the underlying piezoelectric layer. We analyzed the strain-mediated skyrmion switching dynamics based on both phase-field simulations and analytical theorities. We will discuss the possibility of performing high-throughput phase-field simulations of magnetic switching of multiferroic heterostructures. In particular, using a combination finite-element calculations and phase-field simulations, we studied the 3D geometrical effects of Ni islands on the strain relaxation and magnetic domain structures and switching in Ni/PMN-PT nanoscale heterostructures under an external electric field. Finally, we will also discuss potential applications of a dynamical phase-field model to understanding the ultrafast dynamics of ferroic domains in multiferroics.

Spinel magnetostrictive cobalt ferrite (CoFe₂O₄, CFO) can vertically integrate nanopillars embedded in a matrix of perovskite piezoelectric bismuth ferrite (BiFeO₃, BFO), self-assembling the ‘1-3’-type nanocomposites [1, 2]. These two phases exhibit a considerable magnetoelectric (ME) effect due to the coupling via strain at the interfaces [3]. The ME effect of CFO with either rhombohedral or tetragonal BFO has been studied [4, 5].
To further increase the ME coefficient, such ‘1-3’-type nanocomposites were epitaxially deposited on piezoelectric single crystal substrates (i.e. Pb(Mg_1/3Nb_2/3)O₃-x%PbTiO₃, PMN-xPT). An applied electric field (E) of 3 kV/cm was able to induce a giant magnetization change (~93% for ΔM/M_r), equivalent to a converse ME coefficient of 1.3×10^-7 s/m [6]. Moreover, multiple distinguishable magnetization states were obtained, which were stable on removal of the E [6]. This multi-state ME nanocomposite thus has potential for neuromorphic-like computing and multilevel-cell memory devices.
From an energy-saving perspective, CuFe₂O₄ (CuFO) that shows low loss and high mechanical quality factor was used to replace the typical CFO in our ME nanocomposites [7]. The CuFO-BFO/PMN-PT nanocomposites exhibited notably slimmer M-H loops than CFO-BFO/PMN-PT ones, and a significantly larger ME coupling effect than pure CuFO/PMN-PT heterostructures [7, 8]. The findings demonstrate a trade-off between ME effects and energy losses that can be modified by material choices and nanocomposite topography.
Our investigations on CFO-BFO and CuFO-BFO shed light on the feasibility to approach non-volatility in nanocomposites, even though the individual phases/substrate had only volatile properties [8]. It simplifies materials selection for multi-state systems, averting difficulties with compositional non-uniformity and property repeatability in particular with regards to PMN-xPT crystal substrates.

[1] L. Yan, Y. Yang, Z. Wang, Z. Xing, J. Li, D. Viehland, Review of magnetoelectric perovskite–spinel self-assembled nano-composite thin films, Journal of Materials Science, 44 (2009) 5080-5094.
[2] J.L. MacManus-Driscoll, P. Zerrer, H. Wang, H. Yang, J. Yoon, A. Fouchet, R. Yu, M.G. Blamire, Q. Jia, Strain control and spontaneous phase ordering in vertical nanocomposite heteroepitaxial thin films, Nature Materials, 7 (2008) 314.
[3] Z. Wang, Y. Li, R. Viswan, B. Hu, V.G. Harris, J. Li, D. Viehland, Engineered Magnetic Shape Anisotropy in BiFeO₃–CoFe₂O₄ Self-Assembled Thin Films, ACS Nano, 7 (2013) 3447-3456.
[4] M. Gao, R. Viswan, X. Tang, C.M. Leung, J. Li, D. Viehland, Magnetoelectricity of CoFe₂O₄ and tetragonal phase BiFeO₃ nanocomposites prepared by pulsed laser deposition, Scientific Reports, 8 (2018) 323.

Spin-orbit coupling (SOC), the interaction between the electron spin and the orbital angular momentum, can unlock rich phenomena at interfaces, in particular interconverting spin and charge currents. Conventional heavy metals have been extensively explored due to their strong SOC of conduction electrons. However, spin-orbit effects in classes of materials such as epitaxial 5d-electron transition metal complex oxides, which also host strong SOC, remain largely unreported. In addition to strong SOC, these complex oxides can also provide the additional tuning knob of epitaxy to control the electronic structure and the engineering of spin-to-charge conversion by crystalline symmetry. Here, we demonstrate room-temperature generation of spin-orbit torque on a ferromagnet with extremely high efficiency via the spin-Hall effect in epitaxial metastable perovskite SrIrO₃. We first predict a large intrinsic spin-Hall conductivity in orthorhombic bulk SrIrO₃ arising from the Berry curvature in the electronic band structure. By manipulating the intricate interplay between SOC and crystalline symmetry, we control the spin-Hall torque ratio by engineering the tilt of the corner-sharing oxygen octahedra in perovskite SrIrO₃ through epitaxial strain. This allows the presence of an anisotropic spin-Hall effect due to a characteristic structural anisotropy in SrIrO₃ with orthorhombic symmetry. Our experimental findings demonstrate the heteroepitaxial symmetry design approach to engineer spin-orbit effects. We therefore anticipate that these epitaxial 5d transition-metal oxide thin films can be an ideal building block for low-power spintronics.

Achieving novel physical properties, such as superconductivity, ferromagnetism, and ferroelectricity, in two-dimensional (2D) materials should enable numerous functionalities in nanoscale devices. In recent years, interests in high-performance 2D ferroelectric materials have also grown rapidly across multiple scientific and engineering disciplines. Here, two new 2D polar materials are predicted based on physical analysis and by using density functional theory calculations. The first one is a 2D type-II multiferroic magnetoelectric material (MXene Hf₂VC₂F₂) and and the second one is a noncollinear ferrielectricity materials (dioxydihalides MO₂X₂ family, where M= Mo and W; X= Cl and Br).

For multiferroic MXene Hf₂VC₂F₂ monolayer, its ferroelectricity originates directly from its magnetism [1]. The noncollinear 120^o Y-type spin order generates a polarization perpendicular to the spin helical plane. Remarkably, the multiferroic transition is estimated to occur above room temperature.

For dioxydihalides MO₂X₂ monolayer, it is predicted to display noncollinear ferrielectricity, induced by competing ferroelectric and antiferroelectric soft modes [2]. More importantly, this intrinsic noncollinearity of dipoles generates unique physical properties, such as Z₂×Z₂ topological domains, atomic-scale dipole vortices/anti-vortices, and negative piezoelectricity.

Our studies will open the door to a new branch of 2D materials in the pursuit of intrinsically strong magnetoelectricity and noncollinear ferrielectricity.

References
[1] J. J. Zhang, L. F. Lin, Y. Zhang, M. Wu, B. I. Yakobson, S. Dong, J. Am. Chem. Soc. 140 (2018) 9768-9773.
[2] L. F. Lin, Y. Zhang, A. Moreo, E. Dagotto, S. Dong, under review.

Magnetoelectric effects, magnetic-field-mediated polarization and/or electric-field-controllable magnetism, have attracted enormous attentions owing to the growing demands of new-generation information technologies. However, large direct and/or converse magnetoelectric coupling in thin-film materials is challenging (Nat. Mater. 18, 203 (2019)), which inhibits the practical application in future high-density and high-throughput electronic/spintronic devices. Here, via artificially design of correlated oxides at atomic scale, large magnetoelectric response by controlling the interfacial chemistry and physics has been achieved, accompanying the strong interplay of spin, charge, orbital and lattice degrees of freedom. This atomic tailoring of the quantum order parameters in oxide interfaces provides an alternative pathway towards realizing strong and controllable magnetoelectric effects with thin-film integrations.

The bulk properties of the prototypical linear magneto-electric (ME) antiferromagnet Cr₂O₃ have been extensively studied. Here, we report on a muon spin rotation (μSR) study using spin polarized muons to probe the local ME effect. This was motivated by the recent prediction that a point charge inside a linear ME creates a monopolar magnetic field distribution [1]. μSR is a unique and novel way to test such predictions since the muon may act both as a test charge and a sensitive probe of the induced local magnetic field .
Prerequisite for a search for such local or muon induced magnetoelectricity is a thorough understanding of the local electronic structure and magnetic interaction of the muon within the host material. We find that the muon, a light interstitial probe, occupies several distinct stopping sites in Cr₂O₃, and displays a rich dynamic behavior that we interpret in the context of local muon hopping, thermally activated site transitions and the formation of a charge-neutral muon-polaron complex.
Furthermore, when Cr₂O₃ is prepared in a single magnetic domain, a shift in the local magnetic field is observed in response to an applied electric field, with the sign of the shift depending both on the field direction and domain state. The origin of this apparent magneto-electric effect is discussed.

[1] D. I. Khomskii, Nature Communications 5, 4793 (2014)

Magnetoelectric materials are of the great interest due to their unique coupling of the magnetic and electrical order parameters. In these materials magnetic states can be manipulated via electric field and vice versa, offering exciting prospectives for energy efficient memory, logic and sensor devices. However, a sizeable magnetoelectric coupling for technologically relevant applications is obtained for a limited set of single-phase bulk materials. Typically, this restriction can be removed by using two-phase materials, containing strain-coupled magnetoelectric heterostructures based on piezoelectric-magnetostrictive bilayers. Although the concept is promising, there is a clear limitation regarding the fact that strain-induced changes result in the modification of all intrinsic magnetic parameters, in particular anisotropic couplings. Ideally, it would be advantageous that electric field control of magnetic state is achieved without change of global intrinsic magnetic parameters.

We propose a novel type of artificial magnetoelectric material [1], which allows an electric field-induced deterministic switching between magnetic states without influencing intrinsic magnetic parameters. The proposal refers to geometrically curved helimagnets [2,3] embedded in a piezoelectric matrix or sandwiched between two piezoelectric layers. In contrast to typical strain-coupled magnetoelectric heterostructures, we exploit the geometric coupling between the piezoelectric matrix and curvilinear helimagents. Namely, a small geometrical deformation causes a drastic modification of magnetic state of the helimagnet through a magnetic phase transition between a homogeneous magnetic state and a periodical one. Resulting transformations of the average magnetization from non-zero to zero value can be uniquely assigned to logical “1” and “0”. This paves the way towards the realization of novel magnetoelectric devices with geometrically tunable and deterministically switchable magnetic states.

We provide not only the general concept but also show analytical validation for a prototypical example of torsional nanospring helimagnets. Furthermore, we put forth a discussion on the feasibility of the experimental realization of the concept including the choice of materials and fabrication approaches.

[1] O. M. Volkov et al., J. Phys. D: Appl. Phys. (2019). doi:10.1088/1361-6463/ab2368.
[2] O. M. Volkov et al., Scientific Reports 8, 866 (2018).
[3] R. Streubel et al., J. Phys. D: Appl. Phys. (Topical Review) 49, 363001 (2016).

Multiferroic BiFeO₃ (BFO) films with periodic stripe domains produce anomalously large open circuit voltages under visible light illumination with exciting promise in solar energy applications. However, the microscopic origin of this photovoltaic effect has not been understood yet. Here, we make contact-free measurements of light-induced currents in stripe domain BFO films with 71° domain walls using terahertz emission spectroscopy. With this approach, we avoid undesired modifications to the ferroelectric polarization due to the physical electrodes, hence allowing us to disentangle different photovoltaic responses intrinsic to the BFO films. Analyzing the radiated terahertz fields, we find that the current flows perpendicular to the domain walls. This observation strongly indicates the predominant role of the domain walls in the separation of photogenerated charges and enables quantitative estimates of the currents and built-in fields associated with the domain walls. Furthermore, samples with increasing domain wall density show enhanced terahertz emission providing additional evidence for domain wall-mediated charge separation. On the other hand, we find that bulk photovoltaic effects only play a minor role in the photovoltaic response of the stripe domain BFO films with a contribution smaller than 5%. This work enables new fundamental understanding of photoferroelectric responses and defines novel opportunities for ferroelectric-based optoelectronics and efficient bias-free terahertz emitters.

With room temperature coupling between magnetic and electric degrees of freedom, BiFeO₃ (BFO) has attracted much attention as a leading candidate for magnetoelectric applications. These include a variety of spintronic applications such as a magnetoelectric spin orbit (MESO) logic device where there exist stringent requirements on switching speed. Although there does not exist a wealth of true time-dynamics studies of ferroelectrics, a previous study on Pb(Nb_0.04Zr_0.28Ti_0.68)0₃ found ultrafast ferroelectric switching at ~220ps. Time-resolved BFO measurements have, to date, not been reported. There has been extensive work studying the equilibrium or quasi-static magnetoelectric coupling in BFO, but dynamics and fundamental limitations on switching have yet to be explored. Via pulsed ferroelectric (PUND) I-V measurements we provide novel data probing the limits of magnetoelectric switching speeds and study the effect of film thickness on dynamics. We probe the effects of chemical composition on magnetoelectric switching by studying two model systems, namely BFO and La-doped BFO. The data reveal low-nanosecond switching, much faster than previously reported in (La)BFO. Importantly, by exploring lateral scaling of device size, we demonstrate a pathway to the switching speeds required for spintronic applications. By employing giant magneto-resistance (GMR) and longitudinal magneto-optical Kerr effect (MOKE) studies, we are working towards direct observation of voltage control of magnetization on such timescales.

Understanding magnetic domain structures and their responses to electric fields in multiferroic heterostructures is critical to the design of electric-field-driven spintronic devices. In this presentation, I will show high-throughput finite-element and phase-field simulations of piezoelectric strain anisotropy and its relaxation, magnetic domain structures and their responses to applied voltages as function of the in-plane dimensions and thickness of the magnetic Ni nanoislands grown on a Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) membrane. The piezoelectric strain anisotropy is found to increase with the in-plane aspect ratio, but it can be significantly relaxed, as large as >80%, in nanoislands of thickness larger than >15 nm. Magnetic domain diagrams are established to identify the domain structures for Ni nanoislands of different lengths of in-plane major and minor axis, as well as thickness. When a voltage is applied to the multiferroic heterostructure, the single-domain magnetic domain can be switched by the piezoelectric strain, whereas the vortex domain is not switched. However, for a multiferroic heterostructure with a thick nanoisland wherein most of the piezoelectric strain is relaxed, the single-domain magnetic domain shows a weak response to the voltage and cannot be switched by the voltage.
I will also show how to use a phase-field approach to identifying the magnetic anisotropy, which is similar to experimental rotating magneto-optical Kerr effect method and anisotropic magnetoresistance measurement, By performing high-throughput phase-field simulations to obtain thousands of M-H loops, hundreds of remanent magnetization and coercive field versus external magnetic-field direction polar diagrams can be established, which are used to further construct directional angle and coercive field of the easy axis versus strain anisotropies diagrams. Thermodynamic energy analysis are performed to understand the simulation results, which revealed that the strain anisotropy impacts the directional angle and the coercive field of the easy axis by modifying the valley and peak positions, as well as the barrier of the free energy profile. This presentation will contribute to a computational understanding of strain anisotropy and voltage-controlled magnetic properties in strain-mediated multiferroic heterostructures.
References:
Jian-Jun Wang, Tian-Nan Yang, Jacob A. Zorn, Emily Wang, Julian Irwin, Shane Lindemann, Mark S. Rzchowski, Jia-Mian Hu, Chang-Beom Eom, Long-Qing Chen, “Strain Anisotropy and Magnetic Domain Structures in Multiferroic Heterostructures: High-throughput Finite-Element and Phase-field Studies”, Acta Materialia (2019).

BiFeO₃ (Bismuth Ferrite or BFO) is a complex oxide that has been widely investigated for multiferroic properties as well as solar energy harvesting. In the recent times, various different nanostructures such as nanoparticles, thin films, nanowires and nanorods have been reported. In the case of 1D nanostructures such as nanorods and wires, vertical alignment from the underlying substrate holds special importance because of resulting ordered morphology and higher surface to volume ratio. Theoretically it is shown that the intrinsic stress generated due to the quantum confinement may result in shifting the ferroelectric and ferromagnetic transition temperature and also can induce giant magnetoelectric effect. Literature review suggests that, although different morphologies of 1D structures of BFO have been fabricated, highly aligned and vertically oriented nanorods have not been fabricated in template free conditions. Such structures hold special importance, particularly in solar energy and piezoelectric nanogenerator applications.
This work presents first successful fabrication of vertically aligned BiFeO₃ nanorods on FTO coated glass substrate using chemical bath technique followed by the deposition of Bismuth layer by DC sputtering. The significance lies in achieving such structures in template free condition that allows tailoring of the diameter and length and also the formation of heterostructures during the growth process. The crystallinity, microstructure, morphology and chemical composition have been confirmed by XRD, TEM, SEM and XPS. The multiferroic properties have been investigated by Piezoelectric Force Microscopy (PFM), P-E loop measurements and Vibrating sample magnetometry. The maximum obtained value of d₃₃ is 54 pm/V whereas a weak remnant magnetization of around 3 emu/cm³ is observed even at low temperature of 5 K. Finally, the optical and photocatalytic characteristics have been investigated by optical absorption and photo-electrochemical water splitting experiments. At 1 Sun illumination, a photocurrent density of around 55 μA/cm² (0.8 V vs Ag/AgCl) is observed which is found to be stable with time. The developments open up new avenues for BiFeO₃ where 1D nanostructures are desired.

Coupling of ferroelastic and ferromagnetic order parameters in materials offers a means to achieve novel multiferroic applications. We report the observation of one-dimensional ferroelastic domains in LaCoO₃ thin films that are intimately linked to magnetization. Unidirectional structural modulation is achieved by selective choice of substrate or growth plane, which produces broken in-plane rotational symmetry. Structural modification either through film growth or application of pressure is shown to affect the magnetism. Modification perturbs the crystal field energy, which leads to unexpected in-plane anisotropy of the orbital configuration and the magnetization.
Work funded by the U.S. Department of Energy, Basic Energy Sciences

In this talk, we examine multiple ways to control magnetism in solution processed nanostructured materials using an applied electrical bias. We begin with traditional multiferroic coupling, where electricity and magnetism are coupled through strain, and consider two systems. The first is a nanoporous magnetic or ferroelectric network produced using polymer templating of sol-gel oxides. The pores are then conformally filled with the opposite phase (either ferroelectric or magnetic) using atomic layer deposition to produce a three dimensional nanoscale composite. For these materials, we find the largest multiferroic response in materials with partly filled pores, emphasizing the role of residual porosity in controlling the elastic behavior and thus the multiferroic coupling of these composite materials. We next consider strain mediated switching in monolayer nanocrystal arrays. Here we show that nanocrystals can be controllably and reversibly switched from a superparamagnetic state, which has no time-averaged magnetic moment, to a ferromagnetic state using an applied bias. Finally, we explore a new type of multiferroic material, termed granular multiferroics, where exchange coupling between closely spaced magnetic nanocrystals can be modified by tuning the dielectric environment around the nanocrystals using either temperature or an applied electric field. In this work on nickel nanocrystals coupled to a soft ferroelectric, both temperature and field dependent changes in magnetism are observed in the vicinity of the ferroelectric Currie temperature, indicating that magnetism in nanocrystal arrays can indeed be tuned using dielectric changes.

One of the central challenges in realizing magnetoelectric (ME) devices lies in finding a deterministic way to modulate magnetism in integrated circuits with a circuit-operation voltage. Ionic liquid (IL) gating on magnetic thin films with abundant electronic, chemical and magnetic interactions at the interface has become an emerging technology for controlling magnetism in a fast, compact and energy-efficient way. Compared with conventional strain effect dominated piezo/ferroelectric layer multiferroics, IL gating method has advantages like small gating voltage (V_g<5 V), easy-to-integration and compatibility with varied substrates such as Si, flexible substrates etc. In additional, unlike the oxide structures require a high temperature to overcome the oxidation energy barrier, the IL gating control process can be operated at room temperature, suitable for applications in room temperature environment. Here, we will summarize our recent progresses of IL gating control of magnetism in varied magnetic heterostructures, as well as in dfferent manners.^[1-5] As IL gating process, proven to be a truly powerful and compatible gating method, enables giant ME tunability in different heterostructures and provides a tremendous potential in next generation of voltage-tunable spintronics/electronics.
1. S. Zhao, Z. Zhou, B. Peng, M. Zhu, M. Feng, Q. Yang, Y. Yan, W. Ren, Z.G. Ye, and Y. Liu. M. Liu, Adv. Mater. 29, 1606478 (2017)
2. Q. Yang, L. Wang, Z. Zhou, S. Zhao, G. Dong, Y. Chen, T. Min, M. Liu, Nature Communication, 9, 991 (2018)
3. S. Zhao, L. Wang, Z. Zhou, C. Li, G. Dong, L. Zhang, B. Peng, T. Min, Z. Hu, J. Ma, W. Ren, Z.-G. Ye, W. Chen, P. Yu, C-W Nan, M. Liu, Adv. Mater. DOI: 10.1002/adma.201801639 (2018)
4. M. Guan, L. Wang, S. Zhao, Z. Zhou*, G. Dong, W. Su, T. Min, J. Ma, Z. Hu, W. Ren, Z.-G. Ye, C.-W. Nan, M. Liu, Adv. Mater. DOI: 10.1002/adma.201802902 (2018)
5. Q. Yang, Z. Zhou, L. Wang, H. Zhang, Y. Cheng, Z. Hu, B. Peng, and M. Liu, Adv. Mater., 30, 1800449, (2018)

Recently, magnetoelastic coupling has been exploited to detect ferromagnetic resonance (FMR) using surface acoustic waves (SAWs)¹, a technique known as ADFMR. GHz-frequency SAWs are produced and detected electrically using pairs of interdigital transducers (IDTs). A magnetic material is placed in the path of the SAWs, which can then interact with magnetic moments via magnetoelastic coupling. Absorption of the SAWs occurs at FMR, modulating the measured output. Landau-Lifshitz-Gilbert theory describes the interaction in terms of the external magnetic field with a characteristic four-lobe pattern, from which the magnetic anisotropy field, FMR resonance field, and magnetoelastic coupling coefficient can be inferred.
We study the effects of a variety of magnetic materials including Ni, FeCo, FeGaB, and FeCoSiB, to examine the dependence of ADFMR patterns on material parameters and geometries. We use a range of SAW frequencies from ~300 to 2000 MHz to explore spectral effects of the IDT design on transduction efficiency, and the frequency dependence of the resonance field. We also calculate the FMR linewidth and quality factor- figures of merit useful for determining the efficacy of future ADFMR devices.
¹Weiler, M. et al. (2011). Elastically driven ferromagnetic resonance in nickel thin films. Phys. Rev. Lett., 106(11), 117601.

For the measurement of bio-magnetic signals there are high demands on the performance of the respectively utilized sensors. The requirements for these specific applications are a limit of detection (LOD) of significantly below 100 pT/Hz^-1/2, a frequency bandwidth of 1 Hz to at least 20 Hz, better 100 Hz or above and a reduced size for achieving high spatial resolution or for the case of limited installation space. Further the device should be operable at room temperature and without magnetic shielding.

Promising candidates for fulfilling all these requirements are surface acoustic wave (SAW) magnetic field sensors, especially those using shear horizontal SAWs¹. These so-called Love waves result from a piezoelectric substrate generating shear horizontal waves capped with a guiding layer with lower acoustic impedance confining the acoustic wave at the surface. As the sensitive element in these devices amorphous magnetostrictive thin films such as the alloy (Fe₉₀Co₁₀)₇₈Si₁₂B₁₀ seem to be particularly promising due to their high magnetostriction with simultaneously having a low magnetic anisotropy. Operating in a delay line configuration the magnetoelastically-induced change of shear modulus of the sensitive layer (ΔE/ΔG-effect) yields in a corresponding phase shift of the acoustic wave. While the application of amorphous magnetostrictive thin films in magnetoelectric cantilever composites was already studied extensively, the sensitivity and noise behavior in SAW sensors is not quite understood yet. However the quartz based Love wave devices with a working frequency of approximately 150 MHz show already very high sensitivities of up to 2000 °/mT and an equivalent LOD of 70 pT/Hz^-1/2 at 10 Hz and 30 pT/Hz^-1/2 at 100 Hz.



[1] A. Kittmann, P. Durdaut, S. Zabel, J. Reermann, J. Schmalz, B. Spetzler, D. Meyners, N. X. Sun, J. McCord, M. Gerken, G. Schmidt, M. Höft, R. Knöchel, F. Faupel, and E. Quandt, “Wide Band Low Noise Love Wave Magnetic Field Sensor System,” Scientific Reports, vol. 8, no. 1, pp. 278–287, 2018.

This work was funded by the German Research Foundation (DFG) through the Collaborative Research Centre CRC 1261 “Magnetoelectric Sensors – From Composite Materials to Biomagnetic Diagnostics”.

Prior studies have shown the effectiveness of piezoelectric/magnetostrictive MEMS magnetometers featuring a nano plate resonator and using the ΔE effect for highly sensitive detection of DC magnetic fields. By monitoring changes in resonance frequency from applied magnetic fields, a sensitivity of a few Hz/nT has been achieved in unshielded lab environments. With dimensions of ≤ 200 μm across, and < 1 μm thick, these devices offer the advantages of small scale, including portability and low power consumption, as well as high spatial resolution in sensor arrays. However, a thorough understanding of magnetic properties and other performance aspects in these magnetometers has not been developed. In this study we report on the strong effects of resonator plate curvature on sensor performance. It was found that the total resonance frequency shift dropped off exponentially with increasing curvature by as much as two orders of magnitude. By fabricating a ΔE effect magnetometer with low curvature in the nano plate resonator, we achieved maximum magnetic field sensitivity of 4.98 Hz/nT. This is as much as two orders of magnitude higher frequency sensitivity than in other recently reported magnetometers, that also utilize the delta E effect, but which are composed of a cantilever structure and generally operate at lower frequencies.

Optical double cantilever beam magnetometer was designed, fabricated and demonstrated its ability to measure magnetostriction (in-plane and out-of-plane), magnetization and magnetocrystalline anisotropy of ferromagnetic thin films as a function of the magnetic field. Notably, for the first time, here we demonstrate the well-established and simple cantilever beam technique for electric field modification of magnetization of ferromagnetic/ferroelectric heterostructures by considering the induced strains in ferromagnetic thin films through the converse piezoelectric effect in piezoelectric films. Moreover, this set-up also allows measuring the electromechanical properties such as transversal piezoelectric strain (d₃₁) and stress (e₃₁) coefficients of piezoelectric thin films. This magnetometer is simple in construction, inexpensive to manufacture, easy to operate along with noise subtraction provision and having sensitivity nearly 8 nm in the determination of cantilever beam deflection.

Magnetoelectric composites have been studied intensively in the recent past because of their preeminent magnetoelectric coupling, opening the path to highly sensitive magnetic field sensors operating at room temperature [1]. Moreover, this sensor approach yields the advantage to optimize the constituting phases independently. In the context of magnetic field sensing, the role of the magnetostrictive component is to respond to an external magnetic field by generating mechanical stress onto the piezoelectric phase. At least as important, irregular magnetization changes resulting from hysteretic effects have to be avoided in order to prevent significant noise contribution to the sensor output [2]. The multilayer approach increases the parameter space, which can be used to jointly attune the magnetoelastic response towards high sensitivity and improved control of magnetization reversal [3].
The presented work focuses on thin film magnetostrictive phase composites, composed of (Fe₉₀Co₁₀)₇₈Si₁₂B₁₀ multilayers. These layers are based on the stacking order unit Ta 5 / Cu 3 / MnIr 8 / FeCoSiB x [thickness in nm] with varying the FeCoSiB layer thickness x. The total thickness of the FeCoSiB layers is adjusted to 1 µm, 2 µm and 4 µm. The piezoelectric component is a 2 µm thick AlN layer. The constituents are grown on a Si substrate, one each side, by RF and pulsed DC sputter deposition respectively. Using UV lithography combined with dry and wet etching, magnetoelectric cantilever structures are fabricated with typical lateral dimensions of 2.3 mm x 25 mm. Applying a multistep thermal treatment, parallel and antiparallel alignment of magnetizations in adjacent ferromagnetic layers is achieved in order to formulate a stable domain configuration in the magnetic layers [4].
The presentation reports on recent advances achieved by utilizing magnetostrictors that are exchange biased layer by layer. Applying magnetic frequency conversion yields minimum detectable magnetic fields as low as 50 pT/ Hz^1/2 at 10 Hz [5], demonstrating the significantly improved performance of such macroscopic magnetoelectric field sensors.
Funding of the project work by the German Research Foundation through the CRC 1261 Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics is gratefully acknowledged.

[1] Leung, C. M. et al.: A review on applications of magnetoelectric composites: from heterostructural uncooled magnetic sensors, energy harvesters to highly efficient power converters, J. Phys. D 51 (2018), 263002.
[2] Urs, N. O. et al.: Origin of hysteretic magnetoelastic behavior in magnetoelectric 2-2 composites, Appl. Phys. Lett. 105 (2014), 202406
[3] Röbisch, V. et al.: Pushing the detection limit of thin film magnetoelectric heterostructures, J. Mater. Res. 32 (2017), 1009.
[4] M. Jovičević Klug, L. Thormählen et al.: Antiparallel exchange biased multilayers for low magnetic noise magnetic field sensors, Appl. Phys. Lett. 114 (2019), 192410.
[5] Salzer, S. et al.: Noise Limits in Thin-Film Magnetoelectric Sensors with Magnetic Frequency Conversion, IEEE Sensors Journal, 18, (2018) no. 2, pp. 596-604.

Magnetic field sensors based on magnetoelectric composites can offer powerful diagnosis of biomagnetic signals originating in the brain and torso. Thin-film magnetoelectric (ME) composites comprised of FeCoSiB as the magnetostrictive layer and AlN as the piezoelectric layer have demonstrated sensors with a high sensitivity and low limit of detection at biomagnetic relevant frequencies. A magnetoelectric sensor will have its highest sensitivity when operated at its mechanical resonant frequency. Some medical operations, such as deep brain stimulation, would require a ME sensor with a tunable resonant frequency to locate the stimulated area of the brain during operation. This is a major limitation as most state-of-the-art ME sensors are mechanically designed to operate at one single resonant frequency. In this work, we show introducing a functionalized shape memory alloy (SMA) layer to the ME composite can offer a tunable ME sensor through a reversible gradual change in the Young’s modulus (E) of SMA material.

Here, AlN / FeCoSiB thin-film magnetoelectric composites are fabricated and characterized on functional TiNiCu SMA substrates. Through an induced temperature pulse, it is possible to gradually change the Young’s modulus of the SMA, thereby changing the mechanical resonant frequency of the magnetoelectric sensor [1]. Our analytical calculations show that the resonant frequency can be altered by 6% for a magnetoelectric sensor (2 µm AlN / 2 µm FeCoSiB / 25 µm TiNiCu / 100 µm Si) through a temperature induced reversible solid-to-solid phase transformation of the SMA from martensite (E = 41 GPa) to austenite (E = 83 GPa). For a 2.5 mm x 20 mm cantilever the resonant frequency can be gradually increased from 318 Hz to 340 Hz purely through this transformation. The difference in frequency between martensite and austenite phases becomes enhanced by increasing TiNiCu film thickness, or by eliminating the Si substrate.

Currently, MEMS compatible fabrication routes are being explored to build a full magnetoelectric sensor onto a TiNiCu substrate for experimentally verification. Characterization results on the effect of heat treatment and TiNiCu surface roughness on the growth and material performance of the piezoelectric layer (AlN) through piezoforce microscopy (PFM), X-ray diffraction (XRD), double beam laser interferometry (DBLI) and transmission electron microscopy (TEM) will be presented.

Funding from the DFG is greatly acknowledged for this work

[1] Röbisch, Volker, et al. "Frequency-tunable nickel-titanium substrates for magnetoelectric sensors." AIP Advances 8.12 (2018): 1253

400MHz frequency band is the most commonly used band for Medical Implant Communications Service (MICS). However, most of recent antenna researches for this application are based on electromagnetic wave resonance, which lead to a comparable size to the wavelength [1, 2]. To solve this problem, a new antenna design based on the magnetoelectric coupling is introduced by our group [3], which is a thin-film bulk acoustic resonance (FBAR) antenna for 2.5GHz applications, based on the AlN resonator and magnetoelectric coupling. Since acoustic wave has smaller wavelength than electromagnetic wave, the antenna size can be easily scaled down by more than 100 times. However, for most AlN resonator designs, impedance matching is a problem because of the high electric impedance. Hence, a parallel antenna array structure is applied to reduce the impedance and increase the antenna gain [4].
In this work, nano-plate resonance (NPR) antenna arrays for 400MHz application are designed, fabricated and tested. This antenna is based on the magnetoelectric coupling between AlN thin film and FeGaB/SiO₂ multilayer. The antenna array structure is shown in Fig. 1 (a) with 8um width and array numbers of 1,4, 8 and 16. This strip structure resonates at in-plane mode, and this resonance is limited in width direction by the strip. The measured S-parameters are shown in Fig. 1 (b), with operating frequency of 371MHz, and 10dB-bandwidth of 3.875MHz. A radiation peak of S₂₁ is observed clearly at the resonant frequency, and a peak antenna gain of -54.82dBi is achieved. This radiation peak is from the vibration and domain rotation of magnetic film. The measured S₁₁ and calculated Z₁₁ are shown in Fig. 2. For fewer array numbers, the impedance is higher, and the related return loss is also higher, so the radiation peak is cover by the baseline. For this reason, array structure with more than 16 strips is necessary for impedance matching and high antenna gain.
Reference
[1] C. Liu, Y.-X. Guo, and S. Xiao, "Compact dual-band antenna for implantable devices," IEEE Antennas and Wireless Propagation Letters, vol. 11, pp. 1508-1511, 2012.
[2] N. Cho, T. Roh, J. Bae, and H.-J. Yoo, "A planar MICS band antenna combined with a body channel communication electrode for body sensor network," IEEE Transactions on Microwave Theory and Techniques, vol. 57, no. 10, pp. 2515-2522, 2009.
[3] T. Nan et al., "Acoustically actuated ultra-compact NEMS magnetoelectric antennas," Nature communications, vol. 8, no. 1, p. 296, 2017.
[4] R. Gardelli, M. Albani, and F. Capolino, "Array thinning by using antennas in a Fabry–Perot cavity for gain enhancement," IEEE Transactions on Antennas and Propagation, vol. 54, no. 7, pp. 1979-1990, 2006.

Multiferroic materials offer exciting pathways to manipulate both magnetic and electronic properties simultaneously, which, in the case of bismuth ferrite, stem from the intrinsic linking of ferroelectric and antiferromagnetic degrees of freedom. By leveraging this fundamental coupling, we can modify and indeed switch the ferromagnetism of materials such as Co_0.9Fe_0.1 that sit adjacent to ferroelectric bismuth ferrite thin films, using only electric fields [1]. Using an oxide ferromagnet instead, such as La_0.7Sr_0.3MnO₃, could potentially confer on these devices an improved fatigue life, as oxidation may be less critical [2].

This electric field control of magnetism through magnetic coupling opens up a vast array of applications, particularly in logic, data, and spintronics, where we may manipulate magnetic memories such as magnetic tunnel junctions with the application of an electric field, instead of through more energy-intensive spin-transfer torque currents [3].

Double perovskites, such as Sr₂FeMoO₆ (SFMO) and Sr₂CrReO₆ (SCRO), are promising candidates for ferromagnets that may provide such coupling with bismuth ferrite. As half-metallic oxides with Curie points higher than room temperature, both materials exhibit spin-polarised currents and potential perpendicular magnetic anisotropy, which are desirable for magnetic tunnel junctions and other magnetoresistance-based devices [4][5]. Additionally, as these films are oxides rather than pure metals, they could provide an interface that is more robust against oxidation, leading to longer fatigue life.

We show the enhancement of these double perovskites’ coercive fields atop bismuth ferrite when compared to their isolated phases, which could uncover clues as to the strength of any magnetic coupling at the interface. We also explore magnetoelectric coupling by studying the effects of electric fields on the magnetisation of these double perovskites on bismuth ferrite, through giant magnetoresistance and anisotropic magnetoresistance measurements, in addition to correlated magnetic force microscopy and piezoresponse force microscopy images. These materials could lead to the next generation of low-power data storage and logic device technology.


[1] Heron, J. T., Trassin, M., Ashraf, K., Gajek, M., He, Q., Yang, S. Y., … Ramesh, R. (2011). Electric-field-induced magnetization reversal in a ferromagnet-multiferroic heterostructure. Physical Review Letters, 107(21). https://doi.org/10.1103/PhysRevLett.107.217202

[2] Wu, S. M., Cybart, S. A., Yu, P., Rossell, M. D., Zhang, J. X., Ramesh, R., & Dynes, R. C. (2010). Reversible electric control of exchange bias in a multiferroic field-effect device. Nature Materials, 9(9), 756–761. https://doi.org/10.1038/nmat2803

[3] Amiri, P. K., Alzate, J. G., Cai, X. Q., Ebrahimi, F., Hu, Q., Wong, K., … Wang, K. L. (2015). Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling. IEEE Transactions on Magnetics, 51(11). https://doi.org/10.1109/TMAG.2015.2443124

[4] Meetei, O. N., Erten, O., Mukherjee, A., Randeria, M., Trivedi, N., & Woodward, P. (2013). Theory of half-metallic double perovskites. I. Double exchange mechanism. Physical Review B - Condensed Matter and Materials Physics, 87(16). https://doi.org/10.1103/PhysRevB.87.165104

[5] Vaitheeswaran, G., Kanchana, V., & Delin, A. (2006). Electronic structure of the ferromagnetic double-perovskites Sr 2CrReO6, Sr2CrWO6, and Ba 2FeReO6. Journal of Physics: Conference Series, 29(1), 50–53. https://doi.org/10.1088/1742-6596/29/1/008 Amiri, P. K., Alzate, J. G., Cai, X. Q., Ebrahimi, F., Hu, Q., Wong, K., … Wang, K. L. (2015). Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling. IEEE Transactions on Magnetics, 51(11). https://doi.org/10.1109/TMAG.2015.2443124

Magnetoelectric sensing and efficiency in multiferroics is a function of ferroelectric domain dynamics. Understanding domain configurations at various stages in the switching process can therefore reveal switching mechanisms, a necessary step in ultimately optimizing system performance and efficiency. However, these can be strongly influenced by often experimentally inaccessible sub-surface effects, including thickness dependencies, microstructural or compositional heterogeneities, interfaces, and depending on the device design even simple geometric influences. Therefore, we recently introduced Tomographic-AFM for direct volumetric materials property mapping. 3-D resolved domain configurations and thickness dependent switching are specifically reported for monolithic La doped BiFeO₃, and a BiFeO₃/CoFeO₃ vertically aligned nanocomposite. In each case, ferroelectric domain nucleation and growth are further investigated as a function of thickness. Pure BiFeO₃ clearly obeys Kay-Dunn thickness scaling and a multistep switching process, and such multistep 180° switching is also observed for the La doped and nanocomposite films despite the substantially different local microstructure and even composition. Reconfigurations of domain walls upon electric field annealing furthermore lend insight into domain and domain wall stabilities.

It is now well established that ferroelectric and multiferroic domain walls often have unique functional properties that are completely different from the domains that they surround: they can be semiconducting, metallic and even superconducting when the rest of the material is insulating; they can display magnetic order, which is absent elsewhere, and can possess aligned electrical dipoles when the matrix surrounding them is non-polar. In effect, ferroelectric and multiferroic domain walls represent a new class of nanoscale functional material. Crucially, they can be controllably shunted from point to point, created, or made to disappear; hence, a completely new paradigm of transient nanoscale devices may be possible, based on a “now-you-see-it, now-you-don’t” dynamic deployment of domain walls.

This talk will explore progress towards the realisation of these kinds of dreams and in particular show how injection, movement and removal of domain walls have enabled active control of nanoscale thermal conduction (in LaAlO₃ and LiNbO₃), the appearance of local negative capacitance (in Cu-Cl boracites), the existence of 1D mobile p-n junctions (in ErMnO₃ and Cu-Cl boracites) and the design of a new form of domain-wall memristor (in LiNbO₃).

Magnetoelectric (ME) antennas based on the suspended ME circular disk consisting of FeGaB/AlN thin-film have been demonstrated recently [1]. However, the FBAR devices are very fragile and delicate due to the releasing structure, although the complicated packaging process [2] is able to solve this problem. From the device point of view, the solid mounted resonator (SMR) with Bragg reflector has attracted much attention as a promising technology for the film bulk acoustic wave resonator (FBAR) devices [3]. Typically, the SMR devices are formed of a piezoelectric film, such as ZnO that is sandwiched between top and bottom metal electrodes, on top of a Bragg reflector. A resonance is created when a radio frequency (RF) signal is applied across the FBAR device. There are a variety of products such as filters [4], duplexers [4], and voltage controlled oscillators (VCOs) [5] based on SMR designs have been developed in past years.
In this work, we designed, fabricated and tested ZnO-based SMR ME antennas that operate at GHz with high gain performance. As shown in Fig. 1, the Bragg reflector consisting of three pairs of SiO₂/W layers was sputter deposited in one run with RF reactive magnetron sputtering for SiO₂ and DC sputtering for W. The piezoelectric ZnO layer was sputter deposited by RF reactive sputtering with Zn target and high-purity Oxygen gas. The XRD curves and rocking curves for ZnO films under different temperature were measured and exhibited in Fig. 2. In order to grow high quality ZnO films, it is vital to have sufficient ion bombardment during deposition. Therefore, high temperature of 450 degrees Celsius is preferred here with rocking curve FWHM of 2.25 degrees. After optimizing the performance of Bragg reflector and piezoelectric ZnO films, we fabricated and tested the SMR ME antennas. A gain enhancement of 12 dB was achieved in this novel design for robust ME antennas. Therefore, it is very promising in practical applications.

[1] T. Nan, H. Lin, Y. Gao, A. Matyushov, G. Yu, H. Chen, et al., "Acoustically actuated ultra-compact NEMS magnetoelectric antennas," Nature communications, vol. 8, p. 296, 2017.
[2] J. D. Larson III, S. Ellis, and Y. Oshmyansky, "Film bulk acoustic resonator (fbar) devices with simplified packaging," ed: Google Patents, 2008.
[3] R. Ruby, "11E-2 Review and comparison of bulk acoustic wave FBAR, SMR Technology," in 2007 IEEE Ultrasonics Symposium Proceedings, 2007, pp. 1029-1040.
[4] R. Ruby, P. Bradley, D. Clark, D. Feld, T. Jamneala, and K. Wang, "Acoustic FBAR for filters, duplexers and front end modules," in 2004 IEEE MTT-S International Microwave Symposium Digest (IEEE Cat. No. 04CH37535), 2004, pp. 931-934.
[5] R. Boudot, M. Li, V. Giordano, N. Rolland, P. Rolland, and P. Vincent, "A solid-mounted resonator-oscillator-based 4.596 GHz frequency synthesis," Review of Scientific Instruments, vol. 82, p. 034706, 2011.

One of the key challenges to state-of-the-art antennas lies in their size miniaturization. Conventional antennas rely on an EM wave resonance and require a size of more than one tenth of the EM wavelength λ₀. In this work, a new miniature magnetoelectric (ME) antenna is presented utilizing the multilayer Bragg reflector structure of a solidly mounted resonator (SMR) to enhance small scale antenna gain up to -16 dBi. The antenna radiates using a magnetoelectric heterostructure consisting of AlN and FeGaB with resonator diameters of 500 um and 1000 um. Different ME resonator shapes and sizes are designed, fabricated and compared along with varying anchor widths and lengths to provide better impedance matching. The acoustic resonance in the heterostructure films operates with an out-of-plane mode of vibration so that the thickness of each layer was optimized to produce a resonant frequency at 1.5 and 2.45 GHz to allow for seamless on-chip integration with WiFi, Bluetooth and GPS devices. The SMR structure replaces the freestanding membrane structure [1] with a Bragg acoustic reflector for concentrated energy confinement. The Bragg reflector was fabricated with a quarter wavelength mode configuration to create maximum particle displacement in the AlN and FeGaB layers. Three periods of alternating low and high acoustic impedance films, using silicon dioxide and tungsten respectively, were deposited on top of a silicon substrate to provide greater acoustic resonance within the ME heterostructure. Antenna gain measurements were performed in an anechoic chamber using a GSG probe and a reference horn antenna linked to a vector network analyzer. The robust features of the SMR has more advantages compared to its FBAR freestanding membrane counterpart allowing for low cost monolithic integration, high operating frequency, better thermal dissipation and structural stability that will be vital for small-scale antennas and other wireless applications.

[1] T. Nan, H. Lin, Y. Gao, A. Matyushov, G. Yu, H. Chen, et al., "Acoustically actuated ultra-compact NEMS magnetoelectric antennas," Nature communications, vol. 8, p. 296, 2017.

Composite multiferroics, composed of a magnetostrictive ferromagnet and a piezoelectric ferroelectric, have widely been targeted for beyond-CMOS logic due to their large coupling coefficients and high operating temperature^1–3. Magnetoelectric multiferroic systems potentially offer the lowest energy dissipation per bit operation in a scalable platform, yet significant materials challenges still exist in the field. For composite multiferroics, this requires finding pathways to enhance piezomagnetic effects and coupling between layers, an effort that has seen relatively little work⁴. Here, we present a means to boost the magnetostriction of Fe_1-xGa_x alloys and magnetoelectric coupling in a Fe_1-xGa_x - (PMN-PT) composite multiferroic heterostructure through epitaxy.
In bulk, the magnetostriction coefficient of Fe_1–xGa_x alloys versus Ga composition peaks near ~18% Ga occurring due to a phase change from the disordered A2 phase to an ordered BCC phase (D0₃), which reduces the magnetostriction coefficient⁵. A distinct advantage of thin film deposition is the potential to access metastable phases through epitaxy, allowing us to promote the chemically disordered BCC (A2) phase in our film at high (22%) Ga concentrations. We demonstrate that thin film epitaxy stabilizes a chemically disordered BCC Fe_0.78Ga_0.22 alloy where the magnetostriction is enhanced by 200-300% relative to the bulk.
Transport-based magnetoelectric characterization shows 90 electrical switch of magnetic anisotropy and one of the largest converse magnetoelectric coefficients ever achieved at room temperature in a composite multiferroic[jH1] and energy dissipation per operation scaling to 5.9 J cm^-2, making our devices competitive with other state-of-the-art beyond CMOS technologies⁶. This hyperactive performance is achieved through epitaxial stabilization of a disordered, metastable phase of earth-abundant and rare-earth-free magnetostrictor, Fe_0.78Ga_0.22. By epitaxially engineering our ferromagnetic layer to prevent the formation of deleterious intermetallic nanoregions, we provide a pathway to engineering new performance levels in rare-earth free magnetoelastic and magnetoelectric heterostuructures.

1. Meisenheimer, P. B., Novakov, S., Vu, N. M. & Heron, J. T. Perspective: Magnetoelectric switching in thin film multiferroic heterostructures. J. Appl. Phys. 123, 240901 (2018).
2. Ramesh, R. & Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nat. Mater. 6, 21–29 (2007).
3. Bibes, M. & Barthélémy, A. Multiferroics: Towards a magnetoelectric memory. Nat. Mater. 7, 425–426 (2008).
4. Shevlin, S. Multiferroics and the path to the market. Nat. Mater. 18, 191 (2019).
5. Du, Y. et al. Relation between Ga ordering and magnetostriction of Fe-Ga alloys studied by x-ray diffuse scattering. Phys. Rev. B 81, 054432 (2010).
6. Manipatruni, S. et al. Scalable energy-efficient magnetoelectric spin–orbit logic. Nature 565, 35–42 (2019).

Growth of single-phase magnetoelectric materials and their analysis of coupling mechanisms between ferroic order parameters (spin and polarization) is important from the point of view of next generation logic and memory devices. Herein, we report fabrication, dielectric, ferroelectric and magnetic measurement of a Pd-doped room-temperature magnetoelectric multiferroic Pb(Zr_0.20Ti_0.80)_0.70Pd_0.30O_3-δ (PZTPd) thin film. Highly oriented PZTPd thin films were deposited on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 substrates by following laser ablation processes in oxygen atmosphere using pulsed laser deposition technique. X-rays diffraction studies revealed that the film had tetragonal phase with c-axis orientation. The surface morphology studies using atomic force and scanning electron microscopic techniques suggest a smooth and homogeneous distribution of grains on the surface of the film with roughness ~1.8 nm. Temperature dependent dielectric measurements were carried out in Pt/ PZTPd /LSMO metal-ferroelectric-metal capacitors that showed diffused behavior over a large range of temperatures at several frequencies, and exhibited a higher dielectric constant ~3000 at 100 Hz at room temperature. The polarization hysteresis loops were obtained at room temperature, attributed to its ferroelectric behavior. Piezo force microscopy suggests a random polarization orientation in a grown film, whereas polarizations could be switched easily with a positive and negative electric bias. A well-saturated magnetization M-H loop with remanent magnetization of 1.6 emu/cm³ was observed at room temperature. The reason for magnetization in PZTPd thin film is found to be due to mixed oxidation states of Pd²⁺/Pd⁴⁺ in the Pb(Zr_0.20Ti_0.80)O₃ matrix as complemented by x-rays photoelectron spectroscopic results. These findings suggest that our thin films are multiferroic (ferroelectric-ferromagnetic) at room temperature. The details will be presented in the meeting.

Resistive and magnetic switching phenomena have recently attracted large interest for ultra-scaled non-volatile memory and logic devices. The incorporation of hydrogen can significantly influence magnetic and electronic properties of functional materials allowing effective tuning magnetic anisotropy, magnetic moment and electroresistance. Despite extensive experimental investigations, very little is known about the phenomena on a molecular level and systematic investigations are needed to provide fundamental insights on the physical origin of phenomena. In this work, we consider a series of heterostructures (Co/Pd, MgO/Fe, etc) exhibiting perpendicular magnetic anisotropy (PMA) and employ density functional theory calculations (DFT) to determine the critical concentration of hydrogen needed for magnetic anisotropy switching. We show that anisotropy switching effect is mainly attributed to the electronic structure changes at the interface, while lattice expansion caused by hydrogen insertion plays a minor role. These results clarify the underlying mechanisms of magnetic anisotropy switching and help optimizing material combinations for hydrogen-based magneto-ionic devices.

Recent conceptual advances in our understanding of magnetoelectrics have led to renewed interest in materials such as Cr₂O₃. Muon spin rotation (µSR) is a promising technique for investigating recently predicted local magnetoelectric phenomena, acting as a probe of the local magnetic field at the muon stopping site(s). The ab initio prediction of muon stopping sites in materials is crucial to correctly interpreting µSR experiments. However, the quantum nature of the muon, together with its short lifetime (~2.2 µs), allow the muon to exhibit a rich range of dynamical phenomena that are challenging to capture using standard first-principles methods. Furthermore, the impact of the positively charged muon on the surrounding lattice can lead to significant changes to the muon behaviour and experienced local field.

Here we report on a µSR study of Cr₂O₃, focussing on the theoretical prediction of muon stopping sites and dynamics. We find that quantum corrections beyond the harmonic approximation are required to obtain accurate energy barriers between muon stopping sites. We also find strong evidence for a charge-neutral muon-polaron complex in Cr₂O₃, stabilised by a Jahn-Teller distortion. The latter finding has important implications for the charge state and behaviour of hydrogen defects in Cr₂O₃ and related materials.