Symposium S.EL12—Ferroic Materials and Heterostructures for Electronics and Data Storage

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 large reduction of in-plane piezoelectric strain by substrate clamping, and in two-terminal devices, the requirement of anisotropic in-plane strain. Here we show that these limitations can be overcome by designing the piezoelectric strain tensor using the boundary interaction between biased and unbiased piezoelectric. We fabricated 500 nm thick, (001) oriented [Pb(Mg1/3Nb2/3)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 electric field-driven magnetization rotation. We develop a method that can apply designed strain patterns to many other materials systems to control properties such as superconductivity, band topology, conductivity, and optical response. We also discuss how these effects depend on the crystallographic orientation.

Ferromagnetic skyrmion is a topologically protected whirling spin texture in nanoscale. Due to its small size, topologically-protected stability, and solitonic nature together, it holds great promise for future spintronics applications. To translate these compelling features into practical spintronic devices, we should face a key challenge in achieving effective control of skyrmion properties, such as size, density, and thermodynamic stability. Recently, we reported the discovery of ferroelectrically tunable skyrmions in ultrathin BaTiO3/SrRuO3 bilayer heterostructures. The ferroelectric proximity effect occurring at the BaTiO3/SrRuO3 heterointerface can trigger a sizable Dzyaloshinskii-Moriya interaction. Thus the interaction between two ferroic materials can stabilize robust skyrmions with diameters of several tens of nanometers.[1] Moreover, by manipulating the ferroelectric polarization direction of the BaTiO3 layer, we could achieve local, switchable and nonvolatile control of both skyrmion density and thermodynamic stability. Such a ferroelectrically tunable skyrmion system can work as a new paradigm to simultaneously enhance the integratability and addressability of skyrmion-based functional devices.

[1] L. Wang et al., Nature Materials 17, 1087 (2018).

Composite multiferroics, composed of a magnetostrictive ferromagnet and a piezoelectric ferroelectric, hold promise for magnetic field sensors and energy efficient beyond-CMOS logic by harnessing magnetoelectric transduction. Enhancing device performance requires highly magnetostrictive materials, however, relatively little attention has been given to engineering magnetostriction in thin films. One promising magnetostrictor system are the Fe_1-xGa_x alloys yet in bulk, the magnetostriction coefficient of Fe_1–xGa_x alloys versus gallium composition peaks near ~18% Ga due to a phase change from the disordered A2 phase to an ordered BCC phase (D0₃). Here we discuss a means to boost (by 200-300% relative to bulk) the magnetostriction of Fe_1-xGa_x thin film alloys by extending the phase stability of the A2 phase to higher Ga compositions. Transport-based magnetoelectric characterization of a Fe_1-xGa_x - [Pb(Mg_1/3Nb_2/3)O₃]_0.7-[PbTiO₃]_0.3 (PMN-PT) composite multiferroic heterostructure shows are reversible 90 deg electrical switch of magnetic anisotropy and a room temperature converse magnetoelectric coefficient of 5.5×10^-6 s m^-1.

Deterministic magnetic switching driven by electric fields rather than power-dissipating currents on the nanoscale is fundamentally challenging yet promising for applications to energy-efficient and high-density spintronic devices. Here, we demonstrate an electric-field-controlled deterministic, robust and reversible 120^o rotation of 'Y'-like magnetic state in a patterned nanoscale multiferroic heterostructure consisting of a concave triangular nanomagnet on the Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ film. The nanomagnet is juxtaposed with an overlying antiferromagnetic layer to acquire an exchange bias from their interface. Using phase-field simulations, we find that the rotation of 'Y'-like magnetic state is controlled by a synergistic effect of strain-mediated electric-field-induced uniaxial magnetoelastic anisotropy, magnetic in-plane shape anisotropy of the concave-triangle-shaped nanomagnet, and an interfacial exchange-bias field. It is also shown that deterministic magnetic state switching can be accomplished by a pulsed strain, the duration of which can span from a few seconds or longer down to a few nanoseconds, providing great design flexibility. We also discussed the dynamics of electric-field-driven switching of 'Y'-like magnetic state as well as the influence of lateral size, thickness and shape variation (i.e. concave radius) of the nanomagnet on the critical strain for the switching. These results offer a new and technologically viable route to designing nanomagnet-based nonvolatile spin memories with high density and low power.

Incorporating ferromagnetic, ferroelectric and ferroelastic orders inherently in a single-phase multiferroic material or manipulating one ferroic order with the others via interfacial coupling effects in multiferroic heterostructures has driven significant research activities aiming at the realization of their technological potential in the field of sensors, RF electronics, logic devices and memory etc. Magnetoelectric (ME) effect is essentially referred to as a change of electrical polarization (P) induced by external magnetic field (H) or a control of magnetization (M) via applied electric field (E). This report first introduces our recent progress in laminated ME composites consisting of a [011]-oriented Pb(Mg,Nb)O₃-Pb(Ti,Zr)O₃ (PMN-PZT) single-crystal fiber laminated with laser-treated amorphous FeBSi alloy (Metglas). High ME coupling coefficient around 10 kV/cm*Oe has been achieved. We will then review recent advances in multi-layered ME composites as well as their device applications including magnetic field/current sensors, dual-stimulus energy harvesters, ME memories, ME gyrators and tunable electronics. Some perspectives in terms of the future of magnetoelectric composites and devices will also be provided.

There is great interest in composites of polymers and multiferroic nanoparticles for advanced electronic applications. There has been very little work done in the rapid prototyping of such composites. We report the fabrication of composites of silicone elastomer, and barium titanate/iron oxide nanoparticles. Multiferroic nanoparticles of Barium titanate/iron oxide were synthesized using sonochemical reaction of iron acetate, urea, barium titanate nanoparticles in a solvent. This results in iron oxide nanoparticles formed on and near the barium titanate nanoparticles. A subsequent sintering process improves the crystalline nature of the iron oxide. The barium titanate/iron oxide nanoparticles are combined with a moldable silicone elastomer (Dowsil MS-4007) at 5% weight ratio and loaded onto an extrusion head of a commercial 3D printer. Printing on a base maintained at 100°C results in the formation of solid nanocomposite layers. Observations will be presented from characterization of these layers by electrical and magnetic studies, microscopy, mechanical and thermal studies.

Symmetry breaking ascribed to the evolution of an order parameter is one of the most important concepts in ferroic materials such as ferromagnets and ferroelectrics, and bears “domain” structures. The domain switching by an external field is the key principle to exploit them for practical applications. In multiferroics, materials possessing multiple ferroic order parameters, the respective order parameters form domains and multiple types of domain states coexist. It is well-accepted that couplings between the multiple order parameters and domain states are essential for macroscopic magnetoelectric phenomena in multiferroics. However, there are few reports that directly compare responses of coexisting domain states to external fields and macroscopic magnetoelectric properties. In this presentation, we show selective observations of multiple magnetic order parameters and domain structures in some single-phase magnetoelectrics. This work has been done in collaboration with H. Ueda, R. Misawa, T. Katsuyoshi, J. K. H. Fischer, K. Kimura, Y. Tanaka, Y. Wakabayashi, T. Honda, J. S. White, M. Kenzelmann, A. B. Harris, N. Leo, and M. Fiebig.

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.

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[6] A. Mahmood, M. Street, W. Echtenkamp, C. P. Kwan, J. P. Bird, and C. Binek, Phys. Rev. Mater. 2, 044401 (2018).
[7] N. Shimomura, S. P. Pati, T. Nozaki, T. Shibata, and M. Sahashi, AIP Adv. 7, 025212 (2017).

Active manipulation of spin waves is essential for the development of magnon-based technologies. One route towards this goal exploits reprogrammable magnonic crystals. In such structures, a varying spatial modulation of magnetic properties using, for instance, electric currents [1] or optical pulses [2] controls the transmission of spin waves. Besides these dynamic approaches, field-induced toggling between ferromagnetic and antiferromagnetic states in magnetic stripe arrays has been shown to modify spin wave transport in a non-volatile way [3].
Multiferroic heterostructures comprising coupled ferromagnetic and ferroelectric layers offer an interesting alternative as programmable hybrids for magnonics. Attractive properties of multiferroic heterostructures include the imprinting of magnetic domain patterns, the creation of abrupt anisotropy boundaries, strong pinning of magnetic domain walls, and electric field control of magnetic anisotropy, magnetic switching, and magnetic domain wall motion [4-6].
Here, we demonstrate that multiferroic heterostructures provide a versatile platform for the emission of short-wavelength spin waves [7], the confinement of spin wave modes in structurally uniform films [8], and active manipulation of spin wave transmission [9]. 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 [9]. In addition to magnetic field control, we also present recent results on active manipulation of spin wave transmission using electric fields. This sought-after effect originates from local strain-induced changes of the magnetic anisotropy. We demonstrate that the transmission of spin waves can be fully turned on and off in certain frequency bands by the application of voltage pulses across the BaTiO₃ substrate. The multiferroic heterostructures allow also for continuous tuning of the spin wave phase and amplitude, offering prospects for voltage-controlled magnonic logic.

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[5] T. H. E. Lahtinen et al., Sci. Rep. 2, 258 (2012).
[6] K. J. A. Franke et al., Phys. Rev. X 5, 011010 (2015).
[7] B. Van de Wiele et al., Sci. Rep. 6, 21330 (2016).
[8] S. J. Hämäläinen et al., Phys. Rev. Appl. 8, 014020 (2017).
[9] S. J. Hämäläinen et al., Nat. Commun. 9, 4853 (2018).

Topological defects can be formed at a phase transition where there is spontaneous symmetry breaking. These features play a central role in understanding phase transitions, ranging from superfluid ⁴He and high-temperature superconductors to cosmological systems. Certain hexagonal multiferroic materials also display topological defects including (charged) domain walls and ferroelectric vortices. In addition to serving as a model for their universality class, these sites can further host emergent phenomena including anomalous conduction and fractional electronic charges or quantized magnetic moments. Here, we construct atomically-abrupt superlattices to study the ferroelectric order as a function of thickness. We observe controlled formation of charged ferroelectric domain walls. Electrostatic confinement of the walls and the associated vortices pushes the material from a Z₆ symmetry to a stripe phase with continuous U(1) symmetry to a paraelectric phase with decreasing thickness. These materials also display unique topological features not observed in the bulk components. Together we demonstrate how dimensional confinement in artificial superlattices is a new probe for stabilizing new topologies in hexagonal ferroelectric systems.

Ferroelectric (FE) hafnium/zirconium oxide (HZO) thin films have shown many advantages in next-generation device applications such as negative capacitance gate oxides and FE random access memory (FERAM) or multi-state memory. In addition, spontaneous formation of atomically sharp phase and domain boundaries is prevalently observed in the HZO thin films due to structural similarity of the various polymorphs based on fluorite parent structure. Here, density functional theory (DFT) is employed to show that the FE o-phase could be stabilized by significant in-plane tension formed during the thermal annealing process, in which the as-deposited amorphous phase is crystallized into the more dense crystalline phases. The in-plane tension being the stabilization mechanism is in agreement with experiments but in contrast to some of the previous calculations which argued that in-plane compression is responsible for the stabilization of the o-phase. Furthermore, DFT is employed to model stacks of FE and paraelectric (PE) phases to provide an atomic model of negative capacitance. It is shown that the unfavorable interaction of the PE phase with the field from vertically polarized FE phase induce polarization rotation, phase change and domain wall formation consistent with negative capacitance. Furthermore, the rotation of polarization initiates at the PE/FE interface and propagates into the FE layer, while mixed FE phases with the vertical/rotated polarizations show shallow energy minima, and are further stabilized by applying an external electric field. Validation of negative capacitance in atomic models will further enable atomic models of stacks with high k value and their switching behavior in response of external electric fields, mechanical strain, and defects.

This work was supported by DARPA SRC JUMP ASCENT theme 1 task 59.

Consumer electronics and electronics is driven by low cost, reliable performance and low energy consumption. To truly enable innovations, new solutions have to be found in new materials. Oxides show multiple functional properties and therefor offer multiple solutions in one material. Moreover, the reconfiguring functionality of materials by using epitaxy or self-assembly etc. (e.g., non-magnetic to ferromagnetic, dielectric to piezoelectric properties or insulating to metallic) has been demonstrated. The success of reconfiguring many of these oxides critically depends on the atomically controlled thin film deposition techniques such as pulsed laser deposition but till now typically laboratory scale single crystalline wafers have been used. Important progress has been made in terms of plasma scanning techniques on large wafers and compatibility with technical materials (Si). However, fundamental knowledge of a scanning plasma and the composition of the thin films, which is essential in obtaining reliable reconfigured functional properties, is lacking.
In this presentation I will discuss examples of manipulation of magnetic properties in thin films and the possibility of reconfigurable functionality, also on technical wafers such as Si. We showed how to manipulate magnetic and electronic anisotropic properties in manganite heterostructures by engineering the oxygen network on the unit-cell level. In addition, the critical temperature of the metal-to-insulator transition as well as the magnetic transition temperature in RENiO₃ can be tuned by choosing spacer layers with different amounts of octahedral tilts in a superlattice configuration.

We suggest that 5d electron systems are promising class of spintronic materials because of its strong spin-orbit coupling (SOC). A strong SOC inherent to 5d Ir oxides recently emerged as a new paradigm for oxide electronics. For example, we investigated novel physics of spin-orbital Mott insulators [1] and possible topological insulators [2] by tuning the electronic phases through superlattice technique. We also demonstrated a large spin Hall effect of IrO₂, one of the simplest 5d oxides, indicating that Ir oxides are promising class of spintronic materials [3].
In this talk, we focus on yet another topic on spintronics – magnetic skyrmion as a topological spin texture. We have studied transport properties of bilayers consisting of m unit cells of ferromagnetic SrRuO₃ and 2 unit cells of SrIrO₃. We observed an anomaly in the Hall resistivity in addition to anomalous Hall effect (AHE); this is attributed to topological Hall effect (THE) [4]. The results suggest that magnetic skyrmions of 10–20 nm are generated by Dzyaloshinskii-Moriya interaction, which might be caused by both broken inversion symmetry at the interface and strong SOC of SrIrO₃. Even more surprising is that we can control both AHE and THE by electric field in the SrRuO₃-SrIrO₃ bilayers [5]. We observed the clear electric-field dependence only when SrIrO₃ is inserted between SrRuO₃ and a gate dielectric. The results established that strong SOC of nonmagnetic materials such as SrIrO₃ is essential in electrical tuning of these Hall effects.
We are also searching for spin-current transport properties at the interfaces consisting of magnetic oxides and nonmagnetic Ir oxides. We will report on the latest results on them. Through these studies on oxide interfaces containing Ir oxides, we may have a chance to approach a triple point for topology, correlation, and spin-orbit coupling.

References:
[1] J. Matsuno et al., Phys. Rev. Lett. 114, 247209 (2015).
[2] D. Hirai, J. Matsuno, and H. Takagi, APL Mater. 3, 041508 (2015).
[3] K. Fujiwara et al., Nat. Commun. 4, 2893 (2013).
[4] J. Matsuno et al., Sci. Adv. 2, e1600304 (2016).
[5] Y. Ohuchi et al., Nat. Commun. 9, 213 (2018).

A tactile memory, which can sense and memorize the external stimuli, has been reported up to date with great potential in emerging wearable electronics. To realize bi-functional systems for sensing and memorizing, two device platforms integrated with tactile sensor and memory have drawbacks from physically bulky systems as well as increase of circuit resistance from wire connection, giving rise to low sensing sensitivity. In particular, a binary memory with bi-level state of ON/OFF has limitation in storing multi-level sensing information that requires multiple memory array. A multi-level memory-based single device platform without the integration is, therefore, on demand.
Herein, we demonstrate a non-volatile multi-level tactile memory based on a single ferroelectric-gate field-effect transistor (Fe-FET) allowing for intelligent electronic skins (e-skins), which can sense, memorize, and then retrieve the tactile information. Pressure-sensitive top-gate shape formation can precisely sense and memorize the external pressure stimuli by making an underlying ferroelectric film polarized according to the size of the contact area between top-gate and ferroelectric film, giving rise to variation of gate voltage supply. Our tactile 1-transistor memory exhibits high sensitivity of >90 kPa^-1 at low pressure stimuli of <1.5 kPa, and shows good reliability in multiple programming and erasing up to 50 cycles and long-term stability even with bending radius of 6 mm. Tactile memory arrays showing precise spatial positioning of external stimuli with non-volatile multi-level sensing, memorizing, and then retrieving after one week have broad applications suitable for wearable human-interactive electronics.

Nanosized materials, such as nanoparticles, nanowires, nanotubes, and nanocomposites are currently the focus of intense investigations due both to the physics they involve and to their potential for revolutionary technological applications. The applications are expected to be in a wide variety of diverse areas, such as logic circuits, magneto-electronic
devices, magnetic data storage, medicine, and biotechnology. The phenomena and properties behind these applications are unique to nanodimensions, such as quantum confinement effects and enhanced surface to volume ratio. So, we have investigated the effect of particle size/sintering temperature on structural, magnetic and transport properties in Nd_0.5Ca_0.5MnO₃ system. Different size materials have been synthesized using Sol-gel method and sintered at 900 , 1100 and 1300 degree Centigrade to get the samples of different particle sizes. Phase pureness of the samples has been confirmed by Rietveld refinement of the powder X- Ray diffraction (XRD) data, which authenticates orthorhombic structure with Pnma space group. All three lattice constants (a, b and c), bond lengths and bond angles decrease with increasing particle size. The magnetization measurements (H@500 Oe) as a function of temperature exhibit paramagnetic (PM) to ferromagnetic (FM) transition and also brings out that (i) the divergence between the zero field cooled (ZFC) and field cooled cooling (FCC) magnetization decreases with increasing particle size, (ii) all samples show charge ordering at 250 K, (iii) the sharp drop of ZFC magnetization for all samples. At low temperatures a glassy behaviour is visible at higher particle size. The isothermal magnetization loops confirm the increase in the FM component at higher particle size at low temperature (20 K). The temperature dependent resistivity of all samples have been measured without magnetic field and resistivity measurements show no insulator to metal transition at lower sintering temperature but at higher sintering temperature the metal to insulator transition has been found. The data of resistivity have been fitted by using variable range hopping model (VRH). Characteristic temperature (T_o), localization length (1/α) and average hopping distance (R) have been calculated by using this model. X-ray photoemission spectra of Mn2p and Mn2s core-level confirm dual valence state of Mn (Mn³⁺ and Mn⁴⁺), which is responsible for the magnetic behaviour of NCMO system. The correlation between the transport and observed magnetic properties of sintered NCMO samples will be described and discussed in paper.

Domain size is known to play a critical role in electromechanical properties of domain-engineered ferroelectric single crystals. For lead-based relaxor-PbTiO₃ crystals, nanosized polar regions are believed to be the origin of giant piezoelectricity. For lead-free systems such as [111]-poled tetragonal ferroelectric BaTiO₃ crystals, enhanced piezoelectricity has been reported by reducing the domain size from hundreds to several microns. These examples lead to a long-standing belief that the smaller the domain size, the higher the piezoelectricity. Our recent study reveals an alternative route to improve piezoelectricity by increasing the size of domains through AC poling in relaxor-PbTiO₃. In this poster, we will discuss the general domain size effect on piezoelectricity in prototypical non-lead perovskites in tetragonal, orthorhombic, and rhombohedral ferroelectric phases, such as BaTiO₃, K_0.5Na_0.5NbO₃, and BiFeO₃, by examining the effective piezoelectric coefficients of engineered domain structures using a combination of thermodynamic analysis and phase-field simulations.
We find that the domain size effect on piezoelectricity depends on the symmetry of the ferroelectric phase. The size effect becomes significant when the domain is reduced to a critical size (~ sub-100 nm) which depends on the proximity to a ferroelectric-ferroelectric phase transition and on the width of domain walls. Moreover, the domain size effect has a strong anisotropy, giving rise to opposite trends for different piezoelectric tensor components as well as varying along different probing directions. By separating the contributions from domains and domain walls, we reveal that the size effect is majorly originated from the modification of thermodynamic states of domains due to the presence of domain walls. We also reveal that the domain size effect on piezoelectricity can be understood as a competition between the domain size effects on the dielectric permittivity and on the polarization rotation. The former is related to the flattening of free energy landscapes, while the latter is due to the built-in fields between domain walls. Besides, we predict a convenient way of controlling the domain size by poling via an alternating electric field. This work offers a thermodynamic-based understanding of the domain size effect and brings new insights into the fundamental knowledge of the processing-microstructure-property relationship in piezoelectric single crystals.

Oxide materials exhibit a broad range of tunable phenomena, including magnetism, multiferroicity, and superconductivity. Oxide interfaces are particularly intriguing. Their low local symmetry combined with the sensitivity to electrostatics and strain leads to unusual physical properties beyond the bulk properties. Recently, ferroelectric domain walls have attracted attention as a novel type of oxide interface; the walls are spatially mobile and allow controlling electronic signals at the atomic scale, holding great potential as multifunctional 2D systems for future nanoelectronics.

In my talk, I will present unique features that occur at improper ferroelectric domain walls in hexagonal manganites and discuss how these walls can be used to emulate the behavior of key electronic components. For our studies, we choose the p-type semiconductor ErMnO₃ as it naturally develops all fundamental types of ferroelectric domain wall at room temperature, namely neutral (side-by-side) as well as negatively (tail-to-tail) and positively charged (head-to-head) wall configurations. The walls are explicitly robust and, hence, represent an ideal template onto which the desired electronic behavior can be imposed. I will show how the electronic properties can be optimized and controlled, and discuss the possibility to use such walls for designing 2D digital switches and half-wave rectifiers, bringing us one step closer to domain-wall based devices and networks for next-generation nanotechnology.

Piezoresponse force microscopy (PFM) has become the standard tool for imaging and spectroscopy of ferroelectric and multiferroic materials, providing wealth of information on domain structures and switching behaviors. However, until now PFM, similar to other scanning probe microscopy techniques, was based on the classical rectangular scans and predefined voltage patterns. In this presentation, I introduce our recent work on combining PFM with automated experimentation, exploring image-based feedback, non-rectangular and adaptable scans, and autonomous experiments. As one such example, we demonstrate a fundamentally new approach for exploring polarization dynamics of specific domain elements and topological structures in ferroelectric materials manipulation based on in-situ image-based feedback in PFM. This approach was demonstrated for probing highly pinned domain wall dynamics in polydomain PZT, one of the traditionally most difficult objects for PFM studies. We demonstrate the creation of frustrated phases with significantly enhanced electromechanical response prior to the onset of the wall motion and discuss future strategies for fabrication of predefined topological defects and exploration of their dynamic properties. We further illustrate the incorporation of non-rectangular scans and discuss the strategies for automated experimentation in PFM based on the Gaussian Process modelling. Finally, we explore the opportunities for exploring the fundamental physics of ferroelectric materials enabled by the combination of the automated PFM experiment and combinatorial synthesis.

This research is supported by the by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, BES DOE.

Ferroelectricity has often been associated with and observed in materials that are insulating or semiconducting rather than metallic because conduction electrons in metals screen out the static internal fields arising from a long-range dipolar order. In the 1960s, Anderson and Blount proposed materials with these seemingly incompatible characteristics, i.e., metals with a polar axis termed ferroelectric metals. Since then, ferroelectric metals were merely a theoretical construct until recent experimental observations suggesting otherwise. Despite the progress, electrically switchable intrinsic electric polarization, together with the direct observation of ferroelectric domains, has not yet been realized in a bulk crystalline metal, although incomplete screening by mobile conduction charges should, in principle, be possible.
Here, we provide evidence that native metallicity and ferroelectricity coexist in bulk crystalline van der Waals WTe₂. We show that, despite being a Weyl semimetal, WTe₂ has switchable spontaneous polarization and a natural ferroelectric domain structure at room temperature. This new class of materials has tantalizing potential for functional nanoelectronics applications.

In the quest towards commercialization of ferroelectric-based non-volatile nanoelectronic devices, the realization of robust polarization retention is of great significance. However, ferroelectric materials always suffer from polarization relaxation during the time, typically happening within from days to a few weeks, which leads to a final retention loss for the data storage. In this work, by exploiting the domain wall pinning through designer-defect engineering in epitaxial BiFeO₃ films, we have demonstrated an ultra-long polarization with virtually no degradation in switched nanoscale domains for periods longer than two years. This already represents a more than 4000 % improvement over the best values hitherto reported and is believed to have an ongoing retention. Transmission electron microscopy (TEM) with atomic resolution reveals that nanoscale defect pockets pervade the entire film thickness homogeneously. These defects act as highly efficient domain wall pinning centres, resulting in the superb retention. This is also corroborated by scanning probe microscopy-based dynamic switching measurements as the results show a remarkably increased activation field for domain wall movement, originating from the defect-engineered domain wall pinning. Our findings demonstrate that defects can be exploited in a positive manner to solve reliability issues in ferroelectric films used in functional devices.

Cold sintering is a revolutionary new sintering process which densifies ceramics at < 200 ^oC using a combination of pressure and a transient aqueous phase. The most significant progress towards component/device fabrication has been in the area of MW technology and several new materials have recently emerged, including a range of temperature stable, low loss ceramics with permittivies from 8 to 40, as well as new magnetodielectrics. This contribution presents an overview of new developments in cold sintering for MW technology and suggests future directions that might result in ceramics directly deposited and integrated onto polymer circuit boards.

Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ is a relaxor ferroelectric material that exhibits interesting dielectric and piezoelectric properties. Specifically, complex domain architectures are formed in compositions around the morphotropic phase boundary. The polar structure of thin films is additionally affected by the epitaxial strain. In order to understand the structure of these films, multi-scale analysis is needed, considering limitations of individual analytical technique. It will be presented how a combination of high-resolution X-ray diffraction, selected area electron diffraction, atomic-resolution scanning transmission electron microscopy, polarized Raman scattering and wavelength-dispersive X-ray spectroscopy can be used to elucidate the structure of this material. Furthermore, it will be shown how macroscopic piezoelectric and dielectric measurements in different configurations can aid in understanding the film structural characteristics. Growth-related aspects will also be discussed, with emphasis on the challenges and peculiarities occurring in the growth of films with volatile components. Routes for the preparation of single-phase films with near-nominal stoichiometry will be presented, along with the specific defects that appear with off-stoichiometry. The role of the substrate on the phase purity and film quality will also be discussed. It will be shown why in some cases a rough interface is preferred.

Since the discovery of ferroelectricity in HfO₂ thin films that either doped or alloyed with other metal oxide components, HfO₂ based ferroelectric field-effect transistors (FeFETs) have gained increasing interest as a candidate emerging memory due to their ability to scale to very small device dimensions, and their CMOS compatibility. For a FeFET with metal-ferroelectric-insulator-semiconductor (MFIS) gate stack, the insulator layer interposed between the ferroelectric and the semiconductor channel plays an important role to achieve a wide memory window (MW) and good endurance and retention. In this work, we experimentally demonstrate FeFETs with very large MW of 3.3 V by utilization of HfO₂ interlayers both on top of and under the ferroelectric Hf_0.5Zr_0.5O₂ (HZO) layer.

Based on the studies of metal-ferroelectric-metal (MFM) capacitors and MFIS gate stacks with ferroelectric HZO of 10 nm, the thicknesses of top and bottom HfO₂ interlayers are optimized to be 2 nm and 1 nm, respectively, in order to achieve maximum MW and minimum polarization “wake-up” effect. The MFIS without HfO₂ interlayers shows MW of 2.0 V, while the MFIS with HfO₂ interlayers shows MW greater than 3.0 V. The thickness of bottom HfO₂ layer of 2 nm is required to improve the MW, and the possible effect is that it forms a thin continuous film that facilitate the later ALD growth of HZO, which may benefit the orthorhombic phase formation.

It has been reported that FeFETs with 20 nm thick HfO₂ layer have a MW of 2.9 V, switched by +5 V and -5 V pulses on the gate.[i] By utilizing HfO₂ interlayers, our FeFETs has only 13 nm of HZO of HfO₂ layers in total, and exhibit a wider MW of 3.3 V, switched by +4 V and -4 V. In addition, our FeFET devices display good retention at 85°C and maintain a MW of 1.8 V extrapolated for 10 years. As for write disturb, the devices can withstand disturbs in a V_dd/2 scheme up to the tested 10⁶ cycles without degradation of I_on/I_off ratio.


[i] H. Mulaosmanovic, E. T. Breyer, T. Mikolajick and S. Slesazeck, "Ferroelectric FETs With 20-nm-Thick HfO2 Layer for Large Memory Window and High Performance," in IEEE Transactions on Electron Devices, vol. 66, no. 9, pp. 3828-3833, Sept. 2019. doi: 10.1109/TED.2019.2930749

SrTiO₃ is known to exhibit quantum paraelectricity in which quantum fluctuations suppress ferroelectric ordering at low temperature¹. Despite the intrinsic paraelectric nature of SrTiO₃, it is possible to stabilize ferroelectricity via a variety of means such as substrate-induced strain tuning², cation substitution³, ¹⁸O isotope substitution⁴, and defect engineering⁵, etc. In particular, the epitaxial strain applied via lattice mismatch in thin film heterostructures can significantly enhance the ferroelectric transition temperature T_c and even stabilize ferroelectricity at room temperature. However, since a given substrate provides a fixed and discrete lattice parameter that lacks mechanical flexibility, this approach is limited in the ability to dynamically control the strain state in a widely accessible range. In order to tune strain in a dynamic and continuous fashion, here we utilize freestanding SrTiO₃ crystalline membranes to extend the lattice control of ferroelectric ordering in such materials⁶. By dissolving a water-soluble epitaxial buffer layer, we can release SrTiO₃ films from substrates, and transfer the resulting freestanding membranes onto flexible polymer substrates that can be stretched to induce various strain states. Using optical second harmonic generation microscopy, we probe the ferroelectric phase transition as a function of strain wherein we observe T_c is significantly enhanced by strain. Upon applying 1.5% uniaxial strain, we observe robust room-temperature ferroelectricity with the notable feature of 180° ferroelectric domains using piezoresponse force microscopy. First-principle calculations and molecular dynamics simulations further reveal the structural nature at each given strain state and the order-disorder character of the phase transition in strained SrTiO₃ membranes. Overall, our work provides new insights regarding strain induced phenomena in oxide membranes and also demonstrates a new approach to dynamically control ferroelectric ordering in SrTiO₃.

References
1. Müller, K. A. & Burkard, H. SrTiO₃: An intrinsic quantum paraelectric below 4 K. Phys. Rev. B 19, 3593–3602 (1979).
2. Haeni, J. H. et al. Room-temperature ferroelectricity in strained SrTiO₃. Nature 430, 758–761 (2004).
3. Mitsui, T. & Westphal, W. B. Dielectric and X-ray studies of Ca_xBa_1-xTiO₃ and Ca_xSr_1-xTiO₃. Phys. Rev. 124, 1354–1359 (1961).
4. Itoh, M. et al. Ferroelectricity induced by oxygen isotope exchange in strontium titanate perovskite. Phys. Rev. Lett. 82, 3540–3543 (1999).
5. Lee, S. et al. Emergence of room-temperature ferroelectricity at reduced dimensions. Science. 1314, 1–20 (2015).
6. Lu, D. et al. Synthesis of freestanding single-crystal perovskite films and heterostructures by etching of sacrificial water-soluble layers. Nat. Mater. 15, 1255–1260 (2016).

The microstructure of lead zirconate titanate (PZT) at its morphotropic phase boundary (MPB) has attracted much interest due to its exceptional dielectric and ferroelectric properties. One long-held hypothesis is that a monoclinic phase exists at the narrow composition regime near the MPB of PZT to bridge the transition between rhombohedral and tetragonal phases. However till today, there remains no agreement regarding the specific monoclinic structure. Motivated by the disordering in the chemical composition of a solid solution, we propose an extension of the random field Ising model with dipolar state that includes long-range dipole-dipole interactions to study MPB in PZT. A Monte Carlo approach is then adopted to obtain low-energy configurations of the model. The method is formulated to suit the GPU architecture, hence allowing to solve large-scale cases that better capture composition disordering and domain pattern. Our model predicts a sharp transition from rhombohedral phase to tetragonal phase as the composition of lead titanate in PZT is increased. At the MPB, it shows a frustrated state mixed with domains of both rhombohedral and tetragonal phases. The average domain size undergoes a sudden dip while the domain walls no longer follow the low-order crystallographic directions at this region. These findings are consistent with experimental observations. Overall this model presents a picture of the competition between composition disordering and long-range interactions, and clarifies the structural inhomogeneity at the MPB.

Ferroelectric hafnia-based films show large electrical polarization at largely reduced dimensions. In addition they present low leakage as well as CMOS compatibility,[1] making them ideal candidates for memory and logic devices. Here we show that multiferroic tunnel junctions (MTJs) with ferroelectric Hf_0.5Zr_0.5O₂ barriers show both tunneling magnetoresistance effect (TMR) and tunneling electroresistance effect (TER), displaying four resistance states by magnetic and electric field switching.[2] Moreover, under electric field cycling of large enough magnitude, the TER can reach values as large as 10⁶%. Concomitant with this TER enhancement, the devices develop electrical control of spin polarization, with sign reversal of the TMR effect.[3] Currently, this intermediate state exists for a limited number of cycles and understanding the origin of these phenomena is key to improve its stability. Our study points to the magneto-ionic effect as the origin of the large TER and strong magneto-electric coupling, showing that ferroelectric polarization switching of the tunnel barrier is not the main contribution. The interplay between ferroelectric polarization and magneto-ionic coupling will be discussed.

[1] U. Schroeder, C. S. Hwang, and H. Funakubo, Ferroelectricity in doped hafnium oxide: materials, properties and devices,Woodhead Publishing, 2019.
[2] Y. Wei, S. Matzen, T. Maroutian, G. Agnus, M. Salverda, P. Nukala, Q. Chen, J. Ye, P. Lecoeur, and B. Noheda, “Magnetic tunnel junctions based on ferroelectric Hf_0.5Zr_0.5O₂ tunnel barriers,” Physical Review Applied 12, 031001, (2019))
[3] Y. Wei, S. Matzen, C. P. Quinteros, T. Maroutian, G. Agnus, P. Lecoeur, B. Noheda “Magneto-ionic control of spin polarization in magnetic tunnel junctions”, https://arxiv.org/abs/1907.12111

I

We introduce a concept of programmable ferroelectric devices comprised of two-dimensional (2D) and ferroelectric (FE) materials. It enables precise modulation of the in-plane conductivity of a 2D channel material through nanoengineering of FE domains with out-of-plane polarization. The functionality of these new devices has been demonstrated using field-effect transistors (FETs) fabricated with monolayer molybdenum disulfide (MoS₂) channels on the Pb(Zr,Ti)O₃ substrates. Using piezoresponse force microscopy, we show that local switching of FE polarization by a conductive probe can be used to tune the conductivity of the MoS₂ channel. Specifically, patterning of the nanoscale domains with downward polarization creates conductive paths in a resistive MoS₂ channel so that the conductivity of an FET is determined by the number and length of the paths connecting source and drain electrodes. In addition to the device programmability, we demonstrate the device ON/OFF cyclic endurance by successive writing and erasing of conductive paths in a MoS₂ channel. These findings may inspire the development of advanced energy-efficient programmable synaptic devices based on combination of 2D and FE materials.

Recent studies have reported that freestanding epitaxial complex oxide thin films [1, 2] exhibit physical properties significantly different from original, as-grown films. For instance, the collapse of crystalline order has been reported in ultrathin, freestanding SrTiO₃ layers, induced by the electron-volt-level energy of bond breaking that is released from the interfacial layer during the transfer process. How such an energy landscape and bond rearrangement in one layer would necessarily affect other layers of a single crystal complex oxide thin film, i.e., in a three-dimensional object, is poorly understood. Unlike 2D materials such as graphene, complex oxides have strong interlayer bonds, which allows for long-range, three-dimensional mechanical energy propagation. The interfacial bond energy in typical complex oxide heterostructures is quite high and therefore, atomistic changes at the interface will alter the properties of all the layers, resulting in a very different mechanical and electrostatic energy landscape in the entirety of the material.

In this work, [3] we present the effect of interfacial bond-breaking on the structural and ferroelectric (FE) properties of epitaxially-grown PbZr_0.2Ti_0.8O₃ (PZT) thin films at mesoscopic length scale. PZT film grown on La_0.7Sr_0.3MO₃ (LSMO) buffered STO substrate is released by breaking the bond at the LSMO-PZT interface using a layer transfer technique. Utilizing the synchrotron X-ray nanoprobe technique we reveal that the bond-breaking results in nanoscale structural ripples in atomic planes of the FE material. The ripples are formed due to the lack of mechanical boundaries in all three dimensions. We estimate that such ripples can create a strong local strain gradient and flexoelectric field that is comparable to the coercive field of the FE film. In-situ piezo force microscopy and electronic transport measurements show that the flexoelectric effect generated from such ripples in atomic planes strongly blocks the FE domain wall motion and reduces the velocity by several orders magnitude. We will present and discuss the effect of these ripples on the energy landscape that controls domain wall motion and polarization reversal behavior.

Reference:
[1] S. R. Bakaul et al., Nat. Commun. 2016, 7, 10547.
[2] D. Lu et al., Nat. Mater. 2016, 15, 1255.
[3] S. R. Bakaul et al., manuscript under review (2019).

Spin textures are the foundation of properties and behavior of magnetic materials and drive the functionality of magnetic devices. Topology and frustration that impact spin textures have recently attracted significant scientific interest and led to intense research e.g. in magnetic skyrmions (Sk) and artificial spin ice systems (ASI) addressing a broad spectrum of challenging scientific and technological questions, including stability, dynamics, nucleation, and transport. So far, SK and ASI have been treated foremost as two-dimensional spin textures, however, recent investigations have opened the door to a conceptually next leap, which are three-dimensional nanoscale size magnetic spin textures (3D-spin-textures) e.g., chiral bobbers, magnetic hopfions, and skyrmion tubes.
Advanced characterization tools that provide magnetic sensitivity to spin textures at high spatial resolution, ultimately at buried interfaces and in all three dimensions [1], and at high temporal resolution to capture the spin dynamics across scales, are therefore of large scientific interest.
Magnetic soft X-ray spectro-microscopies [2] provide unique characterization opportunities to study the statics and dynamics of spin textures in magnetic materials combining X-ray magnetic circular dichroism (X-MCD) as element specific, quantifiable magnetic contrast mechanism with spatial and temporal resolutions down to fundamental magnetic length, time, and energy scales.
Current developments of x-ray sources aim to increase dramatically the coherence of x-rays opening the path to new techniques, such as ptychography [3] or x-ray photo-correlation spectroscopy (XPCS) [4] that allow unprecedented studies of nanoscale heterogeneity, complexity, and fluctuations.
I will review recent achievements and future opportunities with magnetic x-ray spectro-microscopies. Examples will include static properties and dynamic behavior of various magnetic skyrmion [5,6] textures with potential application to novel magnetic logic and storage devices, as well as results from an XPCS study at LCLS with a novel 2-pulse scheme that allowed to discover an unexpected and drastic change of the correlation times in nanoscale spin fluctuations near phase boundaries, i.e., in the skyrmion phase, and near the boundary with the stripe phase of a multilayered Fe/Gd system [4].
Finally, I will present a study on ferromagnetic liquid droplets, which could establish a complete paradigm shift in magnetic materials, that combine characteristics of liquid, such as reconfigurability and short range spatial and temporal correlations with ferromagnetism, which so far has been confined to condensed matter [7].
This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division Contract No. DE-AC02-05-CH1123 in the Non-Equilibrium Magnetic Materials Program (MSMAG).
References
[1] A. Fernandez-Pacheco, et al., Nature Comm 8 15756 (2017)
[2] P. Fischer and H. Ohldag, Report on Progress in Physics 78 094501 (2015)
[3] X. Shi, et al, Appl Phys Letter 108, 094103 (2016)
[4] M. H. Seaberg, et al, Phys Rev Lett 119 067403 (2017)
[5] S. Woo, et al., Nature Materials 15 501 (2016)
[6] N. Kent et al, Appl Phys Lett (2019) under review
[7] X. Liu et al, Science 365 264 (2019)

Artificial neural networks are performing tasks, such as image recognition and natural language processing, that offer great promises for artificial intelligence. However, these algorithms run on traditional computers and consume orders of magnitude more energy more than the brain does at the same task. One promising path to reduce the energy consumption is to build dedicated hardware to perform artificial intelligence. Spintronics devices are particularly interesting because they allow for complex functionality with low energy consumption and small size.

I will discuss the role of magnetic tunnel junctions as building blocks for artificial neural networks in hardware. These bi-stable nanodevices which magnetic state can be both read and written electrically. Their high endurance, reliability and CMOS-compatibility have made them flagship devices for novel forms of computing. While their main use is as non-volatile memories (MRAM), they can also exhibit rich non-linear dynamics which emulate the basic functionalities of neurons.

In particular, I will focus on the superparamagnetic regime of magnetic tunnel junctions. Here, they behave as stochastic oscillators capable of harnessing the energy of thermal noise. While stochasticity is traditionally unwanted for computing, I will show how it can be leveraged to reduce energy consumption. I will draw an analogy between stochastic magnetic tunnel junctions and stochastic spiking neurons, which emit randomly spaced voltage spikes. I will show how these devices can be assembled into an artificial neural network an perform computing. Such a system would be competitive with conventional electronics implementations, both in terms of area on chip and energy consumption.

These results open the path to low energy hardware implementation of neural networks, making it possible to perform cognitive tasks on chip.

Perpendicular magnetization reversal through spin orbit torque (SOT) is of great recent interest due to its potential application in spin-orbit torque magnetoresistive random access memory (SOT-MRAM). Typically, a simultaneous application of in-plane bias magnetic field is required to break in-plane symmetry, which would cause crosstalk between neighboring memory units and thus impedes the development of high-density SOT-MRAM. Here, we propose a new approach to realize field-free SOT-mediated perpendicular magnetization reversal using micromagnetic simulations implemented in both our in-house package and MuMax3. Using realistic materials parameters from documented experimental measurements, our simulations suggest that such reversal can occur in a MgO/CoFeB/Pt multilayer stack when the disk size of CoFeB, the strength of Dyzaloshinskii-Moriya interaction at the CoFeB/Pt interface, and the magnitude of the driving in-plane current are all controlled within intermediate range. The kinetic mechanism of such new approach is presented and also understood by analogy to classical nucleation and growth theory.

Iron (II) complexes have a ligand field energy and exchange-interaction energy that are within a few eV; therefore, the spin crossover (SCO) from low spin (LS) to high spin (HS) can be controlled using an external stimuli—temperature, pressure, light, etc. Similar to other bistable materials, iron (II) SCO complexes could find applications in technologies such as quantum logic operators and memory storage devices. However, most research on SCO complexes is performed on powder samples, which are incompatible with the industry’s needs for thin films with thickness control. In this study, we aim at developing a layer-by-layer assembly method to fabricate iron (II) SCO thin films. This thin film fabrication method has several advantages, including the precise control of film thickness by simply adjusting the number of layers. Prior to thin film fabrication, [Fe(pyrazine){M(CN)₄}] (M = Pt or Pd) bulk powders were synthesized so that their SCO properties could be used as a reference for their thin film counterparts. The SCO behavior of the Pt and Pd powders was confirmed with a vibrating sample magnetometer (VSM), and both powders displayed abrupt, room temperature hysteresis with widths of 45 K centered around 300 K. The abrupt spin transition is indicative of strong cooperativity between the metal centers. Rietveld analysis of x-ray diffraction (XRD) data further confirmed that the Pt and Pd powders were crystalline, showing nearly identical diffraction patterns in space group P4/m. Next, we employed the layer-by-layer solution deposition method to fabricate [Fe(pyrazine){M(CN)₄}] (M = Pt or Pd) thin films on gold substrates. The resultant Pt and Pd thin films were analyzed using x-ray absorption spectroscopy (XAS) to demonstrate the differences in spin state ratios during heating and cooling cycles. A 25-layer Pd thin film was determined to have a thickness of 0.760 ± 0.04 μm using a scanning electron microscope (SEM). The SCO behavior of the Pd thin film was analyzed using a superconducting quantum interference device (SQUID), and the results suggest the presence of an incomplete spin transition with some evidence of hysteresis. Based on the results from comparing the SCO behavior of thin films with varying number of layers (i.e., film thickness), the SCO behavior appears to improve with film thickness. This alludes to the fact that by increasing the number of layers to beyond 25, this method can potentially be used to fabricate thin films with optimal SCO behavior. When compared to the bulk powder results, the hysteresis loop of the thin film was not abrupt and the spin transition was incomplete, but the general shape was similar and both loops were centered around room temperature. Although further optimization is required to improve the quality of the films, our initial results have demonstrated the use of this simple solution-phase deposition method in the fabrication of SCO thin films.

Oxide materials exhibit a broad range of tunable phenomena, including magnetism, multiferroicity, and superconductivity. Oxide interfaces are particularly intriguing. Their low local symmetry combined with the sensitivity to electrostatics and strain leads to unusual physical properties beyond the bulk properties. Recently, ferroelectric domain walls have attracted attention as a novel type of oxide interface; the walls are spatially mobile and allow controlling electronic signals at the atomic scale, holding great potential as multifunctional 2D systems for future nanoelectronics.

In my talk, I will present unique features that occur at improper ferroelectric domain walls in hexagonal manganites and discuss how these walls can be used to emulate the behavior of key electronic components. For our studies, we choose the p-type semiconductor ErMnO₃ as it naturally develops all fundamental types of ferroelectric domain wall at room temperature, namely neutral (side-by-side) as well as negatively (tail-to-tail) and positively charged (head-to-head) wall configurations. The walls are explicitly robust and, hence, represent an ideal template onto which the desired electronic behavior can be imposed. I will show how the electronic properties can be optimized and controlled, and discuss the possibility to use such walls for designing 2D digital switches and half-wave rectifiers, bringing us one step closer to domain-wall based devices and networks for next-generation nanotechnology.

Ferroelectric hafnia-based films show large electrical polarization at largely reduced dimensions. In addition they present low leakage as well as CMOS compatibility,[1] making them ideal candidates for memory and logic devices. Here we show that multiferroic tunnel junctions (MTJs) with ferroelectric Hf_0.5Zr_0.5O₂ barriers show both tunneling magnetoresistance effect (TMR) and tunneling electroresistance effect (TER), displaying four resistance states by magnetic and electric field switching.[2] Moreover, under electric field cycling of large enough magnitude, the TER can reach values as large as 10⁶%. Concomitant with this TER enhancement, the devices develop electrical control of spin polarization, with sign reversal of the TMR effect.[3] Currently, this intermediate state exists for a limited number of cycles and understanding the origin of these phenomena is key to improve its stability. Our study points to the magneto-ionic effect as the origin of the large TER and strong magneto-electric coupling, showing that ferroelectric polarization switching of the tunnel barrier is not the main contribution. The interplay between ferroelectric polarization and magneto-ionic coupling will be discussed.

[1] U. Schroeder, C. S. Hwang, and H. Funakubo, Ferroelectricity in doped hafnium oxide: materials, properties and devices,Woodhead Publishing, 2019.
[2] Y. Wei, S. Matzen, T. Maroutian, G. Agnus, M. Salverda, P. Nukala, Q. Chen, J. Ye, P. Lecoeur, and B. Noheda, “Magnetic tunnel junctions based on ferroelectric Hf_0.5Zr_0.5O₂ tunnel barriers,” Physical Review Applied 12, 031001, (2019))
[3] Y. Wei, S. Matzen, C. P. Quinteros, T. Maroutian, G. Agnus, P. Lecoeur, B. Noheda “Magneto-ionic control of spin polarization in magnetic tunnel junctions”, https://arxiv.org/abs/1907.12111

I

Piezoresponse force microscopy (PFM) has become the standard tool for imaging and spectroscopy of ferroelectric and multiferroic materials, providing wealth of information on domain structures and switching behaviors. However, until now PFM, similar to other scanning probe microscopy techniques, was based on the classical rectangular scans and predefined voltage patterns. In this presentation, I introduce our recent work on combining PFM with automated experimentation, exploring image-based feedback, non-rectangular and adaptable scans, and autonomous experiments. As one such example, we demonstrate a fundamentally new approach for exploring polarization dynamics of specific domain elements and topological structures in ferroelectric materials manipulation based on in-situ image-based feedback in PFM. This approach was demonstrated for probing highly pinned domain wall dynamics in polydomain PZT, one of the traditionally most difficult objects for PFM studies. We demonstrate the creation of frustrated phases with significantly enhanced electromechanical response prior to the onset of the wall motion and discuss future strategies for fabrication of predefined topological defects and exploration of their dynamic properties. We further illustrate the incorporation of non-rectangular scans and discuss the strategies for automated experimentation in PFM based on the Gaussian Process modelling. Finally, we explore the opportunities for exploring the fundamental physics of ferroelectric materials enabled by the combination of the automated PFM experiment and combinatorial synthesis.

This research is supported by the by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, BES DOE.

We suggest that 5d electron systems are promising class of spintronic materials because of its strong spin-orbit coupling (SOC). A strong SOC inherent to 5d Ir oxides recently emerged as a new paradigm for oxide electronics. For example, we investigated novel physics of spin-orbital Mott insulators [1] and possible topological insulators [2] by tuning the electronic phases through superlattice technique. We also demonstrated a large spin Hall effect of IrO₂, one of the simplest 5d oxides, indicating that Ir oxides are promising class of spintronic materials [3].
In this talk, we focus on yet another topic on spintronics – magnetic skyrmion as a topological spin texture. We have studied transport properties of bilayers consisting of m unit cells of ferromagnetic SrRuO₃ and 2 unit cells of SrIrO₃. We observed an anomaly in the Hall resistivity in addition to anomalous Hall effect (AHE); this is attributed to topological Hall effect (THE) [4]. The results suggest that magnetic skyrmions of 10–20 nm are generated by Dzyaloshinskii-Moriya interaction, which might be caused by both broken inversion symmetry at the interface and strong SOC of SrIrO₃. Even more surprising is that we can control both AHE and THE by electric field in the SrRuO₃-SrIrO₃ bilayers [5]. We observed the clear electric-field dependence only when SrIrO₃ is inserted between SrRuO₃ and a gate dielectric. The results established that strong SOC of nonmagnetic materials such as SrIrO₃ is essential in electrical tuning of these Hall effects.
We are also searching for spin-current transport properties at the interfaces consisting of magnetic oxides and nonmagnetic Ir oxides. We will report on the latest results on them. Through these studies on oxide interfaces containing Ir oxides, we may have a chance to approach a triple point for topology, correlation, and spin-orbit coupling.

References:
[1] J. Matsuno et al., Phys. Rev. Lett. 114, 247209 (2015).
[2] D. Hirai, J. Matsuno, and H. Takagi, APL Mater. 3, 041508 (2015).
[3] K. Fujiwara et al., Nat. Commun. 4, 2893 (2013).
[4] J. Matsuno et al., Sci. Adv. 2, e1600304 (2016).
[5] Y. Ohuchi et al., Nat. Commun. 9, 213 (2018).

Topological defects can be formed at a phase transition where there is spontaneous symmetry breaking. These features play a central role in understanding phase transitions, ranging from superfluid ⁴He and high-temperature superconductors to cosmological systems. Certain hexagonal multiferroic materials also display topological defects including (charged) domain walls and ferroelectric vortices. In addition to serving as a model for their universality class, these sites can further host emergent phenomena including anomalous conduction and fractional electronic charges or quantized magnetic moments. Here, we construct atomically-abrupt superlattices to study the ferroelectric order as a function of thickness. We observe controlled formation of charged ferroelectric domain walls. Electrostatic confinement of the walls and the associated vortices pushes the material from a Z₆ symmetry to a stripe phase with continuous U(1) symmetry to a paraelectric phase with decreasing thickness. These materials also display unique topological features not observed in the bulk components. Together we demonstrate how dimensional confinement in artificial superlattices is a new probe for stabilizing new topologies in hexagonal ferroelectric systems.

Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ is a relaxor ferroelectric material that exhibits interesting dielectric and piezoelectric properties. Specifically, complex domain architectures are formed in compositions around the morphotropic phase boundary. The polar structure of thin films is additionally affected by the epitaxial strain. In order to understand the structure of these films, multi-scale analysis is needed, considering limitations of individual analytical technique. It will be presented how a combination of high-resolution X-ray diffraction, selected area electron diffraction, atomic-resolution scanning transmission electron microscopy, polarized Raman scattering and wavelength-dispersive X-ray spectroscopy can be used to elucidate the structure of this material. Furthermore, it will be shown how macroscopic piezoelectric and dielectric measurements in different configurations can aid in understanding the film structural characteristics. Growth-related aspects will also be discussed, with emphasis on the challenges and peculiarities occurring in the growth of films with volatile components. Routes for the preparation of single-phase films with near-nominal stoichiometry will be presented, along with the specific defects that appear with off-stoichiometry. The role of the substrate on the phase purity and film quality will also be discussed. It will be shown why in some cases a rough interface is preferred.

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 large reduction of in-plane piezoelectric strain by substrate clamping, and in two-terminal devices, the requirement of anisotropic in-plane strain. Here we show that these limitations can be overcome by designing the piezoelectric strain tensor using the boundary interaction between biased and unbiased piezoelectric. We fabricated 500 nm thick, (001) oriented [Pb(Mg1/3Nb2/3)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 electric field-driven magnetization rotation. We develop a method that can apply designed strain patterns to many other materials systems to control properties such as superconductivity, band topology, conductivity, and optical response. We also discuss how these effects depend on the crystallographic orientation.

Ferromagnetic skyrmion is a topologically protected whirling spin texture in nanoscale. Due to its small size, topologically-protected stability, and solitonic nature together, it holds great promise for future spintronics applications. To translate these compelling features into practical spintronic devices, we should face a key challenge in achieving effective control of skyrmion properties, such as size, density, and thermodynamic stability. Recently, we reported the discovery of ferroelectrically tunable skyrmions in ultrathin BaTiO3/SrRuO3 bilayer heterostructures. The ferroelectric proximity effect occurring at the BaTiO3/SrRuO3 heterointerface can trigger a sizable Dzyaloshinskii-Moriya interaction. Thus the interaction between two ferroic materials can stabilize robust skyrmions with diameters of several tens of nanometers.[1] Moreover, by manipulating the ferroelectric polarization direction of the BaTiO3 layer, we could achieve local, switchable and nonvolatile control of both skyrmion density and thermodynamic stability. Such a ferroelectrically tunable skyrmion system can work as a new paradigm to simultaneously enhance the integratability and addressability of skyrmion-based functional devices.

[1] L. Wang et al., Nature Materials 17, 1087 (2018).

Spin textures are the foundation of properties and behavior of magnetic materials and drive the functionality of magnetic devices. Topology and frustration that impact spin textures have recently attracted significant scientific interest and led to intense research e.g. in magnetic skyrmions (Sk) and artificial spin ice systems (ASI) addressing a broad spectrum of challenging scientific and technological questions, including stability, dynamics, nucleation, and transport. So far, SK and ASI have been treated foremost as two-dimensional spin textures, however, recent investigations have opened the door to a conceptually next leap, which are three-dimensional nanoscale size magnetic spin textures (3D-spin-textures) e.g., chiral bobbers, magnetic hopfions, and skyrmion tubes.
Advanced characterization tools that provide magnetic sensitivity to spin textures at high spatial resolution, ultimately at buried interfaces and in all three dimensions [1], and at high temporal resolution to capture the spin dynamics across scales, are therefore of large scientific interest.
Magnetic soft X-ray spectro-microscopies [2] provide unique characterization opportunities to study the statics and dynamics of spin textures in magnetic materials combining X-ray magnetic circular dichroism (X-MCD) as element specific, quantifiable magnetic contrast mechanism with spatial and temporal resolutions down to fundamental magnetic length, time, and energy scales.
Current developments of x-ray sources aim to increase dramatically the coherence of x-rays opening the path to new techniques, such as ptychography [3] or x-ray photo-correlation spectroscopy (XPCS) [4] that allow unprecedented studies of nanoscale heterogeneity, complexity, and fluctuations.
I will review recent achievements and future opportunities with magnetic x-ray spectro-microscopies. Examples will include static properties and dynamic behavior of various magnetic skyrmion [5,6] textures with potential application to novel magnetic logic and storage devices, as well as results from an XPCS study at LCLS with a novel 2-pulse scheme that allowed to discover an unexpected and drastic change of the correlation times in nanoscale spin fluctuations near phase boundaries, i.e., in the skyrmion phase, and near the boundary with the stripe phase of a multilayered Fe/Gd system [4].
Finally, I will present a study on ferromagnetic liquid droplets, which could establish a complete paradigm shift in magnetic materials, that combine characteristics of liquid, such as reconfigurability and short range spatial and temporal correlations with ferromagnetism, which so far has been confined to condensed matter [7].
This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division Contract No. DE-AC02-05-CH1123 in the Non-Equilibrium Magnetic Materials Program (MSMAG).
References
[1] A. Fernandez-Pacheco, et al., Nature Comm 8 15756 (2017)
[2] P. Fischer and H. Ohldag, Report on Progress in Physics 78 094501 (2015)
[3] X. Shi, et al, Appl Phys Letter 108, 094103 (2016)
[4] M. H. Seaberg, et al, Phys Rev Lett 119 067403 (2017)
[5] S. Woo, et al., Nature Materials 15 501 (2016)
[6] N. Kent et al, Appl Phys Lett (2019) under review
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Symmetry breaking ascribed to the evolution of an order parameter is one of the most important concepts in ferroic materials such as ferromagnets and ferroelectrics, and bears “domain” structures. The domain switching by an external field is the key principle to exploit them for practical applications. In multiferroics, materials possessing multiple ferroic order parameters, the respective order parameters form domains and multiple types of domain states coexist. It is well-accepted that couplings between the multiple order parameters and domain states are essential for macroscopic magnetoelectric phenomena in multiferroics. However, there are few reports that directly compare responses of coexisting domain states to external fields and macroscopic magnetoelectric properties. In this presentation, we show selective observations of multiple magnetic order parameters and domain structures in some single-phase magnetoelectrics. This work has been done in collaboration with H. Ueda, R. Misawa, T. Katsuyoshi, J. K. H. Fischer, K. Kimura, Y. Tanaka, Y. Wakabayashi, T. Honda, J. S. White, M. Kenzelmann, A. B. Harris, N. Leo, and M. Fiebig.

Incorporating ferromagnetic, ferroelectric and ferroelastic orders inherently in a single-phase multiferroic material or manipulating one ferroic order with the others via interfacial coupling effects in multiferroic heterostructures has driven significant research activities aiming at the realization of their technological potential in the field of sensors, RF electronics, logic devices and memory etc. Magnetoelectric (ME) effect is essentially referred to as a change of electrical polarization (P) induced by external magnetic field (H) or a control of magnetization (M) via applied electric field (E). This report first introduces our recent progress in laminated ME composites consisting of a [011]-oriented Pb(Mg,Nb)O₃-Pb(Ti,Zr)O₃ (PMN-PZT) single-crystal fiber laminated with laser-treated amorphous FeBSi alloy (Metglas). High ME coupling coefficient around 10 kV/cm*Oe has been achieved. We will then review recent advances in multi-layered ME composites as well as their device applications including magnetic field/current sensors, dual-stimulus energy harvesters, ME memories, ME gyrators and tunable electronics. Some perspectives in terms of the future of magnetoelectric composites and devices will also be provided.

Artificial neural networks are performing tasks, such as image recognition and natural language processing, that offer great promises for artificial intelligence. However, these algorithms run on traditional computers and consume orders of magnitude more energy more than the brain does at the same task. One promising path to reduce the energy consumption is to build dedicated hardware to perform artificial intelligence. Spintronics devices are particularly interesting because they allow for complex functionality with low energy consumption and small size.

I will discuss the role of magnetic tunnel junctions as building blocks for artificial neural networks in hardware. These bi-stable nanodevices which magnetic state can be both read and written electrically. Their high endurance, reliability and CMOS-compatibility have made them flagship devices for novel forms of computing. While their main use is as non-volatile memories (MRAM), they can also exhibit rich non-linear dynamics which emulate the basic functionalities of neurons.

In particular, I will focus on the superparamagnetic regime of magnetic tunnel junctions. Here, they behave as stochastic oscillators capable of harnessing the energy of thermal noise. While stochasticity is traditionally unwanted for computing, I will show how it can be leveraged to reduce energy consumption. I will draw an analogy between stochastic magnetic tunnel junctions and stochastic spiking neurons, which emit randomly spaced voltage spikes. I will show how these devices can be assembled into an artificial neural network an perform computing. Such a system would be competitive with conventional electronics implementations, both in terms of area on chip and energy consumption.

These results open the path to low energy hardware implementation of neural networks, making it possible to perform cognitive tasks on chip.

Composite multiferroics, composed of a magnetostrictive ferromagnet and a piezoelectric ferroelectric, hold promise for magnetic field sensors and energy efficient beyond-CMOS logic by harnessing magnetoelectric transduction. Enhancing device performance requires highly magnetostrictive materials, however, relatively little attention has been given to engineering magnetostriction in thin films. One promising magnetostrictor system are the Fe_1-xGa_x alloys yet in bulk, the magnetostriction coefficient of Fe_1–xGa_x alloys versus gallium composition peaks near ~18% Ga due to a phase change from the disordered A2 phase to an ordered BCC phase (D0₃). Here we discuss a means to boost (by 200-300% relative to bulk) the magnetostriction of Fe_1-xGa_x thin film alloys by extending the phase stability of the A2 phase to higher Ga compositions. Transport-based magnetoelectric characterization of a Fe_1-xGa_x - [Pb(Mg_1/3Nb_2/3)O₃]_0.7-[PbTiO₃]_0.3 (PMN-PT) composite multiferroic heterostructure shows are reversible 90 deg electrical switch of magnetic anisotropy and a room temperature converse magnetoelectric coefficient of 5.5×10^-6 s m^-1.

Active manipulation of spin waves is essential for the development of magnon-based technologies. One route towards this goal exploits reprogrammable magnonic crystals. In such structures, a varying spatial modulation of magnetic properties using, for instance, electric currents [1] or optical pulses [2] controls the transmission of spin waves. Besides these dynamic approaches, field-induced toggling between ferromagnetic and antiferromagnetic states in magnetic stripe arrays has been shown to modify spin wave transport in a non-volatile way [3].
Multiferroic heterostructures comprising coupled ferromagnetic and ferroelectric layers offer an interesting alternative as programmable hybrids for magnonics. Attractive properties of multiferroic heterostructures include the imprinting of magnetic domain patterns, the creation of abrupt anisotropy boundaries, strong pinning of magnetic domain walls, and electric field control of magnetic anisotropy, magnetic switching, and magnetic domain wall motion [4-6].
Here, we demonstrate that multiferroic heterostructures provide a versatile platform for the emission of short-wavelength spin waves [7], the confinement of spin wave modes in structurally uniform films [8], and active manipulation of spin wave transmission [9]. 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 [9]. In addition to magnetic field control, we also present recent results on active manipulation of spin wave transmission using electric fields. This sought-after effect originates from local strain-induced changes of the magnetic anisotropy. We demonstrate that the transmission of spin waves can be fully turned on and off in certain frequency bands by the application of voltage pulses across the BaTiO₃ substrate. The multiferroic heterostructures allow also for continuous tuning of the spin wave phase and amplitude, offering prospects for voltage-controlled magnonic logic.

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