Kirk H. Bevan, McGill University
Sohrab Ismail-Beigi, Yale University
T. Zac Ward, Oak Ridge National Laboratory
Zhenyu Zhang, University of Science and Technology of China
Symposium Support MDC Vacuum Products, LLC
NBM Design Inc.
Princeton Scientific Corp.
Quantum Design, Inc.
SURFACE Systems amp; Technologies GmbH amp; Co. KG
P2: Strongly Correlated Thin Films II
Monday PM, December 02, 2013
Hynes, Level 2, Room 201
2:30 AM - P2.01
Coherent X-Ray Scattering from Striped Serpentine Nanodomains in a Ferroelectric/Dielectric Superlattice
Qingteng Zhang 1 Pice Chen 1 Zhonghou Cai 2 Ross Harder 2 Sara Callori 3 Matthew Dawber 3 Paul Evans 1
1University of Wisconsin-Madison Madison USA2Argonne National Laboratory Argonne USA3Stony Brook University Stony Brook USAShow Abstract
Ferroelectric thin films form striped polarization nanodomains in order to minimize the electrostatic energy. The boundaries of between domains have unique properties such as electromechanical clamping and electrical conductance and can also have crystallographic structures that are distinct from bulk materials. Experimental studies of the configuration and structure of the nanodomains of ultra-thin ferroelectric films in applied electric fields are challenging because the fabrication of top electrodes can destroy the domain pattern and the thinness of the ferroelectric layers leads to large leakage currents. Ferroelectric/dielectric superlattices, however, form nanodomains similar to ultra-thin ferroelectric films because the neighboring ferroelectric layers are weakly coupled due to the electrical isolation of the dielectric layers. The study of ferroelectric nanodomains in ferroelectric/dielectric superlattices, especially their spatial configuration and switching dynamics, provides insight into the structure and dynamics of a more general class of striped domains.
We have studied nanodomain structures in a PbTiO3/SrTiO3 superlattice using synchrotron coherent x-ray diffraction. The degree of transverse coherence of the x-ray probe is increased by illuminating the focusing optics through a narrow slit with a width matching the coherence length of the x-rays. The superlattice nanodomains produce speckle diffraction patterns which contain depend in detail on the arrangement and structure of the nanodomains. We have observed speckle scattering patterns using x-ray beams prepared with focusing optics based on Kirkpatrick-Baez mirrors and Fresnel zone plate. When the numerical aperture of the focusing optics is small, the spatial profile of the speckles is determined by the illumination function at far-field limit. However, when the numerical aperture is large, the spatial profile of the speckles is determined by the interference of the illumination functions and the angular width can be an order of magnitude smaller than the convergent angle of the focusing optics. The speckle visibility decreases with the increase of slit width and count time in both cases. The degree of correlation between speckle patterns decreases over time due to the drift of the experimental setup. A spatial map of the correlation coefficient indicates that there is no spatial correlation of the in-plane nanodomains, which agrees with the approximation of the coherence length calculated from the envelope of the speckle pattern.
2:45 AM - P2.02
Local Control of Magnetic Anisotropy in Permalloy Thin Films Coupled to Ferroelectric Domains of BaTiO3 Single Crystals
Sean Wu Fackler 1 Tieren Gao 1 Sang-Wook Cheong 2 Ichiro Takeuchi 1
1University of Maryland College Park USA2Rutgers, the State University of New Jersey Piscataway USAShow Abstract
We are investigating the coupling between ferroelectric domains of BaTiO3 (BTO) single crystals and magnetic domains in permalloy (Py) thin films deposited on top of them. We reproducibly fabricate dense magnetic stripes near the transcritical state by controlling sputtering deposition parameters. The films display partial out-of-plane anisotropy which is clearly visible as dense stripe domains in magnetic force microscopy. The stripe domains are used as an indicator to investigate the local coupling behavior between Py thin films and BTO crystals. Piezoresponse force microscopy of the crystals shows clear presence and spatial modulation of a-c domains in BTO. When the Py/BTO crystals are annealed above the Curie temperature of BTO (130#9675;C) and cooled down, stripe domains in Py develop local anisotropy modulation consistent with distribution of a-c domains. In particular, there is a clear and abrupt change in deduced in-plane magnetic anisotropy in Py at the a-c domain boundary of BTO. Magnetic stripe period also changes at the a-c domain boundary which is accounted for by magnetoelastic energy. The detailed coupling behavior achieved under different conditions is described by micromagnetic models.
3:00 AM - *P2.03
Design of a Polar Metal with Highly Anisotropic Thermoelectric Properties
James Rondinelli 1
1Drexel University Philadelphia USAShow Abstract
The metallic features in materials, which provide low-resistance channels for electrical conduction, lead to effective screening of local electric dipole moments. Itinerant electrons disfavor both their formation and cooperative ordering. Consequently, most metals with a finite density of states and partial band occupation exhibit centric (inversion symmetric) crystal structures. Despite this contraindication, noncentrosymmetric metals (NCSM) lacking inversion were proposed more than fifty years ago, with some examples discovered serendipitously later. Here we describe a design framework to alleviate the property disparities and accelerate NCSM discovery: The primary ingredient relies on the removal of inversion symmetry through displacements of atoms whose electronic degrees of freedom are decoupled from the states at the Fermi level. Density functional theory calculations validate our crystal-chemistry strategy, and predict a polar perovskite ruthenate to be metallic and robust to spin-orbit interactions. Motivated by recent suggestions that degenerately doped ferroelectrics exhibit advantageous thermoelectric responses, we show that the thermopower in this polar metal exhibits large anisotropy for particular doping regimes along the polar axis. Although the high-density of states is unfavorable for a high power factor, our results indicate highly functional NCSM could be tailored for targeted thermoelectric applications.
This work was performed in collaboration with Dr. Danilo Puggioni and sponsored by the U.S. Army Research Office under grant no. W911NF-12-1-0133.
3:30 AM - P2.04
Fast, Low Temperature Topotactic Valence State Reversal in Strontium Cobaltites
Hyoung Jeen Jeen 1 Woo Seok Choi 1 John W. Freeland 2 Dongwon Shin 1 Jun Hee Lee 3 Valentino R. Cooper 1 Michael D. Biegalski 4 Sung Seok A. Seo 5 Hiromichi Ohta 6 Matthew F. Chisholm 1 Karin M. Rabe 7 Ho Nyung Lee 1
1Oak Ridge National Laboratory Oak Ridge USA2Argonne National Laboratory Argonne USA3Princeton University Princeton USA4Oak Ridge National Laboratory Oak Ridge USA5University of Kentucky Lexington USA6Hokkaido University Sapporo Japan7Rutgers University Piscataway USAShow Abstract
Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for the enhancement of the overall performance and lifetime of many energy and environmental technologies, including solid oxide fuel cells, batteries and catalytic converters. Despite many efforts to search for new materials and routes to overcome the technical challenges, the robust nature of the cation&’s oxidation state and the high thermodynamic barrier have hindered the realization of fast catalytic reactions and bulk diffusion at low temperatures. Here, we report a low-temperature topotactic phase transition in SrCoOx grown directly by pulsed laser epitaxy as one of two distinct crystalline phases, either the perovskite SrCoO3-δ or the brownmillerite SrCoO2.5. Based on real-time temperature dependent characterizations with x-ray diffraction and optical spectroscopy, we found that the two topotactic phases can be reversibly switched at a remarkably reduced temperature (200~300 oC) in a considerably short time (< 1 min) without destroying the parent framework. This phase reversal accompanies distinct electronic (metal vs. insulator) and magnetic (ferromagnetic and antiferromagnetic) transitions. Therefore, our results on low temperature topotactic valence state reversal provide valuable insight not only in understanding the structure-physical property relationship in multivalent oxides, but also for identifying new opportunities for technological applications, such as low temperature catalysts.
The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.
3:45 AM - P2.05
Crystallographic Orientation and Strain Dependence of Oxygen Vacancy Order and Electronic Transport at Cobaltite Interfaces
Shameek Bose 1 Manish Sharma 1 Maria A. Torija 1 Jaume Gazquez 2 3 Maria Varela 2 3 Valeria Lauter 4 Haile Ambaye 4 Richard J. Goyette 4 Michael R. Fitzsimmons 5 Josh Schmitt 1 Chris Leighton 1
1University of Minnesota Minneapolis USA2Oak Ridge National Laboratory Oak Ridge USA3Universidad Complutense de Madrid Madrid Spain4Oak Ridge National Laboratory Oak Ridge USA5Los Alamos National Laboratory Los Alamos USAShow Abstract
Thin films and heterostructures of the perovskite cobaltites are of great interest, not only from the point of view of fundamental physics and materials science, but also for technological applications such as solid oxide fuel cells, gas membranes and multiferroics. Their properties are, however, severely deteriorated from the bulk, being dominated by the presence of interfacial “dead layers”. Working with the prototypical SrTiO3(001)/La1-xSrxCoO3 (LSCO) system, we recently discovered that this degradation in the magnetic and electronic transport at the interface is caused by nanoscopic magneto-electronic phase separation. This was shown to occur primarily due to accumulation of oxygen vacancies near the interface, driven by the fascinating interplay between the strain state and oxygen vacancy ordering [1, 2]. In the present work we show how this understanding allows for engineering of the interfacial magnetic and electronic transport properties via manipulation of this oxygen vacancy superstructure [3, 4]. Using reciprocal space mapping, we demonstrate the ability to control, via the vacancy ordering, the critical strain relaxation thickness by changing the sign of the strain (from tensile on SrTiO3 to compressive on LaAlO3) and crystallographic orientation ((001) vs. (110)). Furthermore, SQUID magnetometry, polarized neutron reflectometry (PNR) and magneto-transport confirm the concomitant mitigation of the interfacial degradation for LSCO films grown on LaAlO3(001) and SrTiO3(110), as compared to films grown on SrTiO3 (001). Finally, we provide cross sectional electron energy loss spectroscopy (EELS) data showing the preservation of both oxygen and hole carrier concentration at the LaAlO3(001)/LSCO and SrTiO3(110)/LSCO interfaces, strikingly different to the severely depleted SrTiO3(001)/LSCO interface Our work thus opens up a new route to tailor interfacial electronic transport and magnetic properties, thereby engineering complex oxide device performance.
Work supported by NSF and DOE (neutron scattering). Research at ORNL supported by U.S. DOE-BES, MS&E Division, at UCM by ERC Starting Investigator Award.
 Torija et al., Adv. Mater. 23, 2711 (2011)
 Gazquez et al., Nano Lett. 11, 973 (2011)
 Gazquez, Bose et al., APL Materials 1, 012105 (2013)
 Bose et al. (in preparation)
4:30 AM - P2.06
Engineering Nonlinear I-V Characteristics in Phase Separated Manganite Thin Films
Vijay Raj Singh 1 2 Anil Kumar Rajapitamahuni 1 2 Xia Hong 1 2
1University of Nebraska-Lincoln Lincoln USA2University of Nebraska-Lincoln Lincoln USAShow Abstract
We have investigated the I-V characteristics of 5-10 nm La_0.7Ca_0.3MnO_3 (LCMO) and La_0.5Sr_0.5MnO_3 (LSMO) thin films grown epitaxial on (001) SrTiO_3 (STO), (001) LaAlO_3 (LAO) and (110) NdGaO_3 substrates via off-axis radio frequency magnetron sputtering. These films show high crystallinity and have RMS roughness of 2-3 Å. The LSMO films with thickness above 6 nm show metal-insulator transitions (MIT) at ~275K and exhibit linear I-V characteristics in the whole temperature range investigated (10 K to 350 K). As the film thickness approaches the electrically dead layer thickness, LSMO becomes totally insulating and exhibits a strong nonlinear behavior in I-V. In the LCMO films, we observe a strong correlation between the I-V characteristics and the MIT. Linear I-V dependence has been observed in the paramagnetic insulating phase and nonlinear I-V starts to develop below the MIT transition temperature (~150 K). We discuss the origin of our observations within the phase separation model and explore how to engineer the I-V characteristics in manganite thin films via film thickness, substrate strain, and an electric field effect through a neighboring ferroelectric gate.
We acknowledge the support from NSF Grant CAREER No. DMR-1148783, the Center for NanoFerroic Devices (CNFD) and the Nanoelectronics Research Initiative (NRI).
4:45 AM - P2.07
Nano-Confinement Steep Metal-Insulator Transition Driven by Temperature and Magnetic Field in Extremely Small (La,Pr,Ca)MnO3 Epitaxial Nanowall Prepared by 3D Nano-Template PLD
Hidekazu Tanaka 1 Yasushi Fujiwara 1 Azusa N. Hattori 1 Kohei Fujiwara 1
1Osaka University Ibaraki JapanShow Abstract
Strongly correlated electron materials exhibit rich and unique properties including colossal magnetoresistance (CMR) in manganite system and so on. Since the discovery of the intrinsic inhomogenence, nanoscale electronic phase separation is thought to play an important role in correlated electronic materials, and the interplay between the nanoscale electric phases lead to exotic properties. In (La,Pr,Ca)MnO3 /(LPCMO), the coexistence and competition between nano ferromagnetic metal phase and nano charge ordering insulator phase result in CMR effect . By spatially confining materials to length scales smaller than the electronic nano phases, a huge effect, i.e., a steep MR change would emerge. Thus. it is possible to artificially design their behavior and expected to create drastic changes in magnetic and electric properties by capturing a single nano phase into nanostructure. For this purpose, we have developed 3D nano-template PLD technique [2, 3] to construct extremely small metal oxide nanostructures, and succeed to form LPCMO nanowall wire structures with 50 nm width. The LPCMO nanowall wire exhibited nano-confinement metal-insulator transition properties driven by both temperature and magnetic field. The drastic resistivity change of a single domain corresponds to the first-order insulator-metal transition of a single domain. For example, this nanowall wire sample showed a digital resistance change at 80 K while a film sample displays the gradual resistance change. Above 80 K, in addition to the insulator feature (negative temperature coefficient of resistance) the discrete drops in resistivity were observed. Very steep magnetoresistance change was also observed on LPCMO nanowall wire sample while smooth MR change appeared on thin film sample. We will also report a quantitative correlation between nano-spatial size and nano-confinement MIT properties and discuss the revealed a single phase dynamics in correlated oxide nano-structures.
 M. Uehara et al., Nature 399, 560 (1999)
 Y. Fujiwara, A. N. Hattori, K. Fujiwara and H. Tanaka, Jpn. J. Appl. Phys. 52 (2013)015001
 T. Kushizaki, K. Fujiwara, A. N. Hattori, T. Kanki and H. Tanaka, Nanotechnology 23 (2012) 485308
5:00 AM - P2.08
Direct Quantification of Charge Transfer at Oxide Interfaces Using Atomic-Resolution Electron Energy Loss Spectroscopy
Julia Mundy 1 Yasuyuki Hikita 2 Takeaki Hidaka 3 Takeaki Yajima 2 Takuya Higuchi 3 Harold Hwang 2 David Muller 1 Lena Kourkoutis 1
1Cornell University Ithaca USA2Stanford University Palo Alto USA3The University of Tokyo Kashiwa JapanShow Abstract
Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in thin films poised for next-generation devices. The very nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Interfacial electronic charge transfers to equilibrate chemical potentials and balance interface charges are expected in a metal, and for insulators there is the possibility for either sustained internal electric fields or an interface dipole to cancel an otherwise growing electrostatic potential - the so-called ‘polar catastrophe&’ Here, we probe the charge distribution for manganite-titanate interfaces traversing the metal-to-insulator transition.
Using scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS), we measure the elemental concentration and valence of the cations at a series of La1-xSrxMnO3/SrTiO3 interfaces. We directly quantify both the charge transfer, manifest as valence changes on the interfacial manganese sites, as well as extrinsic defects such as cation interdiffusion and vacancies. We find an intrinsic interfacial electronic reconstruction in the insulating films (x le; 0.2), where the total charge measured quantitatively agrees with that needed to cancel the polar catastrophe. Surprisingly the width of the charge transferred region remains constant for the insulating films, despite differences in cation interdiffusion. As the manganite becomes metallic with increased hole-doping, the total charge build-up and its spatial range drop substantially consistent with screening lengths in metallic La0.7Sr0.3MnO3. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases.
5:15 AM - P2.09
The Influence of Non-Collinear Spin Structures on Magnetotransport in LaNiO3/La0.7Sr0.3MnO3 Superlattices
Jason D Hoffman 1 Brian J Kirby 2 Anand Bhattacharya 1 3
1Argonne National Laboratory Argonne USA2National Institute of Standards and Technology Gaithersburg USA3Argonne National Laboratory Argonne USAShow Abstract
The interlayer exchange coupling between magnetic layers separated by non-magnetic spacers can give rise to new spin structures that are distinct from the bulk constituents. In this work, we investigate the non-collinear spin structures that arise in superlattices containing paramagnetic LaNiO3 and ferromagnetic La0.7Sr0.3MnO3. Using ozone-assisted molecular beam epitaxy, we have fabricated a series of (LaNiO3)n/(La0.7Sr0.3MnO3)9 superlattices where n is varied from 1 to 9 unit cells on (001) SrTiO3 and LSAT substrates. The total superlattice thickness is maintained at 60 nm by varying the number of superlattice repetitions. The magnetic structure of several superlattices was measured using polarized neutron reflectometry as a function of temperature and applied magnetic field. For low fields, we find the magnetization of neighboring La0.7Sr0.3MnO3 layers to be non-collinear due to an antiferromagnetic interlayer exchange coupling, which persists to temperatures above 250 K. In the low-field regime we observe positive magnetoresistance using in-plane transport, which competes with the negative magnetoresistance of La0.7Sr0.3MnO3 at high fields. We discuss underlying mechanisms for the observed effects and possible applications to oxide-based magnetoresistive devices.
5:30 AM - P2.10
Modulating Electronic Transport in the Rare-Earth Nickelates Using a Ferroelectric
Matthew Marshall 1 Andrei Malashevich 1 Ankit Disa 1 Hanghui Chen 1 2 Sohrab Ismail-Beigi 1 Fred Walker 1 Charles Ahn 1
1Yale University New Haven USA2Columbia University New York City USAShow Abstract
The rare-earth perovskite nickelates (RNiO3) have a strong coupling between structure and electronic transport. The nickelates accommodate changes in the unit cell volume by elongating and shortening the in-plane Ni-O bond lengths and bending or flattening the in-plane Ni-O-Ni bond angles. Because transport in RNiO3 depends on these structural parameters, tuning the structural parameters affects electronic transport. The unit cell volume can be changed by applying pressure or by varying the radius of the rare earth ion. For instance, LaNiO3 is a paramagnetic metal in the bulk, while the other rare earth nickelates undergo a metal-insulator transition as the radius of the rare earth ion is changed. Structural parameters in the nickelates have been successfully tuned by layering in superlattices and with epitaxial strain. These are static effects; in order to dynamically tune electronic transport in RNiO3, we use the ferroelectric PbZr0.2TiO3 (PZT) to couple the polarization of the PZT to the electronic transport of RNiO3 films. Switching the polarization of the ferroelectric from up to down has a dramatic effect on electronic transport in the RNiO3, and can be understood within the context of charge and structure.
5:45 AM - P2.11
The Effect of Biaxial Strain on Octahedral Rotation in Rare-Earth Nickelate Superlattices
Patrick M. McBride 1 Cyrus E. Dreyer 1 Anderson Janotti 1 Chris G. Van de Walle 1
1University of California, Santa Barbara Santa Barbara USAShow Abstract
Heterostructures of complex oxides have attracted great attention for the interplay between structure, electronic, and magnetic properties, offering unique opportunities in device applications. Here we investigate the effects of epitaxial strain on the structural properties of LaNiO3. We perform first-principles calculations based on density functional theory to investigate the NiO6 octahedral tilts of strained LaNiO3 layers in LaNiO3/SrTiO3 superlattices. Recent experimental results suggest that octahedral connectivity at LaNiO3/SrTiO3 interfaces determines the local structure, but the effect of epitaxial strain on the LaNiO3 layers has remained unclear. We present quantitative results for the octahedral tilt angles as a function of both biaxial strain and distance from the substrate for LaNiO3 grown on SrTiO3 (001). Our results indicate that LaNiO3 exhibits vanishing octahedral tilt angles under certain strain conditions, a finding that holds important consequences for its electronic properties.
P1: Strongly Correlated Thin Films I
Kirk H. Bevan
T. Zac Ward
Monday AM, December 02, 2013
Hynes, Level 2, Room 201
9:30 AM - P1.01
Creating Room Temperature Multifunctionality through BiFeO3/Manganite Perovskite Superlattices
Judith L. MacManus-Driscoll 1 Eun-Mi Choi 1 T. Fix 1 S. Singh 2 J. Xiong 2 J. X. Zhu 2 Z. Bi 2 M. Fitzsimmons 2 H. Wang 3 Quanxi Jia 2
1University of Cambridge Cambridge United Kingdom2LANL Los Alamos USA3Texas Aamp;M University College Station USAShow Abstract
Understanding interface in complex oxide heterostructure is scientifically and technologically important since interfaces and surfaces determine the physical properties through coupled competing order parameters (magnetic, charge, orbital, etc.). In this talk, we present two types of BiFeO3 related superlattice (SL) systems, BiFeO3/BiMnO3 (BFO/BMO) and La0.7Sr0.3MnO3/BiFeO3 (LSMO/BFO). The structure of SL-BFO/BMO films is equivalent to 90° rotated nanoscale checkerboard (NCB) BFO/BMO. The film properties are in remarkable agreement with the theoretical predictions of a TC = ~ 407 K for NCB-BFO/BMO. Our experimental results provide an explanation of the emergence of magnetism in complex oxide heterostructures and show how to create a room temperature multiferroic. In SL-LSMO/BFO, the BFO layer showed the magnetization of ~ 75 ± 25 kA/m at 10 K. By ab initio calculation based on the density functional theory, SL-LSMO/BFO has much larger interfacial ferromagnetism of > ~ 0.3 mu;B/Fe than the canted moment 0.03 mu;B/Fe in the bulk BiFeO3. SL-BFO/BMO showed a high magnetic transition temperature (TC ~ 400 K).
9:45 AM - P1.02
Probing Strain Modulated Electronic Structure in Mixed Phase Bismuth Ferrite
Jeffery Aguiar 1 Sundaram Sankar Krishnan 2 Quentin Ramasse 3 Dmitry Kepaptsoglou 2 Wen-I Liang 4 Nigel Browning 5 Ying-Hao Chu 4 Paul Munroe 2 Nagarajan Valanoor 2
1Los Alamos National Laboratory Los Alamos USA2University of New South Wales Syndey Australia3SuperSTEM Laboratory Daresbury United Kingdom4National Chiao Tung University Taiwan Taiwan5Pacific Northwest National Laboratory Richland USAShow Abstract
Oxide heterostructures play an important role in the design of next generation functional information storage and electronic devices. Given the complexities of these oxide heterointerfaces, studying their structure and chemistry at the atomic scale and providing vital information relating to their valence and electronic structure is pivotal to explain the observed properties, such as their electromechanical behavior and related magnetic ordering. A combination of scanning transmission electron microscopy and density functional theory is used to probe mixed phase bismuth ferrite and reveals systematic changes in electronic structure across a phase boundary in the film.
Techniques such as X-ray absorption spectroscopy (XAS) and near-edge X-ray fine absorption spectra (NEXAFS) typically detect minute spectral changes, but not well suited to investigate atomic scale interfacial phenomena. Conversely, scanning transmission electron microscopy (STEM) and STEM-based electron energy loss spectroscopy (EELS) is one method that is able to provide the spatial distribution of electronic phenomena at nanostructured oxide interfaces with single atom and vacancy sensitivity. The unique combination of STEM and density functional theory (DFT) computations further provides valuable insight into the electronic structure controlling functional responses of oxide heterostructures. In order to address the effect of strain-induced configurations at the interface and beyond, however, a progressive study traversing the boundary layer is still lacking and must be conducted.
Here we combine aberration-corrected STEM with sub-Ångstrom resolution EELS and DFT methods to reveal strain modulated electronic structure and bonding perturbations in mixed phase bismuth ferrite thin films. In contrast to previous works, we focus on the incremental spectral transitions in the electronic signatures across the boundary layer to explain the interfacial electronic structure and the concomitant role of strain induced structure on the observed properties. High angle annular dark field (HAADF) and EELS chemical mapping is utilized to observe the atomic structure. Non-linear least squares (NLSS) spectral peak fitting and DFT further confirm these observed changes are attributed to changes in bonding environment surrounding the central iron cation and result in a change in electronic structure. Furthermore, DFT analyses suggest the same changes are attributed to a breakage in the structural symmetry across the boundary due to the simultaneous presence of increasing epitaxial strain and off axial symmetry in the T phase within the square-pyramidal oxygen cages. We expect the results presented to have a significant impact on the fundamental approach to and understanding of the effect of epitaxial strain on the resultant ferroelectric, piezoelectric coefficients, and complex phase equilibria in multiferroics.
10:00 AM - *P1.03
Coupling Magnetism to Electricity in Multiferroic Heterostructures
Ramamoorthy Ramesh 1 John T. Heron 2
1University of California, Berkeley Berkeley USA2Cornell University Ithaca USAShow Abstract
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. Over the past decade we have been exploring the science of such materials (for example, colossal magnetoresistance, ferroelectricity, etc) 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. Using our work in the field of ferroelectric(FE) and ferromagnetic oxides as the background, we are now exploring such materials, as epitaxial thin films as well as nanostructures. Specifically, we are studying the role of thin film growth, heteroepitaxy and processing on the basic properties as well as magnitude of the coupling between the order parameters. In our work we are exploring the switchability of the antiferromagnetic order using this coupling.
What is the importance of this work ? Antiferromagnets(AFM) are pervasive in the recording industry. They are used as exchange biasing layers in MTJ&’s etc. However, to date there has been no antiferomagnet that is electrically tunable. We believe that the multiferroic BiFeO3 is one compound where this can be observed at room temperature. The next step is to explore the coupling of a ferromagnet to this antiferromagnet through the exchange biasing concept. Ultimately, this will give us the opportunity to switch the magnetic state in a ferromagnet( and therefore the spin polarization direction) by simply applying an electric field to the underlying antiferromagnetic ferroelectric. In this talk, I will describe our progress to date on this exciting possibility.
10:30 AM - P1.04
Local Conductivity at Domain and Phase Boundaries in Supertetragonal BiFeO3 Thin Films
Saeedeh Farokhipoor 1 Christianne Beekman 2 Wolter Simons 2 Beatriz Noheda 1 Hans M. Christen 2
1Zernike Institute for Advance Materials, University of Groningen Groningen Netherlands2Materials Science and Technology Division, Oak Ridge National Laboratory Oak Ridge USAShow Abstract
In recent years, engineering and characterization of domain walls has developed with a view to future nanoscale device applications. One of the pivotal physical properties of domain walls is enhanced conduction, which has been extensively studied in different types of oxides in the last few years. In ferroelastic materials the domain walls are known to attract oxygen vacancies [E.K.H Salje et al., ChemPhysChem. (2010)]. It has been pointed out [S. Farokhipoor & B. Noheda PRL (2011)] that in BiFeO3 the migration of oxygen vacancies to the walls can lower the electronic injection barrier at the interface with the metal and locally enhance conduction. Here we have investigated highly strained BiFeO3 thin films grown on LaAlO3 substrates [C. Beekman et al., Adv. Mater (2013)]. These films consist of a stripe-like coexistence of two polymorphs that are each different from BiFeO3&’s bulk phase, leading to an interesting strain landscape across the film. By investigating the local conduction at different types of domain walls and phase boundaries by means of scanning probe techniques, we find a clear correlation between the strain state, the oxygen vacancy concentration and the local conductivity.
10:45 AM - P1.05
Coupling between Electron Transport and Ferroelectric Polarization in Domain Engineered BiFeO3 Epitaxial Films
Linghan Ye 1 Alexandra Merkouriou 1 Jon Ihlefeld 2 Carolina Adamo 3 Darrell Schlom 3 Ramamoorthy Ramesh 4 Bryan D. Huey 1
1Institute of Material Sciences Storrs USA2Sandia National Laboratories Albuquerque USA3Cornell University Ithaca USA4University of California, Berkeley Berkeley USAShow Abstract
The domain structure of ferroelectric and multiferroic materials can have a significant effect on electron transport properties. Piezo Force Microscopy allows unique investigations of such nanoscale effects, and can further be implemented to monitor domain switching dynamics. BiFeO3 thin films engineered with distinct strains and interfacial configurations are thereby shown to influence the ferroelectric domain distributions, with up to 50 mu;m of domain boundary per mu;m2. Coupling these results with conductive AFM, correlations between such domain configurations and through-film electron transport are comprehensively studied, identifying conditions for enhancing devices coupling ferroelectric polarization and conductivity. Characteristic switching processes are demonstrated for distinct specimen conditions as well, including nucleation times and growth velocities, domain wall densities at discrete steps during the switching process, and the distribution of 71°, 109°, and 180° initial and ultimate polarization reorientations. These results provide unique insight into the behavior of ferroelectric domains at the nanoscale for interfacial engineering of optimally performing electron transport devices.
11:30 AM - P1.06
Thickness-Dependent Paraelectric-Ferroelectric Phase Transition in La-Doped BiFeO3 Films on Si Substrates
Deyang Chen 1 2 Xiaohong Zhu 1 3 Christopher Tobias Nelson 1 4 Ya Gao 1 5 Claudy Rayan Serrao 1 Di Yi 1 James David Clarkson 1 Jian Liu 1 Ramamoorthy Ramesh 1 4
1University of Carlifornia, Berkeley Berkeley USA2South China University of Technology Guangzhou China3Sichuan University Chengdu China4Lawrence Berkeley National Laboratory Berkeley USA5Tinghua University Beijing ChinaShow Abstract
Chemical substitution is widely used to induce phase transition in BiFeO3 films. Our previous research discovered the phase transition from rhombohedral phase to orthorhombic phase through La doping in BiFeO3 films. Here we report a thickness-dependent paraelectric-ferroelectric phase transition in La-doped BiFeO3 films on Si substrates, opening up a new path to achieving phase transition. Epitaxial 20% La doping BiFeO3 films (LBFO20) with thickness of 10 to 100 nm were grown on (001) SrTiO3-buffered Si substrates with SRO as bottom electrodes (~20 nm) by pulsed laser deposition. X-ray Diffraction (XRD) results show the peak splitting in 60 and 100 nm LBFO20 films, but there are no peak splitting in 10 to 40 nm films, which demonstrates the paraelectric orthorhombic phase to ferroelectric rhombohedral phase transition. Piezoresponse force microscopy (PFM) and piezoelectric property measurement results are consistently with XRD data. Using PFM, we find that 100nm LBFO20 film cannot be switched with -15 V dc bias and 60nm film can be switched slightly, whereas LBFO20 films with thickness from 10 to 40 nm can be switched completely with -8 v dc bias, which further reveals the paraelectric-ferroelectric phase transition with decreasing thickness of films. Transmission electron microscopy (TEM) shows that lots of orthorhombic phase exists in 100 nm LBFO20 film, however, in 20 nm film, there is almost entirely made up of ferroelectric rhombohedral phase. Orthorhombic and rhombohedral phase boundary is also discovered in the films. In summary, our finding provides a new path to drive the paraelectric-ferroelectric phase transition and also offers a possible route to study morphotropic phase boundary (MPB).
11:45 AM - P1.07
Flexoelectric Effect in the Reversal of Self-Polarization and Associated Changes in the Electronic Functional Properties of BiFeO3 Thin Films
Byung Chul Jeon 1 2 Daesu Lee 1 2 Sangdon Bu 4 Mayng Hwan Lee 3 Sang Mo Yang 1 2 Seung Chul Chae 1 2 Tae Kwon Song 3 Jin-Seok Chung 5 Jong-Gul Yoon 6 Tae Won Noh 1 2
1Center for Functional Interfaces of Correlated Electron Systems, Institute for Basic Science (IBS) Seoul Republic of Korea2Department of Physics and Astronomy, Seoul National University Seoul Republic of Korea3School of Nano and Advanced Materials Engineering, Changwon National University Changwon Republic of Korea4Department of Physics, Chonbuk National University Jeonju Republic of Korea5Department of Physics, Soongsil University Seoul Republic of Korea6Department of Physics, University of Suwon Hwaseong Republic of KoreaShow Abstract
Flexoelectric effect is the generation of an electric field by a strain gradient via electro-mechanical coupling. This effect was predicted theoretically by Kogan in 1964 and experimentally observed by Bursian and Zaikovskii in 1968. The phenomenon was given the name ‘flexoelectricity&’ by Indenbom et al. in 1981. However, there have been few studies on flexoelectric effects in solids. For bulk solids, it was believed that flexoelectric effects should be minimal. However, it was recently shown that the strain gradient in nano-structured materials and/or epitaxial oxide thin films could be 6 or 7 orders of magnitude larger than the corresponding bulk values. After this realization, there has been increased interest in this phenomenon. These electromechanical coupling effects have provided answers to many physical phenomena that could not be explained before.
In this presentation, we will address that fully strained BiFeO3 films are self-poled, having a down ward polarization; this indicated that the interfacial effect is dominant. In contrast, the relaxed films have upward self-polarization, indicating that the flexoelectric effect is dominant. Moreover, quantification of the flexoelectric fields are estimated, indicating that the flexoelectric field in relaxed BFO is higher about one order of magnitude, compared with that of the uniaxially strained BFO film. Interestingly enough, the two kinds of films also exhibit different unidirectional current flows, referred to as the diode effect. By understanding the self-poling mechanisms in BiFeO3 films, such as ferroelectric hysteresis and electronic transport characteristics, the configuration of the as-grown films can be optimized to allow full utilization of the ferroelectric functional device.
12:00 PM - *P1.08
Benchmarking Spintronic Devices Based on Magnetoelectric Oxides
Dmitri E. Nikonov 1 Ian A. Young 1 Sasikanth Manipatruni 1
1Intel Corp. Hillsboro USAShow Abstract
As CMOS transistors are scaled towards atomistic sizes, the research is underway to find beyond-CMOS logic devices which would complement CMOS in futures computational circuits. Majority of currently considered logic gates include spintronic devices, i.e. ones involving ferromagnets. With the currently available switching methods based on passing current through these devices - spin torque and spin Hall effects - these gates are expected to switch with longer time and larger energy than comparable electronic gates. Recently, switching of magnetization by application of voltage was demonstrated via magnetoelectric effects relying on interfaces of ferromagnets with oxides. We simulate a variety of spintronic gates switched with both spin torque and magnetoelectric effects. Various cases of switching are considered, values of magnetoelectric coefficients necessary for switching are determined. We show that only magnetoelectric switching results in time and energy comparable to those of CMOS devices.
12:30 PM - P1.09
Different Polarization Dependence of Transient Current in Polycrystalline and Epitaxial Thin Pb(Zr,Ti)O3 Films
Liubov Delimova 1 Ekaterina Gushchina 2 Valentin Yuferev 1 Igor Grekhov 1
1Ioffe Institute of the RAS St. Petersburg Russian Federation2Ioffe Institute of the RAS St. Petersburg Russian FederationShow Abstract
Conduction mechanisms in thin ferroelectric Pb(Zr,Ti)O3(PZT) films are controlled to a great extent by the film/electrode interfaces. In polycrystalline PZT films with heterophase grain boundaries coalescence of grain polarization charge and semiconducting grain boundaries can trigger such transport effects as intergrain photovoltaic effect  and clockwise current hysteresis .
We use direct current-voltage measurements and Scanning Spreading Resistance Microscopy to study transport characteristics of thin PZT films depending on its polarization. In direct current-voltage measurements a bias voltage is applied to the structure as a sequence of steps of the same amplitude and duration. The voltage rise rate is changed within the range of 1-0.001 V/s by variation of the step amplitude and duration. The current measurements are done in preliminary poled films.
The both experimental techniques, we use to study the polycrystalline (111) textured 100-nm-thick PZT film deposited on PbTiO3/Ir/SiO2/Si substrate and epitaxial (001) oriented 210-nm-thick PZT film deposited on SrRuO3/SrTiO3 substrate, revealed the same results at micro and nanoscale ranges. A current response from PZT film to applied bias contains a long relaxation component, which is shown to depend on voltage rise rate and the polarization direction. However the current dependence on the polarization is found to be completely different for the polycrystalline and epitaxial films. The current of polycrystalline film is much larger when the bias is directed against the polarization, which can be associated with screening of polarization charge by traps charge on grain boundaries with bias variation . While the current of epitaxial films is larger if the bias direction coincides with the polarization, which cannot be explained by ferroelectric switching. We consider the measured current of the epitaxial film as the transient current caused by hole&’s capture at or electron&’s emission from traps level. The electron&’s emission probability is controlled by the impurity potential, which becomes asymmetrical in the presence of spontaneous polarization: the probability of electron's emission in the polarization direction is larger than that against the polarization. Since the electron moves again the external bias, the more favorable condition for quick electron emission and the current relaxation takes place when the bias is directed against the polarization. Therefore, the larger current value measured when the bias and polarization directions are coincides reflects the slower emission of electron and slower current relaxation. The clockwise current hysteresis is observed at any preliminary polarization of the both polycrystalline and epitaxial films. The model, which allows us to estimate characteristics of the transient current, is proposed.
 IEEE-TUFFC 58, pp. 2252-2258 (2011).
 MRS Proc. 1292, mrsf10-1292-k03-31(2011).
12:45 PM - P1.10
Room Temperature Negative Capacitance in Ferroelectric-Dielectric Heterostructure
Weiwei Gao 1 Asif Khan 1 Jayakanth Ravichandran 2 Long You 1 Chun Wing Yeung 1 Chenming Hu 1 Ramamoorthy Ramesh 3 4 5 Sayeef Salahuddin 1
1University of California, Berkeley Berkeley USA2University of California, Berkeley Berkeley USA3University of California, Berkeley Berkeley USA4University of California, Berkeley Berkeley USA5Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
As a novel physical concept, Ferroelectric (FE) negative capacitance (NC) can be effectively utilized to reduce the sub-threshold swing (SS) of ultra-low power MOSFET applications below the fundamental physical limit of 60 mV/decade . According to Salahuddin et al., the negative capacitance state of a ferroelectric capacitor is an unstable non-equilibrium state and can be effectively stabilized under static conditions if it is placed in series with dielectric capacitor. Recently, the experimental demonstrations of ferroelectric negative capacitance phenomenon have been reported by several research groups [2, 3]. However, the negative capacitance effect is only observed in a certain high temperature range, which closely depends on the Curie temperature of ferroelectric materials. In order to be compatible with MOSFET applications, it is necessary to look for proper ferroelectric materials, which can provide negative capacitance effect at the MOSFET operating temperature. In this talk, we will introduce the room temperature negative capacitance effect in LaAlO3/Ba0.8Sr0.2TiO3 (LAO/BSTO) superlattice heterostructure.
The artificial superlattices, [(LAO)m/(BSTO)n]11 (m=6, 12, and 18; n=18, 12, and 6) were grown on the SrRuO3-covered (001)-SrTiO3 (STO), (110)-GdScO3 (GSO) and (110)-DyScO3 (DSO) substrates by Laser-Molecular Beam Epitaxy technique. During the deposition, the substrate temperature was kept at 750 °C for the (LAO)m/(BSTO)n superlattice, 700 °C for the SrRuO3 and the oxygen pressure at 50 mTorr. The capacitance of superlattices were measured by 4194A Impedance/Gain-Phase Analyzer with frequency up to 1 MHz at room temperature. These superlattices show an enhanced capacitance compared to single dielectric LAO film of the same thickness equal to the total thickness in the superlattice. This means that the capacitance of the superlattice has gone up, rather than going down, even after adding additional layers to LAO and thereby increasing the total thickness of the overall capacitance. This is the first observation of room temperature negative capacitance effect in ferroelectric/dielectric heterostructures at room temperature. The effect of lattice strain, FE thickness etc. on the negative capacitance will be discussed in this talk.