Interplay of octahedral tilts and polar order in BiFeO3 films

Ultrafast Optically Induced Perturbation of Oxygen Octahedral Rotations in Multiferroic BiFeO3 Thin Films

The functional properties of complex oxides, including magnetism and ferroelectricity, are closely linked to subtle structural distortions. Ultrafast optical excitations provide the means to manipulate structural features and ultimately to affect the functional properties of complex oxides with picosecond-scale precision. We report that the lattice expansion of multiferroic BiFeO3 following above-bandgap optical excitation leads to distortion of the oxygen octahedral rotation (OOR) pattern. The continuous coupling between OOR and strain was probed using time-resolved X-ray free-electron laser diffraction with femtosecond time resolution. Density functional theory calculations predict a relationship between the OOR and the elastic strain consistent with the experiment, demonstrating a route to employing this approach in a wider range of systems. Ultrafast control of the functional properties of BiFeO3 thin films is enabled by this approach because the OOR phenomena are related to ferroelectricity, and via the Fe-O-Fe bond angles, the superexchange interaction between Fe atoms.

Extended Oxygen Octahedral Tilt Proximity near Oxide Heterostructures

The oxide interfaces between materials with different structural symmetries have been actively studied due to their novel physical properties. However, the investigation of intriguing interfacial phenomena caused by the oxygen octahedral tilt (OOT) proximity effect has not been fully exploited, as there is still no clear understanding of what determines the proximity length and what the underlying control mechanism is. Here, we achieved scalability of the OOT proximity effect in SrRuO₃ (SRO) by epitaxial strain near the SRO/SrTiO₃ heterointerface. We demonstrated that the OOT proximity length scale of SRO is extended from 4 unit cells to 14 unit cells by employing advanced scanning transmission electron microscopy. We also suggest that this variation may originate from changes in phonon dispersions due to electron-phonon coupling in SRO. This study will provide in-depth insights into the structural gradients of correlated systems and facilitate potential device applications.

Automated Experiment in 4D-STEM: Exploring Emergent Physics and Structural Behaviors

Automated experiments in 4D scanning transmission electron microscopy (STEM) are implemented for rapid discovery of local structures, symmetry-breaking distortions, and internal electric and magnetic fields in complex materials. Deep kernel learning enables active learning of the relationship between local structure and 4D-STEM-based descriptors. With this, efficient and "intelligent" probing of dissimilar structural elements to discover desired physical functionality is made possible. This approach allows effective navigation of the sample in an automated fashion guided by either a predetermined physical phenomenon, such as strongest electric field magnitude, or in an exploratory fashion. We verify the approach first on preacquired 4D-STEM data and further implement it experimentally on an operational STEM. The experimental discovery workflow is demonstrated using graphene and subsequently extended toward a lesser-known layered 2D van der Waals material, MnPS₃. This approach establishes a pathway for physics-driven automated 4D-STEM experiments that enable probing the physics of strongly correlated systems and quantum materials and devices, as well as exploration of beam-sensitive materials.

Strain and orientation engineering in ABO3perovskite oxide thin films

Perovskite oxides with chemical formula ABO₃are widely studied for their properties including ferroelectricity, magnetism, strongly correlated physics, optical effects, and superconductivity. A thriving research direction using such materials is through their integration as epitaxial thin films, allowing many novel and exotic effects to be discovered. The integration of the thin film on a single crystal substrate, however, can produce unique and powerful effects, and can even induce phases in the thin film that are not stable in bulk. The substrate imposed mechanical boundary conditions such as strain, crystallographic orientation, octahedral rotation patterns, and symmetry can also affect the functional properties of perovskite films. Here, the author reviews the current state of the art in epitaxial strain and orientation engineering in perovskite oxide thin films. The paper begins by introducing the effect of uniform conventional biaxial strain, and then moves to describe how the substrate crystallographic orientation can induce symmetry changes in the film materials. Various material case studies, including ferroelectrics, magnetically ordered materials, and nonlinear optical oxides are covered. The connectivity of the oxygen octahedra between film and substrate depending on the strain level as well as the crystallographic orientation is then discussed. The review concludes with open questions and suggestions worthy of the community's focus in the future.

Seeing Structural Mechanisms of Optimized Piezoelectric and Thermoelectric Bulk Materials through Structural Defect Engineering

Aberration-corrected scanning transmission electron microscopy (AC-STEM) has evolved into the most powerful characterization and manufacturing platform for all materials, especially functional materials with complex structural characteristics that respond dynamically to external fields. It has become possible to directly observe and tune all kinds of defects, including those at the crucial atomic scale. In-depth understanding and technically tailoring structural defects will be of great significance for revealing the structure-performance relation of existing high-property materials, as well as for foreseeing paths to the design of high-performance materials. Insights would be gained from piezoelectrics and thermoelectrics, two representative functional materials. A general strategy is highlighted for optimizing these functional materials’ properties, namely defect engineering at the atomic scale.

Exotic Long-Range Surface Reconstruction on La0.7Sr0.3MnO3 Thin Films

Due to an extremely diverse phase space, La_1-xSr_xMnO₃, as with other manganites, offers a wide range of tunability and applications including colossal magnetoresistance and use as spin-polarized electrodes. Here, we study an unprecedented, exotic surface reconstruction (6 × 6) in La_1-xSr_xMnO₃ (x = 0.3) observed via low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) shows the surface is relatively flat, with unit-cell step heights, and X-ray photoelectron spectroscopy (XPS) reveals a strong degree of Sr segregation at the surface. By combining electron diffraction and first-principles computations, we propose that the long-range surface reconstruction consists of a Sr-segregated surface with La (6 × 6) ordering. This study expands our understanding of manganite systems and underscores their ability to form interesting surface reconstructions, driven largely by cation segregation that can potentially be controlled for tuning surface ordering.

Exploring physics of ferroelectric domain walls via Bayesian analysis of atomically resolved STEM data

The physics of ferroelectric domain walls is explored using the Bayesian inference analysis of atomically resolved STEM data. We demonstrate that domain wall profile shapes are ultimately sensitive to the nature of the order parameter in the material, including the functional form of Ginzburg-Landau-Devonshire expansion, and numerical value of the corresponding parameters. The preexisting materials knowledge naturally folds in the Bayesian framework in the form of prior distributions, with the different order parameters forming competing (or hierarchical) models. Here, we explore the physics of the ferroelectric domain walls in BiFeO₃ using this method, and derive the posterior estimates of relevant parameters. More generally, this inference approach both allows learning materials physics from experimental data with associated uncertainty quantification, and establishing guidelines for instrumental development answering questions on what resolution and information limits are necessary for reliable observation of specific physical mechanisms of interest.

Ferroelectric domain wall profiles can be modeled by phenomenological Ginzburg-Landau theory, with different candidate models and parameters. Here, the authors solve the problem of model selection by developing a Bayesian inference framework allowing for uncertainty quantification and apply it to atomically resolved images of walls. This analysis can also predict the level of microscope performance needed to detect specific physical phenomena.

Spin dynamics, antiferrodistortion and magnetoelectric interaction in multiferroics. The case of BiFeO3

We present a theoretical study of the spin dynamics in perovskite-like multiferroics with homogeneous magnetic order in the presence of external magnetic and electric fields. A particular example of such material is BeFeO3 in which the spin cycloid can be suppressed by application of external magnetic field, doping or by epitaxial strain. Understanding the effect of the external electric field on the spin-wave spectrum of these systems is required for devices based on spin wave interference and other innovative advances of magnonics and spintronics. Thus, we propose a model for BiFeO3 in which the thermodynamic potential is expressed in terms of polarization \boldsymbol{P}, antiferrodistortion \boldsymbol{\Omega}, antiferromagnetic moment \boldsymbol{L} and magnetization \boldsymbol{M}. Based on this model, we derive the corresponding equations of motion and demonstrate the existence of electromagnons, that is, magnons that can be excited by electric fields. These excitations are closely related to the magnetoelectric effect and the dynamics of the antiferrodistortion \boldsymbol{\Omega}. Specifically, the influence of the external electric field on the magnon spectra is due to reorientation of both polarization \boldsymbol{P} and antiferrodistortion \boldsymbol{\Omega} under the influence of the electric field and is linked to emergence of a field-induced anisotropy.

Coupling Lattice Instabilities Across the Interface in Ultrathin Oxide Heterostructures

Oxide heterointerfaces constitute a rich platform for realizing novel functionalities in condensed matter. A key aspect is the strong link between structural and electronic properties, which can be modified by interfacing materials with distinct lattice symmetries. Here, we determine the effect of the cubic-tetragonal distortion of SrTiO3 on the electronic properties of thin films of SrIrO3, a topological crystalline metal hosting a delicate interplay between spin-orbit coupling and electronic correlations. We demonstrate that below the transition temperature at 105 K, SrIrO3 orthorhombic domains couple directly to tetragonal domains in SrTiO3. This forces the in-phase rotational axis to lie in-plane and creates a binary domain structure in the SrIrO3 film. The close proximity to the metal-insulator transition in ultrathin SrIrO3 causes the individual domains to have strongly anisotropic transport properties, driven by a reduction of bandwidth along the in-phase axis. The strong structure-property relationships in perovskites make these compounds particularly suitable for static and dynamic coupling at interfaces, providing a promising route towards realizing novel functionalities in oxide heterostructures.

Polarization Screening Mechanisms at La0.7Sr0.3MnO3-PbTiO3 Interfaces

The structural, electronic, and magnetic properties of interfaces between epitaxial La_0.7Sr_0.3MnO₃ and PbTiO₃ have been explored via atomic resolution transmission electron microscopy of a functional multiferroic tunnel junction. Measurements of the polar displacements and octahedral tilting show the competition between the two distortions at the interface and demonstrate strong dependence on the polarization orientation. The density functional theory provides information on the electronic and magnetic properties, where the interface termination plays a crucial role in the screening mechanisms.

Progress in BiFeO3-based heterostructures: materials, properties and applications

BiFeO3-based heterostructures have attracted much attention for potential applications due to their room-temperature multiferroic properties, proper band gaps and ultrahigh ferroelectric polarization of BiFeO3, such as data storage, optical utilization in visible light regions and synapse-like function. Here, this work aims to offer a systematic review on the progress of BiFeO3-based heterostructures. In the first part, the optical, electric, magnetic, and valley properties and their interactions in BiFeO3-based heterostructures are briefly reviewed. In the second part, the morphologies of BiFeO3 and medium materials in the heterostructures are discussed. Particularly, in the third part, the physical properties and underlying mechanism in BiFeO3-based heterostructures are discussed thoroughly, such as the photovoltaic effect, electric field control of magnetism, resistance switching, and two-dimensional electron gas and valley characteristics. The fourth part illustrates the applications of BiFeO3-based heterostructures based on the materials and physical properties discussed in the second and third parts. This review also includes a future prospect, which can provide guidance for exploring novel physical properties and designing multifunctional devices.

Feature extraction via similarity search: application to atom finding and denoising in electron and scanning probe microscopy imaging

We develop an algorithm for feature extraction based on structural similarity and demonstrate its application for atom and pattern finding in high-resolution electron and scanning probe microscopy images. The use of the combined local identifiers formed from an image subset and appended Fourier, or other transform, allows tuning selectivity to specific patterns based on the nature of the recognition task. The proposed algorithm is implemented in Pycroscopy, a community-driven scientific data analysis package, and is accessible through an interactive Jupyter notebook available on GitHub.

Surface-screening mechanisms in ferroelectric thin films and their effect on polarization dynamics and domain structures

For over 70 years, ferroelectric materials have been one of the central research topics for condensed matter physics and material science, an interest driven both by fundamental science and applications. However, ferroelectric surfaces, the key component of ferroelectric films and nanostructures, still present a significant theoretical and even conceptual challenge. Indeed, stability of ferroelectric phase per se necessitates screening of polarization charge. At surfaces, this can lead to coupling between ferroelectric and semiconducting properties of material, or with surface (electro) chemistry, going well beyond classical models applicable for ferroelectric interfaces. In this review, we summarize recent studies of surface-screening phenomena in ferroelectrics. We provide a brief overview of the historical understanding of the physics of ferroelectric surfaces, and existing theoretical models that both introduce screening mechanisms and explore the relationship between screening and relevant aspects of ferroelectric functionalities starting from phase stability itself. Given that the majority of ferroelectrics exist in multiple-domain states, we focus on local studies of screening phenomena using scanning probe microscopy techniques. We discuss recent studies of static and dynamic phenomena on ferroelectric surfaces, as well as phenomena observed under lateral transport, light, chemical, and pressure stimuli. We also note that the need for ionic screening renders polarization switching a coupled physical-electrochemical process and discuss the non-trivial phenomena such as chaotic behavior during domain switching that stem from this.

Atomically Resolved Electronic States and Correlated Magnetic Order at Termination Engineered Complex Oxide Heterointerfaces

We map electronic states, band gaps, and interface-bound charges at termination-engineered BiFeO₃/La_0.7Sr_0.3MnO₃ interfaces using atomically resolved cross-sectional scanning tunneling microscopy. We identify a delicate interplay of different correlated physical effects and relate these to the ferroelectric and magnetic interface properties tuned by engineering the atomic layer stacking sequence at the interfaces. This study highlights the importance of a direct atomically resolved access to electronic interface states for understanding the intriguing interface properties in complex oxides.

Effects of the Hubbard U on density functional-based predictions of BiFeO3 properties

First principles studies of multiferroic materials, such as bismuth ferrite (BFO), require methods that extend beyond standard density functional theory (DFT). The DFT  +  U method is one such extension that is widely used in the study of BFO. We present a systematic study of the effects of the U parameter on the structural, ferroelectric and electronic properties of BFO. We find that the structural and ferroelectric properties change negligibly in the range of U typically considered for BFO (3-5 eV). In contrast, the electronic structure varies significantly with U. In particular, we see large changes to the character and curvature of the valence band maximum and conduction band minimum, in addition to the expected increase in band gap, as U increases. Most significantly, we find that the [Formula: see text]/[Formula: see text] ordering at the conduction band minimum inverts for U values larger than 4 eV. We therefore recommend a U value of at most 4 eV to be applied to the Fe d orbitals in BFO. More generally, this study emphasises the need for systematic investigations of the effects of the U parameter not merely on band gaps but on the electronic structure as a whole, especially for strongly correlated materials.

Knowledge Extraction from Atomically Resolved Images

Tremendous strides in experimental capabilities of scanning transmission electron microscopy and scanning tunneling microscopy (STM) over the past 30 years made atomically resolved imaging routine. However, consistent integration and use of atomically resolved data with generative models is unavailable, so information on local thermodynamics and other microscopic driving forces encoded in the observed atomic configurations remains hidden. Here, we present a framework based on statistical distance minimization to consistently utilize the information available from atomic configurations obtained from an atomically resolved image and extract meaningful physical interaction parameters. We illustrate the applicability of the framework on an STM image of a FeSe_xTe_1-x superconductor, with the segregation of the chalcogen atoms investigated using a nonideal interacting solid solution model. This universal method makes full use of the microscopic degrees of freedom sampled in an atomically resolved image and can be extended via Bayesian inference toward unbiased model selection with uncertainty quantification.

Melting of Oxygen Vacancy Order at Oxide-Heterostructure Interface

Modifications in oxygen coordination environments in heterostructures consisting of dissimilar oxides often emerge and lead to unusual properties of the constituent materials. Although lots of attention has been paid to slight modifications in the rigid oxygen octahedra of perovskite-based heterointerfaces, revealing the modification behaviors of the oxygen coordination environments in the heterostructures containing oxides with oxygen vacancies have been challenging. Here, we show that a significant modification in the oxygen coordination environments-melting of oxygen vacancy order-is induced at the heterointerface between SrFeO_2.5 (SFO) and DyScO₃ (DSO). When an oxygen-deficient perovskite (brownmillerite structure) SrFeO_2.5 film grows epitaxially on a perovskite DyScO₃ substrate, both FeO₆ octahedra and FeO₄ tetrahedra in the (101)-oriented SrFeO_2.5 thin film connect to ScO₆ octahedra in DyScO₃. As a consequence of accommodating a structural mismatch, the alternately ordered arrangement of oxygen vacancies is significantly disturbed and reconstructed in the 2 nm thick heterointerface region. The stabilized heterointerface structure consists of Fe³⁺ octahedra with an oxygen vacancy disorder. The melting of the oxygen vacancy order, which in bulk SrFeO_2.5 occurs at 1103 K, is induced at the present heterointerface at ambient temperatures.

Thermally Stable Sr2RuO4 Electrode for Oxide Heterostructures

The use of thermally stable Sr₂RuO₄ electrodes in high-temperature synthesis of oxide heterostructures was investigated. Atomically smooth Sr₂RuO₄ thin films were grown on SrTiO₃(001) substrates by pulsed laser deposition and used as a bottom electrode for ferroelectric BaTiO₃ capacitors grown at temperatures of up to 1000 °C. The thermal stability of Sr₂RuO₄ electrodes was verified by structural and electrical measurements of the ferroelectric BaTiO₃ films. The best growth temperature for the BaTiO₃ films was found to be 900 °C, exhibiting the largest spontaneous polarization, dielectric constant, and pyroelectric response. We conclude that Sr₂RuO₄ films are suitable for use as thermally stable electrodes in heterostructures synthesized at temperatures up to at least 1000 °C and oxygen pressures from 10^-6 to 10^-1 Torr. This range of growth film conditions is much wider than that for other common oxide electrode materials such as SrRuO₃, widening the available process window for optimizing the performance of oxide electronic devices.

Interface-induced multiferroism by design in complex oxide superlattices

Interfaces between materials present unique opportunities for the discovery of intriguing quantum phenomena. Here, we explore the possibility that, in the case of superlattices, if one of the layers is made ultrathin, unexpected properties can be induced between the two bracketing interfaces. We pursue this objective by combining advanced growth and characterization techniques with theoretical calculations. Using prototype La_2/3Sr_1/3MnO₃ (LSMO)/BaTiO₃ (BTO) superlattices, we observe a structural evolution in the LSMO layers as a function of thickness. Atomic-resolution EM and spectroscopy reveal an unusual polar structure phase in ultrathin LSMO at a critical thickness caused by interfacing with the adjacent BTO layers, which is confirmed by first principles calculations. Most important is the fact that this polar phase is accompanied by reemergent ferromagnetism, making this system a potential candidate for ultrathin ferroelectrics with ferromagnetic ordering. Monte Carlo simulations illustrate the important role of spin-lattice coupling in LSMO. These results open up a conceptually intriguing recipe for developing functional ultrathin materials via interface-induced spin-lattice coupling.

Quantitative comparison of bright field and annular bright field imaging modes for characterization of oxygen octahedral tilts

Octahedral tilt behavior is increasingly recognized as an important contributing factor to the physical behavior of perovskite oxide materials and especially their interfaces, necessitating the development of high-resolution methods of tilt mapping. There are currently two major approaches for quantitative imaging of tilts in scanning transmission electron microscopy (STEM), bright field (BF) and annular bright field (ABF). In this paper, we show that BF STEM can be reliably used for measurements of oxygen octahedral tilts. While optimal conditions for BF imaging are more restricted with respect to sample thickness and defocus, we find that BF imaging with an aberration-corrected microscope with the accelerating voltage of 300kV gives us the most accurate quantitative measurement of the oxygen column positions. Using the tilted perovskite structure of BiFeO₃ (BFO) as our test sample, we simulate BF and ABF images in a wide range of conditions, identifying the optimal imaging conditions for each mode. We show that unlike ABF imaging, BF imaging remains directly quantitatively interpretable for a wide range of the specimen mistilt, suggesting that it should be preferable to the ABF STEM imaging for quantitative structure determination.

Ultra-high resolution electron microscopy

The last two decades have seen dramatic advances in the resolution of the electron microscope brought about by the successful correction of lens aberrations that previously limited resolution for most of its history. We briefly review these advances, the achievement of sub-Ångstrom resolution and the ability to identify individual atoms, their bonding configurations and even their dynamics and diffusion pathways. We then present a review of the basic physics of electron scattering, lens aberrations and their correction, and an approximate imaging theory for thin crystals which provides physical insight into the various different imaging modes. Then we proceed to describe a more exact imaging theory starting from Yoshioka's formulation and covering full image simulation methods using Bloch waves, the multislice formulation and the frozen phonon/quantum excitation of phonons models. Delocalization of inelastic scattering has become an important limiting factor at atomic resolution. We therefore discuss this issue extensively, showing how the full-width-half-maximum is the appropriate measure for predicting image contrast, but the diameter containing 50% of the excitation is an important measure of the range of the interaction. These two measures can differ by a factor of 5, are not a simple function of binding energy, and full image simulations are required to match to experiment. The Z-dependence of annular dark field images is also discussed extensively, both for single atoms and for crystals, and we show that temporal incoherence must be included accurately if atomic species are to be identified through matching experimental intensities to simulations. Finally we mention a few promising directions for future investigation.

Nanoscale Origins of Ferroelastic Domain Wall Mobility in Ferroelectric Multilayers

The nanoscale origins of ferroelastic domain wall motion in ferroelectric multilayer thin films that lead to giant electromechanical responses are investigated. We present direct evidence for complex underpinning factors that result in ferroelastic domain wall mobility using a combination of atomic-level aberration corrected scanning transmission electron microscopy and phase-field simulations in model epitaxial (001) tetragonal (T) PbZr_xTi_1-xO₃ (PZT)/rhombohedral (R) PbZr_xTi_1-xO₃ (PZT) bilayer heterostructures. The local electric dipole distribution is imaged on an atomic scale for a ferroelastic domain wall that nucleates in the R-layer and cuts through the composition breaking the T/R interface. Our studies reveal a highly complex polarization rotation domain structure that is nearly on the knife-edge at the vicinity of this wall. Induced phases, namely tetragonal-like and rhombohedral-like monoclinic were observed close to the interface, and exotic domain arrangements, such as a half-4-fold closure structure, are observed. Phase field simulations show this is due to the minimization of the excessive elastic and electrostatic energies driven by the enormous strain gradient present at the location of the ferroelastic domain walls. Thus, in response to an applied stimulus, such as an electric field, any polarization reorientation must minimize the elastic and electrostatic discontinuities due to this strain gradient, which would induce a dramatic rearrangement of the domain structure. This insight into the origins of ferroelastic domain wall motion will allow researchers to better "craft" such multilayered ferroelectric systems with precisely tailored domain wall functionality and enhanced sensitivity, which can be exploited for the next generation of integrated piezoelectric technologies.

Directing Matter: Toward Atomic-Scale 3D Nanofabrication

Enabling memristive, neuromorphic, and quantum-based computing as well as efficient mainstream energy storage and conversion technologies requires the next generation of materials customized at the atomic scale. This requires full control of atomic arrangement and bonding in three dimensions. The last two decades witnessed substantial industrial, academic, and government research efforts directed toward this goal through various lithographies and scanning-probe-based methods. These technologies emphasize 2D surface structures, with some limited 3D capability. Recently, a range of focused electron- and ion-based methods have demonstrated compelling alternative pathways to achieving atomically precise manufacturing of 3D structures in solids, liquids, and at interfaces. Electron and ion microscopies offer a platform that can simultaneously observe dynamic and static structures at the nano- and atomic scales and also induce structural rearrangements and chemical transformation. The addition of predictive modeling or rapid image analytics and feedback enables guiding these in a controlled manner. Here, we review the recent results that used focused electron and ion beams to create free-standing nanoscale 3D structures, radiolysis, and the fabrication potential with liquid precursors, epitaxial crystallization of amorphous oxides with atomic layer precision, as well as visualization and control of individual dopant motion within a 3D crystal lattice. These works lay the foundation for approaches to directing nanoscale level architectures and offer a potential roadmap to full 3D atomic control in materials. In this paper, we lay out the gaps that currently constrain the processing range of these platforms, reflect on indirect requirements, such as the integration of large-scale data analysis with theory, and discuss future prospects of these technologies.

Extraction of structural and chemical information from high angle annular dark-field image by an improved peaks finding method

With the development of spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM), high angle annular dark filed (HAADF) imaging technique has been widely applied in the microstructure characterization of various advanced materials with atomic resolution. However, current qualitative interpretation of the HAADF image is not enough to extract all the useful information. Here a modified peaks finding method was proposed to quantify the HAADF-STEM image to extract structural and chemical information. Firstly, an automatic segmentation technique including numerical filters and watershed algorithm was used to define the sub-areas for each atomic column. Then a 2D Gaussian fitting was carried out to determine the atomic column positions precisely, which provides the geometric information at the unit-cell scale. Furthermore, a self-adaptive integration based on the column position and the covariance of statistical Gaussian distribution were performed. The integrated intensities show very high sensitivity on the mean atomic number with improved signal-to-noise (S/N) ratio. Consequently, the polarization map and strain distributions were rebuilt from a HAADF-STEM image of the rhombohedral and tetragonal BiFeO3 interface and a MnO2 monolayer in LaAlO3 /SrMnO3 /SrTiO3 heterostructure was discerned from its neighbor TiO2 layers. Microsc. Res. Tech. 79:820-826, 2016. © 2016 Wiley Periodicals, Inc.

Big Data Analytics for Scanning Transmission Electron Microscopy Ptychography

Electron microscopy is undergoing a transition; from the model of producing only a few micrographs, through the current state where many images and spectra can be digitally recorded, to a new mode where very large volumes of data (movies, ptychographic and multi-dimensional series) can be rapidly obtained. Here, we discuss the application of so-called “big-data” methods to high dimensional microscopy data, using unsupervised multivariate statistical techniques, in order to explore salient image features in a specific example of BiFeO₃ domains. Remarkably, k-means clustering reveals domain differentiation despite the fact that the algorithm is purely statistical in nature and does not require any prior information regarding the material, any coexisting phases, or any differentiating structures. While this is a somewhat trivial case, this example signifies the extraction of useful physical and structural information without any prior bias regarding the sample or the instrumental modality. Further interpretation of these types of results may still require human intervention. However, the open nature of this algorithm and its wide availability, enable broad collaborations and exploratory work necessary to enable efficient data analysis in electron microscopy.

Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide

Strong correlations between electrons, spins and lattices--stemming from strong hybridization between transition metal d and oxygen p orbitals--are responsible for the functional properties of transition metal oxides. Artificial oxide heterostructures with chemically abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. Here we demonstrate the control of the oxygen coordination environment of the perovskite, SrRuO3, by heterostructuring it with Ca0.5Sr0.5TiO3 (0-4 monolayers thick) grown on a GdScO3 substrate. We found that a Ru-O-Ti bond angle of the SrRuO3 /Ca0.5Sr0.5TiO3 interface can be engineered by layer-by-layer control of the Ca0.5Sr0.5TiO3 layer thickness, and that the engineered Ru-O-Ti bond angle not only stabilizes a Ru-O-Ru bond angle never seen in bulk SrRuO3, but also tunes the magnetic anisotropy in the entire SrRuO3 layer. The results demonstrate that interface engineering of the oxygen coordination environment allows one to control additional degrees of freedom in functional oxide heterostructures.

Ferroelectric Metal in Tetragonal BiCoO3/BiFeO3 Bilayers and Its Electric Field Effect

By first-principles calculations we investigate the electronic structure of tetragonal BiCoO₃/BiFeO₃ bilayers with different terminations. The multiferroic insulator BiCoO₃ and BiFeO₃ transform into metal in all of three models. Particularly, energetically favored model CoO₂-BiO exhibits ferroelectric metallic properties, and external electric field enhances the ferroelectric displacements significantly. The metallic character is mainly associated to e_g electrons, while t_2g electrons are responsible for ferroelectric properties. Moreover, the strong hybridization between e_g and O p electrons around Fermi level provides conditions to the coexistence of ferroelectric and metallic properties. These special behaviors of electrons are influenced by the interfacial electronic reconstruction with formed Bi-O electrovalent bond, which breaks O_A-Fe/Co-O_B coupling partially. Besides, the external electric field reverses spin polarization of Fe/Co ions efficiently, even reaching 100%.

Towards 3D Mapping of BO6 Octahedron Rotations at Perovskite Heterointerfaces, Unit Cell by Unit Cell

The rich functionalities in the ABO3 perovskite oxides originate, at least in part, from the ability of the corner-connected BO6 octahedral network to host a large variety of cations through distortions and rotations. Characterizing these rotations, which have significant impact on both fundamental aspects of materials behavior and possible applications, remains a major challenge at heterointerfaces. In this work, we have developed a unique method to investigate BO6 rotation patterns in complex oxides ABO3 with unit cell resolution at heterointerfaces, where novel properties often emerge. Our method involves column shape analysis in ABF-STEM images of the ABO3 heterointerfaces taken in specific orientations. The rotating phase of BO6 octahedra can be identified for all three spatial dimensions without the need of case-by-case simulation. In several common rotation systems, quantitative measurements of all three rotation angles are now possible. Using this method, we examined interfaces between perovskites with distinct tilt systems as well as interfaces between tilted and untilted perovskites, identifying an unusual coupling behavior at the CaTiO3/LSAT interface. We believe this method will significantly improve our knowledge of complex oxide heterointerfaces.

Phase control of a perovskite transition-metal oxide through oxygen displacement at the heterointerface

We overview investigations highlighting the significance of interface engineering of oxygen displacement as a tool for phase control of strained oxides.

Direct observation of ferroelectric field effect and vacancy-controlled screening at the BiFeO3/LaxSr1-xMnO3 interface

The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.

Oxygen octahedral distortions in LaMO3/SrTiO3 superlattices

In this work we study the interfaces between the Mott insulator LaMnO3 (LMO) and the band insulator SrTiO3 (STO) in epitaxially grown superlattices with different thickness ratios and different transport and magnetic behaviors. Using atomic resolution electron energy-loss spectral imaging, we analyze simultaneously the structural and chemical properties of these interfaces. We find changes in the oxygen octahedral tilts within the LaMnO3 layers when the thickness ratio between the manganite and the titanate layers is varied. Superlattices with thick LMO and ultrathin STO layers present unexpected octahedral tilts in the STO, along with a small amount of oxygen vacancies. On the other hand, thick STO layers exhibit undistorted octahedra while the LMO layers present reduced O octahedral distortions near the interfaces. These findings are discussed in view of the transport and magnetic differences found in previous studies.

Oxygen-vacancy-induced polar behavior in (LaFeO3)2/(SrFeO3) superlattices

Complex oxides displaying ferroelectric and/or multiferroic behavior are of high fundamental and applied interest. In this work, we show that it is possible to achieve polar order in a superlattice made up of two nonpolar oxides by means of oxygen vacancy ordering. Using scanning transmission electron microscopy imaging, we show the polar displacement of magnetic Fe ions in a superlattice of (LaFeO3)2/(SrFeO3) grown on a SrTiO3 substrate. Using density functional theory calculations, we systematically study the effect of epitaxial strain, octahedral rotations, and surface terminations in the superlattice and find them to have a negligible effect on the antipolar displacements of the Fe ions lying in between SrO and LaO layers of the superlattice (i.e., within La0.5Sr0.5FeO3 unit cells). The introduction of oxygen vacancies, on the other hand, triggers a polar displacement of the Fe ions. We confirm this important result using electron energy loss spectroscopy, which shows partial oxygen vacancy ordering in the region where polar displacements are observed and an absence of vacancy ordering outside of that area.

Crystal-chemistry guidelines for noncentrosymmetric A2BO4 Ruddlesden-Popper oxides

Noncentrosymmetric (NCS) phases are seldom seen in layered A2BO4 Ruddlesden-Popper (214 RP) oxides. In this work, we uncover the underlying crystallographic symmetry restrictions that enforce the spatial parity operation of inversion and then subsequently show how to lift them to achieve NCS structures. Simple octahedral distortions alone, while impacting the electronic and magnetic properties, are insufficient. We show using group theory that the condensation of two distortion modes, which describe suitable symmetry unique octahedral distortions or a combination of a single octahedral distortion with a "compositional" A or B cation ordering mode, is able to transform the centrosymmetric aristotype into a NCS structure. With these symmetry guidelines, we formulate a data-driven model founded on Bayesian inference that allows us to rationally search for combinations of A- and B-site elements satisfying the inversion symmetry lifting criterion. We describe the general methodology and apply it to 214 iridates with A(2+) cations, identifying RP-structured Ca2IrO4 as a potential NCS oxide, which we evaluate with density functional theory. We find a strong energetic competition between two closely related polar and nonpolar low-energy crystal structures in Ca2IrO4 and suggest pathways to stabilize the NCS structure.