Interface control of bulk ferroelectric polarization

Manipulating chiral spin transport with ferroelectric polarization

A magnon is a collective excitation of the spin structure in a magnetic insulator and can transmit spin angular momentum with negligible dissipation. This quantum of a spin wave has always been manipulated through magnetic dipoles (that is, by breaking time-reversal symmetry). Here we report the experimental observation of chiral spin transport in multiferroic BiFeO3 and its control by reversing the ferroelectric polarization (that is, by breaking spatial inversion symmetry). The ferroelectrically controlled magnons show up to 18% modulation at room temperature. The spin torque that the magnons in BiFeO3 carry can be used to efficiently switch the magnetization of adjacent magnets, with a spin-torque efficiency comparable to the spin Hall effect in heavy metals. Utilizing such controllable magnon generation and transmission in BiFeO3, an all-oxide, energy-scalable logic is demonstrated composed of spin-orbit injection, detection and magnetoelectric control. Our observations open a new chapter of multiferroic magnons and pave another path towards low-dissipation nanoelectronics.

Interfacial Polarization Control Engineering and Ferroelectric PZT/Graphene Heterostructure Integrated Application

Integration and miniaturization are the inevitable trends in the development of electronic devices. PZT and graphene are typical ferroelectric and carbon-based materials, respectively, which have been widely used in various fields. Achieving high-quality PZT/graphene heterogeneous integration and systematically studying its electrical properties is of great significance. In this work, we reported the characterization of a PZT film based on the sol-gel method. Additionally, the thickness of the PZT film was pushed to the limit size (~100 nm) by optimizing the process. The test results, including the remnant and leakage current, show that the PZT film is a reliable and suitable platform for further graphene-integrated applications. The non-destructive regulation of the electrical properties of graphene has been studied based on a domain-polarized substrate and strain-polarized substrate. The domain structures in the PZT film exhibit different geometric structures with ~0.3 V surface potential. The I-V output curves of graphene integrated on the surface of the PZT film exhibited obvious rectification characteristics because of p/n-doping tuned by an interfacial polarized electric field. In contrast, a ~100 nm thick PZT film makes it easy to acquire a larger strain gradient for flexural potential. The tested results also show a rectification phenomenon, which is similar to domain polarization substrate regulation. Considering the difficulty of measuring the flexural potential, the work might provide a new approach to assessing the flexural polarized regulation effect. A thinner ferroelectric film/graphene heterojunction and the polarized regulation of graphene will provide a platform for promoting low-dimension film-integrated applications.

Reversible Optical Control of Polarization in Epitaxial Ferroelectric Thin Films

Light is an effective tool to probe the polarization and domain distribution in ferroelectric materials passively, that is, non-invasively, for example, via optical second harmonic generation (SHG). With the emergence of oxide electronics, there is now a strong demand to expand the role of light toward active control of the polarization. In this work, optical control of the ferroelectric polarization is demonstrated in prototypical epitaxial PbZrx Ti1-x O3 (PZT)-based heterostructures. This is accomplished in three steps, using above-bandgap UV light, while tracking the response of the polarization with optical SHG. First, it is found that UV-light exposure induces a transient enhancement or suppression of the ferroelectric polarization in films with an upward- or downward-oriented polarization, respectively. This behavior is attributed to a modified charge screening driven by the separation of photoexcited charge carriers at the Schottky interface of the ferroelectric thin film. Second, by taking advantage of this optical handle on electrostatics, remanent optical poling from a pristine multi-domain into a single-domain configuration is accomplished. Third, via thermal annealing or engineered electrostatic boundary conditions, a complete reversibility of the optical poling is further achieved. Hence, this work paves the way for the all-optical control of the spontaneous polarization in ferroelectric thin films.

Surface Effect of Thickness-Dependent Polarization and Domain Evolution in BiFeO3 Epitaxial Ultrathin Films

With the trend of device miniaturization, ultrathin ferroelectric films are gaining more and more attention. However, understanding ferroelectricity in this nanoscale context remains a formidable challenge, primarily due to the heightened relevance of surface effects, which often leads to the loss of net polarization. Here, the influence of surface effects on the polarization as a function of thickness in ultrathin BiFeO3 films is investigated using phase-field simulations. The findings reveal a notable increase in ferroelectric polarization with increasing thickness, with a particularly discernible change occurring below the 10 nm threshold. Upon accounting for surface effects, the polarization is marginally lower than the case without such considerations, with the disparity becoming more pronounced at smaller thicknesses. Moreover, the hysteresis loop and butterfly loop of the ultrathin film were simulated, demonstrating that the ferroelectric properties of films remain robust even down to a thickness of 5 nm. Our investigations provide valuable insights into the significance of ferroelectric thin films in device miniaturization.

Defeating depolarizing fields with artificial flux closure in ultrathin ferroelectrics

Material surfaces encompass structural and chemical discontinuities that often lead to the loss of the property of interest in so-called dead layers. It is particularly problematic in nanoscale oxide electronics, where the integration of strongly correlated materials into devices is obstructed by the thickness threshold required for the emergence of their functionality. Here we report the stabilization of ultrathin out-of-plane ferroelectricity in oxide heterostructures through the design of an artificial flux-closure architecture. Inserting an in-plane-polarized ferroelectric epitaxial buffer provides the continuity of polarization at the interface; despite its insulating nature, we observe the emergence of polarization in our out-of-plane-polarized model of ferroelectric BaTiO3 from the very first unit cell. In BiFeO3, the flux-closure approach stabilizes a 251° domain wall. Its unusual chirality is probably associated with the ferroelectric analogue to the Dzyaloshinskii-Moriya interaction. We, thus, see that in an adaptively engineered geometry, the depolarizing-field-screening properties of an insulator can even surpass those of a metal and be a source of functionality. This could be a useful insight on the road towards the next generation of oxide electronics.

Out-of-plane polarization reversal and changes in in-plane ferroelectric and ferromagnetic domains of multiferroic BiFe0.9Co0.1O3 thin films by water printing

BiFe_0.9Co_0.1O₃ is a promising material for an ultra-low-power-consumption nonvolatile magnetic memory device because local magnetization reversal is possible through application of an electric field. Here, changes in ferroelectric and ferromagnetic domain structures in a multiferroic BiFe_0.9Co_0.1O₃ thin film induced by "water printing", which is a polarization reversal method involving chemical bonding and charge accumulation at the interface between the liquid and the film, was investigated. Water printing using pure water with pH = 6.2 resulted in an out-of-plane polarization reversal from upward to downward. The in-plane domain structure remained unchanged after the water printing process, indicating that 71° switching was achieved in 88.4% of the observation area. However, magnetization reversal was observed in only 50.1% of the area, indicating a loss of correlation between the ferroelectric and magnetic domains because of the slow polarization reversal due to nucleation growth.

Interface-engineered ferroelectricity of epitaxial Hf0.5Zr0.5O2 thin films

Ferroelectric hafnia-based thin films have attracted intense attention due to their compatibility with complementary metal-oxide-semiconductor technology. However, the ferroelectric orthorhombic phase is thermodynamically metastable. Various efforts have been made to stabilize the ferroelectric orthorhombic phase of hafnia-based films such as controlling the growth kinetics and mechanical confinement. Here, we demonstrate a key interface engineering strategy to stabilize and enhance the ferroelectric orthorhombic phase of the Hf_0.5Zr_0.5O₂ thin film by deliberately controlling the termination of the bottom La_0.67Sr_0.33MnO₃ layer. We find that the Hf_0.5Zr_0.5O₂ films on the MnO₂-terminated La_0.67Sr_0.33MnO₃ have more ferroelectric orthorhombic phase than those on the LaSrO-terminated La_0.67Sr_0.33MnO₃, while with no wake-up effect. Even though the Hf_0.5Zr_0.5O₂ thickness is as thin as 1.5 nm, the clear ferroelectric orthorhombic (111) orientation is observed on the MnO₂ termination. Our transmission electron microscopy characterization and theoretical modelling reveal that reconstruction at the Hf_0.5Zr_0.5O₂/ La_0.67Sr_0.33MnO₃ interface and hole doping of the Hf_0.5Zr_0.5O₂ layer resulting from the MnO₂ interface termination are responsible for the stabilization of the metastable ferroelectric phase of Hf_0.5Zr_0.5O₂. We anticipate that these results will inspire further studies of interface-engineered hafnia-based systems.

Visualizing Giant Ferroelectric Gating Effects in Large-Scale WSe2/BiFeO3 Heterostructures

Multilayers based on quantum materials (complex oxides, topological insulators, transition-metal dichalcogenides, etc.) have enabled the design of devices that could revolutionize microelectronics and optoelectronics. However, heterostructures incorporating quantum materials from different families remain scarce, while they would immensely broaden the range of possible applications. Here we demonstrate the large-scale integration of compounds from two highly multifunctional families: perovskite oxides and transition-metal dichalcogenides (TMDs). We couple BiFeO₃, a room-temperature multiferroic oxide, and WSe₂, a semiconducting two-dimensional material with potential for photovoltaics and photonics. WSe₂ is grown by molecular beam epitaxy and transferred on a centimeter-scale onto BiFeO₃ films. Using angle-resolved photoemission spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe₂ and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polarization direction in the underlying BiFeO₃. Such a strong shift opens new perspectives in the efficient manipulation of TMD properties by proximity effects.

Interface-Driven Multiferroicity in Cubic BaTiO3-SrTiO3 Nanocomposites

Perovskite multiferroics have drawn significant attention in the development of next-generation multifunctional electronic devices. However, the majority of existing multiferroics exhibit ferroelectric and ferromagnetic orderings only at low temperatures. Although interface engineering in complex oxide thin films has triggered many exotic room-temperature functionalities, the desired coupling of charge, spin, orbital and lattice degrees of freedom often imposes stringent requirements on deposition conditions, layer thickness and crystal orientation, greatly hindering their cost-effective large-scale applications. Herein, we report an interface-driven multiferroicity in low-cost and environmentally friendly bulk polycrystalline material, namely cubic BaTiO₃-SrTiO₃ nanocomposites which were fabricated through a simple, high-throughput solid-state reaction route. Interface reconstruction in the nanocomposites can be readily controlled by the processing conditions. Coexistence of room-temperature ferromagnetism and ferroelectricity, accompanying a robust magnetoelectric coupling in the nanocomposites, was confirmed both experimentally and theoretically. Our study explores the 'hidden treasure at the interface' by creating a playground in bulk perovskite oxides, enabling a broad range of applications that are challenging with thin films, such as low-power-consumption large-volume memory and magneto-optic spatial light modulator.

Control of up-to-down/down-to-up light-induced ferroelectric polarization reversal

Light control of ferroelectric polarization is of interest for the exploitation of ferroelectric thin films in ultrafast data storage and logic functionalities. The rapidly oscillating electric field of light absorbed in a ferroelectric layer can suppress its polarization but cannot selectively reverse its direction. Here we take advantage of the built-in asymmetry at ferroelectric/electrode interfaces to break the up/down symmetry in uniaxial ferroelectrics to promote polarization reversal under illumination. It is shown that appropriate ferroelectric/metal structures allow the direction of the imprint electric field to be selected, which is instrumental for polarization reversal. This ability is further exploited by demonstrating the optical control of the resistance states in a ferroelectric capacitor.

Materials for a Sustainable Microelectronics Future: Electric Field Control of Magnetism with Multiferroics

This article is written on behalf of many colleagues, collaborators, and researchers in the field of complex oxides as well as current and former students and postdocs who continue to enable and undertake cutting-edge research in the field of multiferroics, magnetoelectrics, and the pursuit of electric-field control of magnetism. What I present is something that is extremely exciting from both a fundamental science and applications perspective and has the potential to revolutionize our world, particularly from a sustainability perspective. To realize this potential will require numerous new innovations, both in the fundamental science arena as well as translating these scientific discoveries into real applications. Thus, this article will attempt to bridge the gap between fundamental materials physics and the actual manifestations of the physical concepts into real-life applications. I hope this article will help spur more translational research within the broad materials community.

In-plane quasi-single-domain BaTiO3 via interfacial symmetry engineering

The control of the in-plane domain evolution in ferroelectric thin films is not only critical to understanding ferroelectric phenomena but also to enabling functional device fabrication. However, in-plane polarized ferroelectric thin films typically exhibit complicated multi-domain states, not desirable for optoelectronic device performance. Here we report a strategy combining interfacial symmetry engineering and anisotropic strain to design single-domain, in-plane polarized ferroelectric BaTiO₃ thin films. Theoretical calculations predict the key role of the BaTiO₃/PrScO₃ [Formula: see text] substrate interfacial environment, where anisotropic strain, monoclinic distortions, and interfacial electrostatic potential stabilize a single-variant spontaneous polarization. A combination of scanning transmission electron microscopy, piezoresponse force microscopy, ferroelectric hysteresis loop measurements, and second harmonic generation measurements directly reveals the stabilization of the in-plane quasi-single-domain polarization state. This work offers design principles for engineering in-plane domains of ferroelectric oxide thin films, which is a prerequisite for high performance optoelectronic devices.

Local Kondo scattering in 4d-electron RuOx nanoclusters on atomically-resolved ultrathin SrRuO3 films

Perovskite SrRuO₃ is a unique 4d transition metal oxide with coexisting spin-orbit coupling (SOC) and electron-electron correlation. However, the intrinsic, non-reconstructed surface structure of SrRuO₃ has not been reported so far. Here we report an atomic imaging of the non-reconstructed, SrO-terminated SrRuO₃ surface by scanning tunneling microscopy/spectroscopy. Moreover, a Kondo resonant behavior is revealed in RuO_x clusters located on top of the nonmagnetic SrO surface layer. The density functional theory calculations confirm that RuO_x clusters possess localized 4d-electron-involved spin moments and hybridize with the conduction electrons in the metal host, resulting in the appearance of the Kondo resonance features around the Fermi level. Our work demonstrates that artificially-engineered transition metal oxides provide new opportunities to explore the Kondo physics in 4d multi-orbital systems.

Direct Observation of Interface-Dependent Multidomain State in the BaTiO3 Tunnel Barrier of a Multiferroic Tunnel Junction Memristor

Multiferroic tunnel junctions (MFTJs), normally consisting of a four-state resistance, have been studied extensively as a potential candidate for nonvolatile memory devices. More interestingly, the MFTJs whose resistance can be tuned continuously with applied voltage were also reported recently. Since the performance of MFTJs is closely related to their interfacial structures, it is necessary to investigate MFTJs at the atomic scale. In this work, atomic-resolution HAADF, ABF, and EELS of the La_0.7Sr_0.3MnO₃/BaTiO₃/La_0.7Sr_0.3MnO₃ MFTJ memristor have been obtained with aberration-corrected scanning transmission electron microscopy (STEM). These results demonstrate varied degree of interfacial cation intermixing at the bottom BTO/LSMO interface, which has a direct influence on the polarization of the ferroelectric barrier BTO and the electronic structure of Mn near the interfaces. We also took advantage of a simplified model to explain the relation between the interfacial behavior and polarization states, which could be a contributing factor to the transport properties of this MFTJ.

Controlling Strain Relaxation by Interface Design in Highly Lattice-Mismatched Heterostructure

Strain engineering plays an important role in tuning the microstructure and properties of heterostructures. The key to implement the strain modulation to heterostructures is controlling the strain relaxation, which is generally realized by varying the thickness of thin films or changing substrates. Here, we show that interface polarity can tailor the behavior of strain relaxation in a hexagonal manganite film, whose strain state can be tuned to different extents. Using scanning transmission electron microscopy, a reconstructed atomic layer with elongated interlayer spacing and minor in-plane rotation is observed at the interface, suggesting that the bond hierarchy at interface transits from three-dimension to two-dimension, which accounts for the strain-free heteroepitaxy. Utilizing interface polarity to control the strain relaxation highlights a conceptually opt route to optimize the strain engineering and the realization of strain-free heteroepitaxy in such highly lattice-mismatched heterostructure also provides possibility to transform more bulklike functional oxides to low dimensionality.

Electric field control of magnetism

Electric field control of magnetism is an extremely exciting area of research, from both a fundamental science and an applications perspective and has the potential to revolutionize the world of computing. To realize this will require numerous further innovations, both in the fundamental science arena as well as translating these scientific discoveries into real applications. Thus, this article will attempt to bridge the gap between condensed matter physics and the actual manifestations of the physical concepts into applications. We have attempted to paint a broad-stroke picture of the field, from the macroscale all the way down to the fundamentals of spin–orbit coupling that is a key enabler of the physics discussed. We hope it will help spur more translational research within the broad materials physics community. Needless to say, this article is written on behalf of a large number of colleagues, collaborators and researchers in the field of complex oxides as well as current and former students and postdocs who continue to pursue cutting-edge research in this field.

In situmonitoring of epitaxial ferroelectric thin-film growth

In ferroelectric thin films, the polarization state and the domain configuration define the macroscopic ferroelectric properties such as the switching dynamics. Engineering of the ferroelectric domain configuration during synthesis is in permanent evolution and can be achieved by a range of approaches, extending from epitaxial strain tuning over electrostatic environment control to the influence of interface atomic termination. Exotic polar states are now designed in the technologically relevant ultrathin regime. The promise of energy-efficient devices based on ultrathin ferroelectric films depends on the ability to create, probe, and manipulate polar states in ever more complex epitaxial architectures. Because most ferroelectric oxides exhibit ferroelectricity during the epitaxial deposition process, the direct access to the polarization emergence and its evolution during the growth process, beyond the realm of existing structuralin situdiagnostic tools, is becoming of paramount importance. We review the recent progress in the field of monitoring polar states with an emphasis on the non-invasive probes allowing investigations of polarization during the thin film growth of ferroelectric oxides. A particular importance is given to optical second harmonic generationin situ. The ability to determine the net polarization and domain configuration of ultrathin films and multilayers during the growth of multilayers brings new insights towards a better understanding of the physics of ultrathin ferroelectrics and further control of ferroelectric-based heterostructures for devices.

Engineering of multiferroic BiFeO3 grain boundaries with head-to-head polarization configurations

Confined low dimensional charges with high density such as two-dimensional electron gas (2DEG) at interfaces and charged domain walls in ferroelectrics show great potential to serve as functional elements in future nanoelectronics. However, stabilization and control of low dimensional charges is challenging, as they are usually subject to enormous depolarization fields. Here, we demonstrate a method to fabricate tunable charged interfaces with ~77°, 86° and 94° head-to-head polarization configurations in multiferroic BiFeO₃ thin films by grain boundary engineering. The adjacent grains are cohesively bonded and the boundary is about 1 nm in width and devoid of any amorphous region. Remarkably, the polarization remains almost unchanged near the grain boundaries, indicating the polarization charges are well compensated, i.e., there should be two-dimensional charge gas confined at grain boundaries. Adjusting the tilt angle of the grain boundaries enables tuning the angle of polarization configurations from 71° to 109°, which in turn allows the control of charge density at the grain boundaries. This general and feasible method opens new doors for the application of charged interfaces in next generation nanoelectronics.

Layer and spontaneous polarizations in perovskite oxides and their interplay in multiferroic bismuth ferrite

We review the concept of surface charge, first, in the context of the polarization in ferroelectric materials and, second, in the context of layers of charged ions in ionic insulators. While the former is traditionally discussed in the ferroelectrics community and the latter in the surface science community, we remind the reader that the two descriptions are conveniently unified within the modern theory of polarization. In both cases, the surface charge leads to electrostatic instability-the so-called "polar catastrophe"-if it is not compensated, and we review the range of phenomena that arise as a result of different compensation mechanisms. We illustrate these concepts using the example of the prototypical multiferroic bismuth ferrite, BiFeO₃, which is unusual in that its spontaneous ferroelectric polarization and the polarization arising from its layer charges can be of the same magnitude. As a result, for certain combinations of polarization orientation and surface termination, its surface charge is self-compensating. We use density functional calculations of BiFeO₃ slabs and superlattices, analysis of high-resolution transmission electron micrographs, and examples from the literature to explore the consequences of this peculiarity.

Tracking atomic structure evolution during directed electron beam induced Si-atom motion in graphene via deep machine learning

Using electron beam manipulation, we enable deterministic motion of individual Si atoms in graphene along predefined trajectories. Structural evolution during the dopant motion was explored, providing information on changes of the Si atom neighborhood during atomic motion and providing statistical information of possible defect configurations. The combination of a Gaussian mixture model and principal component analysis applied to the deep learning-processed experimental data allowed disentangling of the atomic distortions for two different graphene sublattices. This approach demonstrates the potential of e-beam manipulation to create defect libraries of multiple realizations of the same defect and explore the potential of symmetry breaking physics. The rapid image analytics enabled via a deep learning network further empowers instrumentation for e-beam controlled atom-by-atom fabrication. The analysis described in the paper can be reproduced via an interactive Jupyter notebook at https://git.io/JJ3Bx.

Multiferroic heterostructures for spintronics

For next-generation technology, magnetic systems are of interest due to the natural ability to store information and, through spin transport, propagate this information for logic functions. Controlling the magnetization state through currents has proven energy inefficient. Multiferroic thin-film heterostructures, combining ferroelectric and ferromagnetic orders, hold promise for energy efficient electronics. The electric field control of magnetic order is expected to reduce energy dissipation by 2–3 orders of magnitude relative to the current state-of-the-art. The coupling between electrical and magnetic orders in multiferroic and magnetoelectric thin-film heterostructures relies on interfacial coupling though magnetic exchange or mechanical strain and the correlation between domains in adjacent functional ferroic layers. We review the recent developments in electrical control of magnetism through artificial magnetoelectric heterostructures, domain imprint, emergent physics and device paradigms for magnetoelectric logic, neuromorphic devices, and hybrid magnetoelectric/spin-current-based applications. Finally, we conclude with a discussion of experiments that probe the crucial dynamics of the magnetoelectric switching and optical tuning of ferroelectric states towards all-optical control of magnetoelectric switching events.

Interface and surface stabilization of the polarization in ferroelectric thin films

With an ever-increasing societal demand for energy for electronic devices and in the face of the current climate issues, the need for low-energy-consuming electronics has never been greater. Ferroelectrics are promising energy-efficient device components for digital information storage, with the functionality relying on the manipulation of their polarization in ultrathin films. Polar discontinuities at the thin film interfaces and surfaces, however, can cause loss of polarization and thus functionality. Here we show how the interface and surface influence the overall polarization of the thin film. We show that the structure of the interface and surface can be tailored toward a specific polarization direction and strength, and that great control in the engineering of ferroelectrics thin films can be achieved.

Ferroelectric perovskites present a switchable spontaneous polarization and are promising energy-efficient device components for digital information storage. Full control of the ferroelectric polarization in ultrathin films of ferroelectric perovskites needs to be achieved in order to apply this class of materials in modern devices. However, ferroelectricity itself is not well understood in this nanoscale form, where interface and surface effects become particularly relevant and where loss of net polarization is often observed. In this work, we show that the precise control of the structure of the top surface and bottom interface of the thin film is crucial toward this aim. We explore the properties of thin films of the prototypical ferroelectric lead titanate (PbTiO₃) on a metallic strontium ruthenate (SrRuO₃) buffer using a combination of computational (density functional theory) and experimental (optical second harmonic generation) methods. We find that the polarization direction and strength are influenced by chemical and electronic processes occurring at the epitaxial interface and at the surface. The polarization is particularly sensitive to adsorbates and to surface and interface defects. These results point to the possibility of controlling the polarization direction and magnitude by engineering specific interface and surface chemistries.

In-situ monitoring of interface proximity effects in ultrathin ferroelectrics

The development of energy-efficient nanoelectronics based on ferroelectrics is hampered by a notorious polarization loss in the ultrathin regime caused by the unscreened polar discontinuity at the interfaces. So far, engineering charge screening at either the bottom or the top interface has been used to optimize the polarization state. Yet, it is expected that the combined effect of both interfaces determines the final polarization state; in fact the more so the thinner a film is. The competition and cooperation between interfaces have, however, remained unexplored so far. Taking PbTiO₃ as a model system, we observe drastic differences between the influence of a single interface and the competition and cooperation of two interfaces. We investigate the impact of these configurations on the PbTiO₃ polarization when the interfaces are in close proximity, during thin-film synthesis in the ultrathin limit. By tailoring the interface chemistry towards a cooperative configuration, we stabilize a robust polarization state with giant polarization enhancement. Interface cooperation hence constitutes a powerful route for engineering the polarization in thin-film ferroelectrics towards improved integrability for oxide electronics in reduced dimension.

How to maintain a robust polarization in ferroelectrics despite its inherent suppression when going to the thin-film limit is a long-standing issue. Here, the authors propose the concept of competitive and cooperative interfaces and establish robust polarization states in the ultrathin regime.

Continuously controllable photoconductance in freestanding BiFeO3 by the macroscopic flexoelectric effect

Flexoelectricity induced by the strain gradient is attracting much attention due to its potential applications in electronic devices. Here, by combining a tunable flexoelectric effect and the ferroelectric photovoltaic effect, we demonstrate the continuous tunability of photoconductance in BiFeO3 films. The BiFeO3 film epitaxially grown on SrTiO3 is transferred to a flexible substrate by dissolving a sacrificing layer. The tunable flexoelectricity is achieved by bending the flexible substrate which induces a nonuniform lattice distortion in BiFeO3 and thus influences the inversion asymmetry of the film. Multilevel conductance is thus realized through the coupling between flexoelectric and ferroelectric photovoltaic effect in freestanding BiFeO3. The strain gradient induced multilevel photoconductance shows very good reproducibility by bending the flexible BiFeO3 device. This control strategy offers an alternative degree of freedom to tailor the physical properties of flexible devices and thus provides a compelling toolbox for flexible materials in a wide range of applications.

Imaging and quantification of charged domain walls in BiFeO3

Charged domain walls in ferroelectrics hold great promise for the design of novel electronic devices due to their enhanced local conductivity. In fact, charged domain walls show unique properties including the possibility of being created, moved and erased by an applied voltage. Here, we demonstrate that the charged domain walls are constituted by a core region where most of the screening charge is localized and such charge accumulation is responsible for their enhanced conductivity. In particular, the link between the local structural distortions and charge screening phenomena in 109° tail-to-tail domain walls of BiFeO3 is elucidated by a series of multiscale analysis performed by means of scanning probe techniques, including conductive atomic force microscopy (cAFM) and atomic resolution differential phase contrast scanning transmission electron microscopy (DPC-STEM). The results prove that an accumulation of oxygen vacancies occurs at the tail-to-tail domain walls as the leading charge screening process. This work constitutes a new insight in understanding the behavior of such complex systems and lays down the fundaments for their implementation into novel nanoelectronic devices.

Unusual Hole and Electron Midgap States and Orbital Reconstructions Induced Huge Ferroelectric Tunneling Electroresistance in BaTiO3/SrTiO3

Oxide heterostructures have attracted a lot of interest because of their rich exotic phenomena and potential applications. Recently, a greatly enhanced tunneling electroresistance (TER) of ferroelectric tunnel junctions (FTJs) has been realized in such heterostructures. However, our understanding on the electronic structure of resistance response with polarization reversal and the origin of huge TER is still lacking. Here, we report on electronic structures, particularly at the interface and surface, and the control of the spontaneous polarization of BaTiO₃ films by changing the termination of a SrTiO₃ substrate. Interestingly, unusual electron and hole midgap states are concurrently formed and accompanied by orbital reconstructions, which determine the ferroelectric polarization orientation in the BaTiO₃/SrTiO₃. Such unusual midgap states, which yield a strong electronic screening effect, reduce the ferroelectric barrier width and height, and pin the ferroelectric polarization, lead to a dramatic enhancement of the TER effect. The midgap states are also observed in BaTiO₃ films on electron-doped Nb/SrTiO₃ revealing its universality. Our result provides new insight into the origin of the huge TER effect and opens a new route for designing ferroelectric tunnel junction-based devices with huge TER through interface engineering.

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.

Domain and Switching Control of the Bulk Photovoltaic Effect in Epitaxial BiFeO3 Thin Films

Absence of inversion symmetry is the underlying origin of ferroelectricity, piezoelectricity, and the bulk photovoltaic (BPV) effect, as a result of which they are inextricably linked. However, till now, only the piezoelectric effects (inverse) have been commonly utilized for probing ferroelectric characteristics such as domain arrangements and resultant polarization orientation. The bulk photovoltaic effect, despite sharing same relation with the symmetry as piezoelectricity, has been mostly perceived as an outcome of ferroelectricity and not as a possible analytical method. In this work, we investigate the development of BPV characteristics, i.e. amplitude and angular dependency of short-circuit current, as the ferroelastic domain arrangement is varied by applying electric fields in planar devices of BiFeO₃ films. A rather sensitive co-dependency was observed from measurements on sample with ordered and disordered domain arrangements. Analysis of the photovoltaic response manifested in a mathematical model to estimate the proportion of switched and un-switched regions. The results unravel the potential utility of BPV effect to trace the orientation of the polarization vectors (direction and amplitude) in areas much larger than that can be accommodated in probe-based techniques.

Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures

The current burst of device concepts based on nanoscale domain-control in magnetically and electrically ordered systems motivates us to review the recent development in the design of domain engineered oxide heterostructures. The improved ability to design and control advanced ferroic domain architectures came hand in hand with major advances in investigation capacity of nanoscale ferroic states. The new avenues offered by prototypical multiferroic materials, in which electric and magnetic orders coexist, are expanding beyond the canonical low-energy-consuming electrical control of a net magnetization. Domain pattern inversion, for instance, holds promises of increased functionalities. In this review, we first describe the recent development in the creation of controlled ferroelectric and multiferroic domain architectures in thin films and multilayers. We then present techniques for probing the domain state with a particular focus on non-invasive tools allowing the determination of buried ferroic states. Finally, we discuss the switching events and their domain analysis, providing critical insight into the evolution of device concepts involving multiferroic thin films and heterostructures.

Atomic-Scale Control of Magnetism at the Titanite-Manganite Interfaces

Complex oxide thin-film heterostructures often exhibit magnetic properties different from those known for bulk constituents. This is due to the altered local structural and electronic environment at the interfaces, which affects the exchange coupling and magnetic ordering. The emergent magnetism at oxide interfaces can be controlled by ferroelectric polarization and has a strong effect on spin-dependent transport properties of oxide heterostructures, including magnetic and ferroelectric tunnel junctions. Here, using prototype La_2/3Sr_1/3MnO₃/BaTiO₃ heterostructures, we demonstrate that ferroelectric polarization of BaTiO₃ controls the orbital hybridization and magnetism at heterointerfaces. We observe changes in the enhanced orbital occupancy and significant charge redistribution across the heterointerfaces, affecting the spin and orbital magnetic moments of the interfacial Mn and Ti atoms. Importantly, we find that the exchange coupling between Mn and Ti atoms across the interface is tuned by ferroelectric polarization from ferromagnetic to antiferromagnetic. Our findings provide a viable route to electrically control complex magnetic configurations at artificial multiferroic interfaces, taking a step toward low-power spintronics.

Advances in magnetoelectric multiferroics

The manipulation of magnetic properties by an electric field in magnetoelectric multiferroic materials has driven significant research activity, with the goal of realizing their transformative technological potential. Here, we review progress in the fundamental understanding and design of new multiferroic materials, advances in characterization and modelling tools to describe them, and the exploration of devices and applications. Focusing on the translation of the many scientific breakthroughs into technological innovations, we identify the key open questions in the field where targeted research activities could have maximum impact in transitioning scientific discoveries into real applications.

Tailoring Magnetoelectric Coupling in BiFeO3 /La0.7 Sr0.3 MnO3 Heterostructure through the Interface Engineering

Electric field control of magnetism ultimately opens up the possibility of reducing energy consumption of memory and logic devices. Electric control of magnetization and exchange bias are demonstrated in all-oxide heterostructures of BiFeO₃ (BFO) and La_0.7 Sr_0.3 MnO₃ (LSMO). However, the role of the polar heterointerface on magnetoelectric (ME) coupling is not fully explored. Here, the ME coupling in BFO/LSMO heterostructures with two types of interfaces, achieved by exploiting the interface engineering at the atomic scale, is investigated. It is shown that both magnetization and exchange bias are reversibly controlled by switching the ferroelectric polarization of BFO. Intriguingly, distinctly different modulation behaviors that depend on the interfacial atomic sequence are observed. These results provide new insights into the underlying physics of ME coupling in the model system. This study highlights that designing interface at the atomic scale is of general importance for functional spintronic devices.

Interfacial Coupling Boosts Giant Electrocaloric Effects in Relaxor Polymer Nanocomposites: In Situ Characterization and Phase-Field Simulation

The electrocaloric effect (ECE) refers to reversible thermal changes of a polarizable material upon the application or removal of electric fields. Without a compressor or cooling agents, all-solid-state electrocaloric (EC) refrigeration systems are environmentally benign, highly compact, and of very high energy efficiency. Relaxor ferroelectric ceramics and polymers are promising candidates as EC materials. Here, synergistic efforts are made by composing relaxor Ba(Zr_0.21 Ti_0.79 )O₃ nanofibers with P(VDF-TrFE-CFE) to make relaxor-relaxor-type polymer nanocomposites. The ECEs of the nanocomposites are directly measured and these relaxor nanocomposites exhibit, so far, the highest EC temperature change at a modest electric field, along with high thermal stability within a broad temperature range span to room temperature. The superior EC performance is attributed to the interfacial coupling between dipoles across the filler/polymer interfaces. The thermodynamics and kinetics of interfacial coupling are investigated in situ by piezoresponse force microscopy while the real-time evolution of interfacial coupling is simulated and visualized by phase-field modeling.

Self-Assembled Ferroelectric Nanoarray

Self-assembled heteroepitaxial nanostructures have played an important role for miniaturization of electronic devices, e.g., the ultrahigh density ferroelectric memories, and cause for great concern. Our first principle calculations predict that the materials with low formation energy of the interface ( E_f) tend to form matrix structure in self-assembled heteroepitaxial nanostructures, whereas those with high E_f form nanopillars. Under the guidance of the theoretical modeling, perovskite BiFeO₃ (BFO) nanopillars are swimmingly grown into CeO₂ matrix on single-crystal (001)-SrTiO₃ (STO) substrates by pulsed laser deposition, where CeO₂ has a lower formation energy of the interface ( E_f) than BFO. This work provides a good paradigm for controlling self-assembled nanostructures as well as the application of self-assembled ferroelectric nanoscale memory.

Strain vs. charge mediated magnetoelectric coupling across the magnetic oxide/ferroelectric interfaces

We utilize polarized neutron reflectometry (PNR) in consort with ab initio based density functional theory (DFT) calculations to study magnetoelectric coupling at the interface of a ferroelectric PbZr_0.2Ti_0.8O₃ (PZT) and magnetic La_0.67Sr_0.33MnO₃ (LSMO) heterostructure grown on a Nb-doped SrTiO₃ (001) substrate. Functional device working conditions are mimicked by gating the heterostructure with a Pt top electrode to apply an external electric field, which alters the magnitude and switches the direction of the ferroelectric (FE) polarization, across the PZT layer. PNR results show that the gated PZT/LSMO exhibits interfacial magnetic phase modulation attributed to ferromagnetic (FM) to A-antiferromagnetic (A-AF) phase transitions resulting from hole accumulation. When the net FE polarization points towards the interface (positive), the interface doesn't undergo a magnetic phase transition and retains its global FM ordered state. In addition to changes in the interfacial magnetic ordering, the global magnetization of LSMO increases while switching the polarization from positive to negative and decreases vice versa. DFT calculations indicate that this enhanced magnetization also correlates with an out of plane tensile strain, whereas the suppressed magnetization for positive polarization is attributed to out of plane compressive strain. These calculations also show the coexistence of FM and A-AF phases at zero out of plane strain. Charge modulations throughout the LSMO layer appear to be unaffected by strain, suggesting that these charge mediated effects do not significantly change the global magnetization. Our PNR results and DFT calculations are in consort to verify that the interfacial magnetic modulations are due to co-action of strain and charge mediated effects with the strain and charge effects dominant at different length scale.

We utilize polarized neutron reflectometry in consort with ab initio based density functional theory calculations to study interface magnetoelectric coupling across a ferroelectric PbZr_0.2Ti_0.8O₃ and magnetic La_0.67Sr_0.33MnO₃ heterostructure.

Manipulating the Ferroelectric Domain States and Structural Distortion in Epitaxial BiFeO3 Ultrathin Films via Bi Nonstoichiometry

Exploring and manipulating domain configurations in ferroelectric thin films are of critical importance for the design and fabrication of ferroelectric heterostructures with a novel functional performance. In this study, BiFeO₃ (BFO) ultrathin films with various Bi/Fe ratios from excess Bi to deficient Bi have been grown on (La_0.7Sr_0.3)MnO₃ (LSMO)-covered SrTiO₃ substrates by a laser molecular beam epitaxy system. Atomic force microscopy and piezoresponse force microscopy measurements show that both the surface morphology and ferroelectric polarization of the films are relevant to Bi nonstoichiometry. More significantly, a Bi-excess thin film shows an upward (from substrate to film surface) uniform ferroelectric polarization, whereas a Bi-deficient thin film exhibits a downward uniform polarization, which means the as-grown polarization of BFO thin films can be controlled by changing the Bi contents. Atomic-scale structural and chemical characterizations and second-harmonic generation measurements reveal that two different kinds of structural distortions and interface atomic configurations in the BFO/LSMO heterostructures can be induced by the change of Bi nonstoichiometry, leading to the two opposite as-grown ferroelectric polarizations. It has also been revealed that the band gap of BFO thin films can be modulated via Bi nonstoichiometry. These results demonstrate that Bi nonstoichiometry plays a key role on the ferroelectric domain states and physical properties of BFO thin films and also open a new avenue to manipulate the structure and ferroelectric domain states in BFO thin films.

Three-dimensional atomic scale electron density reconstruction of octahedral tilt epitaxy in functional perovskites

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

In complex oxides, oxygen octahedra are major structural motifs and their tilts sensitively determine the material’s physical properties. Exploiting Coherent Bragg Rod Analysis enables 3D mapping of complex tilt patterns and reveals the means to control polarization through them in CaTiO₃ thin films.

In-Plane Ferroelectricity in Thin Flakes of Van der Waals Hybrid Perovskite

Collective ferroic orders in van der Waals (vdW) crystals are receiving increasing attention in 2D materials research. The interplay between spatial quantum confinement and long-range cooperative phenomena not only broadens the horizon of fundamental physics, but also enables new device paradigms and functionalities built upon vdW heterostructures. Here, the in-plane ferroelectric properties in thin flakes of vdW hybrid perovskite bis(benzylammonium) lead tetrachloride are studied. The ordering of electric dipoles along the layer plane circumvents the depolarization field and preserves the ferroelectricity down to one unit-cell thickness or two vdW layers at room temperature. The superior performance of the electromechanical energy conversion is demonstrated by exploiting its in-plane piezoelectricity. The successful isolation of ferroelectric order in atomically thin vdW hybrid perovskite paves the way for nonvolatile flexible electronic devices with the cross-coupling between strain, charge polarization, and valley degrees of freedom.

Water printing of ferroelectric polarization

Ferroelectrics, which generate a switchable electric field across the solid-liquid interface, may provide a platform to control chemical reactions (physical properties) using physical fields (chemical stimuli). However, it is challenging to in-situ control such polarization-induced interfacial chemical structure and electric field. Here, we report that construction of chemical bonds at the surface of ferroelectric BiFeO3 in aqueous solution leads to a reversible bulk polarization switching. Combining piezoresponse (electrostatic) force microscopy, X-ray photoelectron spectroscopy, scanning transmission electron microscopy, first-principles calculations and phase-field simulations, we discover that the reversible polarization switching is ascribed to the sufficient formation of polarization-selective chemical bonds at its surface, which decreases the interfacial chemical energy. Therefore, the bulk electrostatic energy can be effectively tuned by H+/OH- concentration. This water-induced ferroelectric switching allows us to construct large-scale type-printing of polarization using green energy and opens up new opportunities for sensing, high-efficient catalysis, and data storage.

Control of Domain Structures in Multiferroic Thin Films through Defect Engineering

Domain walls (DWs) have become an essential component in nanodevices based on ferroic thin films. The domain configuration and DW stability, however, are strongly dependent on the boundary conditions of thin films, which make it difficult to create complex ordered patterns of DWs. Here, it is shown that novel domain structures, that are otherwise unfavorable under the natural boundary conditions, can be realized by utilizing engineered nanosized structural defects as building blocks for reconfiguring DW patterns. It is directly observed that an array of charged defects, which are located within a monolayer thickness, can be intentionally introduced by slightly changing substrate temperature during the growth of multiferroic BiFeO₃ thin films. These defects are strongly coupled to the domain structures in the pretemperature-change portion of the BiFeO₃ film and can effectively change the configuration of newly grown domains due to the interaction between the polarization and the defects. Thus, two types of domain patterns are integrated into a single film without breaking the DW periodicity. The potential use of these defects for building complex patterns of conductive DWs is also demonstrated.

Control of Synaptic Plasticity Learning of Ferroelectric Tunnel Memristor by Nanoscale Interface Engineering

Brain-inspired computing is an emerging field, which intends to extend the capabilities of information technology beyond digital logic. The progress of the field relies on artificial synaptic devices as the building block for brainlike computing systems. Here, we report an electronic synapse based on a ferroelectric tunnel memristor, where its synaptic plasticity learning property can be controlled by nanoscale interface engineering. The effect of the interface engineering on the device performance was studied. Different memristor interfaces lead to an opposite virgin resistance state of the devices. More importantly, nanoscale interface engineering could tune the intrinsic band alignment of the ferroelectric/metal-semiconductor heterostructure over a large range of 1.28 eV, which eventually results in different memristive and spike-timing-dependent plasticity (STDP) properties of the devices. Bidirectional and unidirectional gradual resistance modulation of the devices could therefore be controlled by tuning the band alignment. This study gives useful insights on tuning device functionalities through nanoscale interface engineering. The diverse STDP forms of the memristors with different interfaces may play different specific roles in various spike neural networks.

Probing Ferroic States in Oxide Thin Films Using Optical Second Harmonic Generation

Forthcoming low-energy consumption oxide electronics rely on the deterministic control of ferroelectric and multiferroic domain states at the nanoscale. In this review, we address the recent progress in the field of investigation of ferroic order in thin films and heterostructures, with a focus on non-invasive optical second harmonic generation (SHG). For more than 50 years, SHG has served as an established technique for probing ferroic order in bulk materials. Here, we will survey the specific new aspects introduced to SHG investigation of ferroelectrics and multiferroics by working with thin film structures. We show how SHG can probe complex ferroic domain patterns non-invasively and even if the lateral domain size is below the optical resolution limit or buried beneath an otherwise impenetrable cap layer. We emphasize the potential of SHG to distinguish contributions from individual (multi-) ferroic films or interfaces buried in a device or multilayer architecture. Special attention is given to monitoring switching events in buried ferroic domain- and domain-wall distributions by SHG, thus opening new avenues towards the determination of the domain dynamics. Another aspect studied by SHG is the role of strain. We will finally show that by integrating SHG into the ongoing thin film deposition process, we can monitor the emergence of ferroic order and properties in situ, while they emerge during growth. Our review closes with an outlook, emphasizing the present underrepresentation of ferroic switching dynamics in the study of ferroic oxide heterostructures.

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.

Manipulating Ferroelectrics through Changes in Surface and Interface Properties

Ferroelectric materials are used in many applications of modern technologies including information storage, transducers, sensors, tunable capacitors, and other novel device concepts. In many of these applications, the ferroelectric properties, such as switching voltages, piezoelectric constants, or stability of nanodomains, are crucial. For any application, even for material characterization, the material itself needs to be interfaced with electrodes. On the basis of the structural, chemical, and electronic properties of the interfaces, the measured material properties can be determined by the interface. This is also true for surfaces. However, the importance of interfaces and surfaces and their effect on experiments are often neglected, which results in many dramatically different experimental results for nominally identical samples. Therefore, it is crucial to understand the role of the interface and surface properties on internal bias fields and the domain switching process. Here, the nanoscale ferroelectric switching process and the stability of nanodomains for Pb(Zr,Ti)O₃ thin films are investigated by using scanning probe microscopy. Interface and surface properties are modulated through the selection/redesign of electrode materials as well as tuning the surface-near oxygen vacancies, which both can result in changes of the electric fields acting across the sample, and consequently this controls the measured ferroelectric and domain retention properties. By understanding the role of surfaces and interfaces, ferroelectric properties can be tuned to eliminate the problem of asymmetric domain stability by combining the effects of different electrode materials. This study forms an important step toward integrating ferroelectric materials in electronic devices.

Nanoscale design of polarization in ultrathin ferroelectric heterostructures

The success of oxide electronics depends on the ability to design functional properties such as ferroelectricity with atomic accuracy. However, despite tremendous advances in ferroelectric heterostructures, the development towards multilevel architectures with precise layer-by-layer command over the polarization is impeded by the lack of continuous control over the balance of electrostatics, strain, chemistry and film thickness during growth. Moreover, the polarization in the deeper layers becomes inaccessible when these are buried by the ongoing deposition. Taking ferroelectric BaTiO₃ and multiferroic BiFeO₃ as model systems, we observe and engineer the emergence, orientation and interaction of ferroelectric polarization in ultrathin heterostructures with monolayer accuracy. We achieve this by optical second harmonic generation which tracks the evolution of spontaneous polarization in real time throughout the deposition process. Such direct and in situ access to the polarization during growth leads us to heterostructures with user-defined polarization sequences—towards a new class of functional ferroic materials.

Ferroelectric heterostructures exhibit a range of functional properties; however control of their growth remains a challenge. De  Luca et al., demonstrate in-situ optical second harmonic generation to monitor and tailor the polarisation and growth of multilayer barium titanate and bismuth ferrite films.

Engineering magnetism at functional oxides interfaces: manganites and beyond

The family of transition metal oxides (TMOs) is a large class of magnetic materials that has been intensively studied due to the rich physics involved as well as the promising potential applications in next generation electronic devices. In TMOs, the spin, charge, orbital and lattice are strongly coupled, and significant advances have been achieved to engineer the magnetism by different routes that manipulate these degrees of freedom. The family of manganites is a model system of strongly correlated magnetic TMOs. In this review, using manganites thin films and the heterostructures in conjunction with other TMOs as model systems, we review the recent progress of engineering magnetism in TMOs. We first discuss the role of the lattice that includes the epitaxial strain and the interface structural coupling. Then we look into the role of charge, focusing on the interface charge modulation. Having demonstrated the static effects, we continue to review the research on dynamical control of magnetism by electric field. Next, we review recent advances in heterostructures comprised of high T _c cuprate superconductors and manganites. Following that, we discuss the emergent magnetic phenomena at interfaces between 3d TMOs and 5d TMOs with strong spin-orbit coupling. Finally, we provide our outlook for prospective future directions.

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.

Electronic-Reconstruction-Enhanced Tunneling Conductance at Terrace Edges of Ultrathin Oxide Films

Quantum mechanical tunneling of electrons across ultrathin insulating oxide barriers has been studied extensively for decades due to its great potential in electronic-device applications. In the few-nanometers-thick epitaxial oxide films, atomic-scale structural imperfections, such as the ubiquitously existed one-unit-cell-high terrace edges, can dramatically affect the tunneling probability and device performance. However, the underlying physics has not been investigated adequately. Here, taking ultrathin BaTiO₃ films as a model system, an intrinsic tunneling-conductance enhancement is reported near the terrace edges. Scanning-probe-microscopy results demonstrate the existence of highly conductive regions (tens of nanometers wide) near the terrace edges. First-principles calculations suggest that the terrace-edge geometry can trigger an electronic reconstruction, which reduces the effective tunneling barrier width locally. Furthermore, such tunneling-conductance enhancement can be discovered in other transition metal oxides and controlled by surface-termination engineering. The controllable electronic reconstruction can facilitate the implementation of oxide electronic devices and discovery of exotic low-dimensional quantum phases.