Interface control of bulk ferroelectric polarization

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.

Oxygen Partial Pressure during Pulsed Laser Deposition: Deterministic Role on Thermodynamic Stability of Atomic Termination Sequence at SrRuO3/BaTiO3 Interface

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO₃/BaTiO₃/SrRuO₃ (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO₂ or TiO₂-SrO) with oxygen partial pressure (P_O2) during PLD. We found that a uniform SrO-TiO₂ termination sequence at the SRO/BTO interface can be achieved by lowering the P_O2 to 5 mTorr, regardless of the total background gas pressure (P_total), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures.

Ferroelectric switching of band alignments in LSMO/PZT/Co multiferroic tunnel junctions: an ab initio study

Band alignments in ferroelectric tunnel junctions (FTJs) are expected to play a critical role in determining the charge transport across the tunneling barrier. In general, however, the interface band discontinuities and their polarization dependence are not well known in these systems. Using a first-principles density-functional-theory approach, we explore the ferroelectric (FE) polarization dependence of the band alignments in [Formula: see text] (LSMO/PZT/Co) multiferroic tunnel junctions, for which recent experiments indicated an ON/OFF conductivity behavior upon switching the PZT FE polarization. Our results on the pseudomorphic defect-free LSMO/PZT/Co FTJs evidence a major FE switching effect on the band discontinuities at both interfaces. Based on the changes in the band alignments, we provide a possible explanation for the observed trends in the resistive switching.

Resonant electron tunnelling assisted by charged domain walls in multiferroic tunnel junctions

The peculiar features of domain walls observed in ferroelectrics make them promising active elements for next-generation non-volatile memories, logic gates and energy-harvesting devices. Although extensive research activity has been devoted recently to making full use of this technological potential, concrete realizations of working nanodevices exploiting these functional properties are yet to be demonstrated. Here, we fabricate a multiferroic tunnel junction based on ferromagnetic La_0.7Sr_0.3MnO₃ electrodes separated by an ultrathin ferroelectric BaTiO₃ tunnel barrier, where a head-to-head domain wall is constrained. An electron gas stabilized by oxygen vacancies is confined within the domain wall, displaying discrete quantum-well energy levels. These states assist resonant electron tunnelling processes across the barrier, leading to strong quantum oscillations of the electrical conductance.

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.

Local Enhancement of Polarization at PbTiO3/BiFeO3 Interfaces Mediated by Charge Transfer

Ferroelectrics hold promise for sensors, transducers, and telecommunications. With the demand of electronic devices scaling down, they take the form of nanoscale films. However, the polarizations in ultrathin ferroelectric films are usually reduced dramatically due to the depolarization field caused by incomplete charge screening at interfaces, hampering the integrations of ferroelectrics into electric devices. Here, we design and fabricate a ferroelectric/multiferroic PbTiO₃/BiFeO₃ system, which exhibits discontinuities in both chemical valence and ferroelectric polarization across the interface. Aberration-corrected scanning transmission electron microscopic study reveals an 8% elongation of out-of-plane lattice spacing associated with 104%, 107%, and 39% increments of δ_Ti, δ_O1, and δ_O2 in the PbTiO₃ layer near the head-to-tail polarized interface, suggesting an over ∼70% enhancement of polarization compared with that of bulk PbTiO₃. Besides that in PbTiO₃, polarization in the BiFeO₃ is also remarkably enhanced. Electron energy loss spectrum and X-ray photoelectron spectroscopy investigations demonstrate the oxygen vacancy accumulation as well as the transfer of Fe³⁺ to Fe²⁺ at the interface. On the basis of the polar catastrophe model, FeO₂/PbO interface is determined. First-principles calculation manifests that the oxygen vacancy at the interface plays a predominate role in inducing the local polarization enhancement. We propose a charge transfer mechanism that leads to the remarkable polarization increment at the PbTiO₃/BiFeO₃ interface. This study may facilitate the development of nanoscale ferroelectric devices by tailing the coupling of charge and lattice in oxide heteroepitaxy.

Ultra-large non-volatile modulation of magnetic moments in PbZr0.2Ti0.8O3/MgO/La0.7Sr0.3MnO3 heterostructure at room temperature via interfacial polarization mediation

Multiferroic hybrid structures PbZr0.2Ti0.8O3 (PZT)/La0.7Sr0.3MnO3 (LSMO) and PZT/MgO/LSMO were epitaxially deposited on (001) Nb:SrTiO3 crystals. Crystallinity and ferroelectric domain structures were investigated for the PZT/LSMO heterostructure. Interestingly, relatively high non-volatile magnetoelectric coupling effects were observed in both heterostructures at room temperature. The change of chemical valence for Mn and Ti at the PZT/MgO/LSMO interface may play a dominant role rather than external strain or orbital reconstruction, which lead to a large modulation of the magnetization. Correspondingly, the transport behavior of the PZT/MgO/LSMO heterostructure is investigated to confirm the role of oxygen vacancies motion. Our result indicates that the PZT/MgO/LSMO heterostructure have a promising application for future high-density non-volatile memories.

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.

Interface Control of Ferroelectricity in an SrRuO3 /BaTiO3 /SrRuO3 Capacitor and its Critical Thickness

The atomic-scale synthesis of artificial oxide heterostructures offers new opportunities to create novel states that do not occur in nature. The main challenge related to synthesizing these structures is obtaining atomically sharp interfaces with designed termination sequences. In this study, it is demonstrated that the oxygen pressure (PO2) during growth plays an important role in controlling the interfacial terminations of SrRuO₃ /BaTiO₃ /SrRuO₃ (SRO/BTO/SRO) ferroelectric (FE) capacitors. The SRO/BTO/SRO heterostructures are grown by a pulsed laser deposition method. The top SRO/BTO interface, grown at high PO2 (around 150 mTorr), usually exhibits a mixture of RuO₂ -BaO and SrO-TiO₂ terminations. By reducing PO2, the authors obtain atomically sharp SRO/BTO top interfaces with uniform SrO-TiO₂ termination. Using capacitor devices with symmetric and uniform interfacial termination, it is demonstrated for the first time that the FE critical thickness can reach the theoretical limit of 3.5 unit cells.

Theoretical Investigation of the Interfaces and Mechanisms of Induced Spin Polarization of 1D Narrow Zigzag Graphene- and h-BN Nanoribbons on a SrO-Terminated LSMO(001) Surface

The structure of the interfaces and the mechanisms of induced spin polarization of 1D infinite and finite narrow graphene- and h-BN zigzag nanoribbons placed on a SrO-terminated La_1-xSr_xMnO₃ (LSMO) (001) surface were studied using density functional theory (DFT) electronic structure calculations. It was found that the π-conjugated nanofragments are bonded to the LSMO(001) surface by weak disperse interactions. The types of coordination of the fragments, the strength of bonding, and the rate of spin polarization depend upon the nature of the fragments. Infinite and finite graphene narrow zigzag nanoribbons are characterized by the lift of the spin degeneracy and strong spin polarization caused by interface-induced structural asymmetry and oxygen-mediated indirect exchange interactions with Mn ions of LSMO support. Spin polarization changes the semiconducting nature of infinite graphene nanoribbons to half-metallic state with visible spin-up density of states at the Fermi level. The h-BN nanoribbon binding energy is weaker than graphene nanoribbon ones with noticeably shorter interlayer distance. The asymmetry effect and indirect exchange interactions cause spin polarization of h-BN nanoribbon as well with formation of embedded states inside the band gap. The results show a possibility to use one-atom thick nanofragments to design LSMO-based heterostructures for spintronic nanodevices with h-BN as an inert spacer to develop different potential barriers.

PbTiO3-based perovskite ferroelectric and multiferroic thin films

Ferroelectric thin films, especially PbTiO₃-based perovskite thin films which possess robust spontaneous electrical polarization, are widely investigated and applied in various devices.

Controllable growth of ultrathin BiFeO3 from finger-like nanostripes to atomically flat films

BiFeO3 (BFO) ultrathin films with nominal thicknesses from 2 to 12 nm were grown with a SrRuO3 (SRO) buffer layer on TiO2-terminated (001) SrTiO3 (STO) substrates using pulsed laser deposition. The surface morphologies and domain configurations of the thin films were investigated using atomic force microscopy and piezoelectric force microscopy. Periodical one-dimensional finger-like nanostripes of BFO on the SRO covered STO substrates were observed. With increasing thickness, the BFO ultrathin films develop from the finger-like nanostripes to an atomically flat surface. The formation of the finger-like nanostructures of BFO is related to the atomic step or terrace structure of the substrate. The BFO nanostripes and the atomically flat thin films both show good ferroelectricity. The as-grown domain orientations of the BFO ultrathin films are ascribed to the chemical terminations at the surface of the SRO layer. These results indicate that the surface morphologies and the domain configurations of BFO ultrathin films can be artificially designed by using substrates with optimized terrace structures and chemical termination, and these films are potentially useful in multifunctional nanoelectronic devices.

Tunable photoelectrochemical performance of Au/BiFeO3 heterostructure

Ferroelectric photoelectrodes, other than conventional semiconductors, are alternative photo-absorbers in the process of water splitting. However, the capture of photons and efficient transfer of photo-excited carriers remain as two critical issues in ferroelectric photoelectrodes. In this work, we overcome the aforementioned issues by decorating the ferroelectric BiFeO3 (BFO) surface with Au nanocrystals, and thus improving the photoelectrochemical (PEC) performance of BFO film. We demonstrate that the internal field induced by the spontaneous polarization of BFO can (1) tune the efficiency of the photo-excited carriers' separation and charge transfer characteristics in bare BFO photoelectrodes, and (2) modulate an extra optical absorption within the visible light region, created by the surface plasmon resonance excitation of Au nanocrystals to capture more photons in the Au/BFO heterostructure. This study provides key insights for understanding the tunable features of PEC performance, composed of the heterostructure of noble metals and ferroelectric materials.

Inversion of Ferrimagnetic Magnetization by Ferroelectric Switching via a Novel Magnetoelectric Coupling

Although several multiferroic materials or heterostructures have been extensively studied, finding strong magnetoelectric couplings for the electric field control of the magnetization remains challenging. Here, a novel interfacial magnetoelectric coupling based on three components (ferroelectric dipole, magnetic moment, and antiferromagnetic order) is analytically formulated. As an extension of carrier-mediated magnetoelectricity, the new coupling is shown to induce an electric-magnetic hysteresis loop. Realizations employing BiFeO_{3} bilayers grown along the [111] axis are proposed. Without involving magnetic phase transitions, the magnetization orientation can be switched by the carrier modulation driven by the field effect, as confirmed using first-principles calculations.

New modalities of strain-control of ferroelectric thin films

Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.

Observation of a three-dimensional quasi-long-range electronic supermodulation in YBa2Cu3O(7-x)/La0.7Ca0.3MnO3 heterostructures

Recent developments in high-temperature superconductivity highlight a generic tendency of the cuprates to develop competing electronic (charge) supermodulations. While coupled with the lattice and showing different characteristics in different materials, these supermodulations themselves are generally conceived to be quasi-two-dimensional, residing mainly in individual CuO₂ planes, and poorly correlated along the c axis. Here we observed with resonant elastic X-ray scattering a distinct type of electronic supermodulation in YBa₂Cu₃O_7−x (YBCO) thin films grown epitaxially on La_0.7Ca_0.3MnO₃ (LCMO). This supermodulation has a periodicity nearly commensurate with four lattice constants in-plane, eight out of plane, with long correlation lengths in three dimensions. It sets in far above the superconducting transition temperature and competes with superconductivity below this temperature for electronic states predominantly in the CuO₂ plane. Our finding sheds light on the nature of charge ordering in cuprates as well as a reported long-range proximity effect between superconductivity and ferromagnetism in YBCO/LCMO heterostructures.

Understanding the nature of competing phases is a key to understanding the superconducting mechanism of unconventional superconductors. Here, the authors demonstrate a three-dimensional charge ordering state which competes with superconductivity in epitaxial YBa₂Cu₃O_7-x thin films grown on La_0.7Ca_0.3MnO₃ substrates.

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%.

Ferroelectricity and Self-Polarization in Ultrathin Relaxor Ferroelectric Films

We report ferroelectricity and self-polarization in the (001) oriented ultrathin relaxor ferroelectric PMN-PT films grown on Nb-SrTiO₃, SrRuO₃ and La_0.7Sr_0.3MnO₃, respectively. Resistance-voltage measurements and AC impedance analysis suggest that at high temperatures Schottky depletion width in a 4 nm thick PMN-PT film deposited on Nb-SrTiO₃ is smaller than the film thickness. We propose that Schottky interfacial dipoles make the dipoles of the nanometer-sized polar nanoregions (PNRs) in PMN-PT films grown on Nb-SrTiO₃ point downward at high temperatures and lead to the self-polarization at room temperature with the assistance of in-plane compressive strain. This work sheds light on the understanding of epitaxial strain effects on relaxor ferroelectric films and self-polarization mechanism.

Low energy consumption spintronics using multiferroic heterostructures

We review the recent progress in the field of multiferroic magnetoelectric heterostructures. The lack of single phase multiferroic candidates exhibiting simultaneously strong and coupled magnetic and ferroelectric orders led to an increased effort into the development of artificial multiferroic heterostructures in which these orders are combined by assembling different materials. The magnetoelectric coupling emerging from the created interface between the ferroelectric and ferromagnetic layers can result in electrically tunable magnetic transition temperature, magnetic anisotropy or magnetization reversal. The full potential of low energy consumption magnetic based devices for spintronics lies in our understanding of the magnetoelectric coupling at the scale of the ferroic domains. Although the thin film synthesis progresses resulted into the complete control of ferroic domain ordering using epitaxial strain, the local observation of magnetoelectric coupling remains challenging. The ability to imprint ferroelectric domains into ferromagnets and to manipulate those solely using electric fields suggests new technological advances for spintronics such as magnetoelectric memories or memristors.

Enhanced Structural and Magnetic Coupling in a Mesocrystal-Assisted Nanocomposite

Benefiting from the advances made in well-controlled materials synthesis techniques, nanocomposites have drawn considerable attention due to their enthralling physics and functionalities. In this work, we report a new heteroepitaxial mesocrystal-perovskite nanocomposite, (NiFe2O4)0.33:(La0.67Ca0.33MnO3)0.67. Elaborate structural studies revealed that tiny NiFe2O4 nanocrystals aggregate into ordered octahedral mesocrystal arrays with {111} facets together with a concomitant structural phase transition of the La0.67Ca0.33MnO3 matrix upon postannealing process. Combined magnetic and X-ray absorption spectroscopic measurements show significant enhancement in the magnetic properties at room temperature due to the structural evolution of magnetic NiFe2O4 and the consequent magnetic coupling at the heterointerfaces mediating via well connected octahedrons of Mn-O6 in La0.67Ca0.33MnO3 and (Ni,Fe)-O6 in NiFe2O4. This work demonstrates an approach to manipulate the exciting physical properties of material systems by integrating desired functionalities of the constituents via synthesis of a self-assembled mesocrystal embedded nanocomposite system.

Observation of the effects of Bi-deficiency on ferroelectric and electrical properties in Bi(1+x)FeO3/La0.65Sr0.35MnO3 heterostructures via atomic force microscopy

Bi_(1+x)FeO₃ thin films with different Bi contents (x = 0%, 5%, and 10%) were grown on (001) SrTiO₃ substrates with La_0.65Sr_0.35MnO₃ (LSMO) buffered layers via pulsed laser deposition.

Space-charge Effect on Electroresistance in Metal-Ferroelectric-Metal capacitors

Resistive switching through electroresistance (ER) effect in metal-ferroelectric-metal (MFM) capacitors has attracted increasing interest due to its potential applications as memories and logic devices. However, the detailed electronic mechanisms resulting in large ER when polarisation switching occurs in the ferroelectric barrier are still not well understood. Here, ER effect up to 1000% at room temperature is demonstrated in C-MOS compatible MFM nanocapacitors with a 8.8 nm-thick poly(vinylidene fluoride) (PVDF) homopolymer ferroelectric, which is very promising for silicon industry integration. Most remarkably, using theory developed for metal-semiconductor rectifying contacts, we derive an analytical expression for the variation of interfacial barrier heights due to space-charge effect that can interpret the observed ER response. We extend this space-charge model, related to the release of trapped charges by defects, to MFM structures made of ferroelectric oxides. This space-charge model provides a simple and straightforward tool to understand recent unusual reports. Finally, this work suggests that defect-engineering could be an original and efficient route for tuning the space-charge effect and thus the ER performances in future electronic devices.

Giant elastic tunability in strained BiFeO3 near an electrically induced phase transition

Elastic anomalies are signatures of phase transitions in condensed matters and have traditionally been studied using various techniques spanning from neutron scattering to static mechanical testing. Here, using band-excitation elastic/piezoresponse spectroscopy, we probed sub-MHz elastic dynamics of a tip bias-induced rhombohedral−tetragonal phase transition of strained (001)-BiFeO₃ (rhombohedral) ferroelectric thin films from ∼10³ nm³ sample volumes. Near this transition, we observed that the Young's modulus intrinsically softens by over 30% coinciding with two- to three-fold enhancement of local piezoresponse. Coupled with phase-field modelling, we also addressed the influence of polarization switching and mesoscopic structural heterogeneities (for example, domain walls) on the kinetics of this phase transition, thereby providing fresh insights into the morphotropic phase boundary in ferroelectrics. Furthermore, the giant electrically tunable elastic stiffness and corresponding electromechanical properties observed here suggest potential applications of BiFeO₃ in next-generation frequency-agile electroacoustic devices, based on the utilization of the soft modes underlying successive ferroelectric phase transitions.

Ferroelectric materials possess spontaneous electrical polarization coupled to their underlying lattice structure, which may be utilized technologically. Here, the authors use band-excitation piezoresponse/elastic spectroscopy to study the sub-megahertz dynamics of a structural phase transition in BiFeO₃.

Polarization-Mediated Thermal Stability of Metal/Oxide Heterointerface

A polarization-mediated heterointerface is designed to research the thermal stability of magnetic metal/oxide interfaces. Using polarization engineering, the thermal stability of the interface between BiFeO3 and CoFeB can be improved by about 100°C. This finding provides new insight into the chemistry of the metal/oxide heterointerface.

Magnetoelectric Coupling Induced by Interfacial Orbital Reconstruction

Reversible orbital reconstruction driven by ferroelectric polarization modulates the magnetic performance of model ferroelectric/ferromagnetic heterostructures without onerous limitations. Mn-d(x2-y2) orbital occupancy and related interfacial exotic magnetic states are enhanced and weakened by negative and positive electric fields, respectively, filling the missing member-orbital in the mechanism of magnetoelectric coupling and advancing the application of orbitals to microelectronics.

Depth profiling charge accumulation from a ferroelectric into a doped Mott insulator

The electric field control of functional properties is a crucial goal in oxide-based electronics. Nonvolatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here, we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca(1-x)Ce(x)MnO3, thus providing insight on how interface-engineering can enhance these switching effects.

Synthetic magnetoelectric coupling in a nanocomposite multiferroic

Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), composite systems seem an attractive solution to realize magnetoelectric coupling between ferromagnetic and ferroelectric order parameters. Despite having antiferromagnetic order, BiFeO₃ (BFO) has nevertheless been a key material due to excellent ferroelectric properties at room temperature. We studied a superlattice composed of 8 repetitions of 6 unit cells of La_0.7Sr_0.3MnO₃ (LSMO) grown on 5 unit cells of BFO. Significant net uncompensated magnetization in BFO, an insulating superlattice, is demonstrated using polarized neutron reflectometry. Remarkably, the magnetization enables magnetic field to change the dielectric properties of the superlattice, which we cite as an example of synthetic magnetoelectric coupling. Importantly, controlled creation of magnetic moment in BFO is a much needed path toward design and implementation of integrated oxide devices for next generation magnetoelectric data storage platforms.

Ultrahigh energy density of polymer nanocomposites containing BaTiO3@TiO2 nanofibers by atomic-scale interface engineering

Atomic-scale interface engineering in BaTiO3@TO2 nanofibers (TiO2 nano-fibers embedded with BaTiO3 nano-particles) leads to concurrent enhancement of electric displacement and breakdown strength in poly(vinylidene fluoride) (PVDF)-based nanocomposites. An ultrahigh energy density of ≈20 J cm(-3) is achieved with only 3 vol% nanofibers, which is by far the highest discharged energy density of PVDF-based nanocomposites.

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.

Flexoelectric control of defect formation in ferroelectric epitaxial thin films

Flexoelectric control of defect formation and associated electronic function is demonstrated in ferroelectric BiFeO3 thin films. An intriguing, so far never demonstrated, effect of internal electric field (Eint ) on defect formation is explored by a means of flexoelectricity. Our study provides novel insight into defect engineering, as well as allows a pathway to design defect configuration and associated electronic function.

Tuning the entanglement between orbital reconstruction and charge transfer at a film surface

The interplay between orbital, charge, spin, and lattice degrees of freedom is at the core of correlated oxides. This is extensively studied at the interface of heterostructures constituted of two-layer or multilayer oxide films. Here, we demonstrate the interactions between orbital reconstruction and charge transfer in the surface regime of ultrathin (La,Sr)MnO₃, which is a model system of correlated oxides. The interactions are manipulated in a quantitative manner by surface symmetry-breaking and epitaxial strain, both tensile and compressive. The established charge transfer, accompanied by the formation of oxygen vacancies, provides a conceptually novel vision for the long-term problem of manganites—the severe surface/interface magnetization and conductivity deterioration. The oxygen vacancies are then purposefully tuned by cooling oxygen pressure, markedly improving the performances of differently strained films. Our findings offer a broad opportunity to tailor and benefit from the entanglements between orbit, charge, spin, and lattice at the surface of oxide films.

Thickness-dependent crossover from charge- to strain-mediated magnetoelectric coupling in ferromagnetic/piezoelectric oxide heterostructures

Magnetoelectric oxide heterostructures are proposed active layers for spintronic memory and logic devices, where information is conveyed through spin transport in the solid state. Incomplete theories of the coupling between local strain, charge, and magnetic order have limited their deployment into new information and communication technologies. In this study, we report direct, local measurements of strain- and charge-mediated magnetization changes in the La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 system using spatially resolved characterization techniques in both real and reciprocal space. Polarized neutron reflectometry reveals a graded magnetization that results from both local structural distortions and interfacial screening of bound surface charge from the adjacent ferroelectric. Density functional theory calculations support the experimental observation that strain locally suppresses the magnetization through a change in the Mn-eg orbital polarization. We suggest that this local coupling and magnetization suppression may be tuned by controlling the manganite and ferroelectric layer thicknesses, with direct implications for device applications.

Magnetic biasing of a ferroelectric hysteresis loop in a multiferroic orthoferrite

In a multiferroic orthoferrite Dy0.7Tb0.3FeO3, which shows electric-field-(E-)driven magnetization (M) reversal due to a tight clamping between polarization (P) and M, a gigantic effect of magnetic-field (H) biasing on P-E hysteresis loops is observed in the case of rapid E sweeping. The magnitude of the bias E field can be controlled by varying the magnitude of H, and its sign can be reversed by changing the sign of H or the relative clamping direction between P and M. The origin of this unconventional biasing effect is ascribed to the difference in the Zeeman energy between the +P and -P states coupled with the M states with opposite sign.

Piezoelectric power generation of vertically aligned lead-free (K,Na)NbO3 nanorod arrays

We demonstrate the potential of eco-friendly nanogenerators based on (K,Na)NbO₃ nanorod arrays for high-output power generation at room temperature and elevated temperature.

Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures

Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing non-trivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three-dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, and with interlayer quantum tunneling. We demonstrate that in the stack of bilayers grown along a (001)-direction, a non-trivial topological phase transition occurs above a critical number of Rashba bilayers. In the topological phase, we find the formation of a single spin-polarized Dirac cone at the -point. This approach offers an accessible way to design artificial topological insulators in a set up that takes full advantage of the atomic layer deposition approach. This design principle is tunable and also allows us to bypass limitations imposed by bulk crystal geometry.

Presently, the design of 3D topological insulators is limited to single-compound synthesis with appropriate symmetries. Here, the authors propose a new design principle for 3D topological insulators based on stacked 2D Fermi gases, which may allow for better control of topological properties.

Flexoelectric effect in the reversal of self-polarization and associated changes in the electronic functional properties of BiFeO(3) thin films

Flexoelectricity can play an important role in the reversal of the self-polarization direction in epitaxial BiFeO3 thin films. The flexoelectric and interfacial effects compete with each other to determine the self-polarization state. In Region I, the self-polarization is downward because the interfacial effect is more dominant than the flexoelectric effect. In Region II, the self-polarization is upward, because the flexoelectric effect becomes more dominant than the interfacial effect.

Ferroelectric control of a Mott insulator

The electric field control of functional properties is an important goal in oxide-based electronics. To endow devices with memory, ferroelectric gating is interesting, but usually weak compared to volatile electrolyte gating. Here, we report a very large ferroelectric field-effect in perovskite heterostructures combining the Mott insulator CaMnO₃ and the ferroelectric BiFeO₃ in its “supertetragonal” phase. Upon polarization reversal of the BiFeO₃ gate, the CaMnO₃ channel resistance shows a fourfold variation around room temperature, and a tenfold change at ~200 K. This is accompanied by a carrier density modulation exceeding one order of magnitude. We have analyzed the results for various CaMnO₃ thicknesses and explain them by the electrostatic doping of the CaMnO₃ layer and the presence of a fixed dipole at the CaMnO₃/BiFeO₃ interface. Our results suggest the relevance of ferroelectric gates to control orbital- or spin-ordered phases, ubiquitous in Mott systems, and pave the way toward efficient Mott-tronics devices.

Tuning the competition between ferromagnetism and antiferromagnetism in a half-doped manganite through magnetoelectric coupling

We investigate the possibility of controlling the magnetic phase transition of the heterointerface between a half-doped manganite La(0.5)Ca(0.5)MnO(3) and a multiferroic BiFeO(3) (BFO) through magnetoelectric coupling. Using macroscopic magnetometry and element-selective x-ray magnetic circular dichroism at the Mn and Fe L edges, we discover that the ferroelectric polarization of BFO controls simultaneously the magnetization of BFO and La(0.5)Ca(0.5)MnO(3) (LCMO). X-ray absorption spectra at the oxygen K edge and linear dichroism at the Mn L edge suggest that the interfacial coupling is mainly derived from the superexchange between Mn and Fe t(2g) spins. The combination of x-ray absorption spectroscopy and mean-field theory calculations reveals that the d-electron modulation of Mn cations changes the magnetic coupling in LCMO, which controls the enhanced canted moments of interfacial BFO via the interfacial coupling. Our results demonstrate that the competition between ferromagnetic and antiferromagnetic instability can be modulated by an electric field at the heterointerface, providing another pathway for the electrical field control of magnetism.

Ultrathin limit of exchange bias coupling at oxide multiferroic/ferromagnetic interfaces

Exchange bias coupling at the multiferroic- ferromagnetic interface in BiFeO₃ /La₀.₇ Sr₀.₃ MnO₃ heterostructures exhibits a critical thickness for ultrathin BiFeO₃ layers of 5 unit cells (2 nm). Linear dichroism measurements demonstrate the dependence on the BiFeO₃ layer thickness with a strong reduction for ultrathin layers, indicating diminished antiferromagnetic ordering that prevents interfacial exchange bias coupling.

Nucleation-induced self-assembly of multiferroic BiFeO3-CoFe2O4 nanocomposites

Large areas of perfectly ordered magnetic CoFe2O4 nanopillars embedded in a ferroelectric BiFeO3 matrix were successfully fabricated via a novel nucleation-induced self-assembly process. The nucleation centers of the magnetic pillars are induced before the growth of the composite structure using anodic aluminum oxide (AAO) and lithography-defined gold membranes as hard mask. High structural quality and good functional properties were obtained. Magneto-capacitance data revealed extremely low losses and magneto-electric coupling of about 0.9 μC/cmOe. The present fabrication process might be relevant for inducing ordering in systems based on phase separation, as the nucleation and growth is a rather general feature of these systems.

Ferroelectric control of the conduction at the LaAlO₃/SrTiO₃ heterointerface

Modulation of band bending at a complex oxide heterointerface by a ferroelectric layer is demonstrated. The as-grown polarization (Pup ) leads to charge depletion and consequently low conduction. Switching the polarization direction (Pdown ) results in charge accumulation and enhances the conduction at the interface. The metal-insulator transition at a conducting polar/nonpolar oxide heterointerface can be controlled by ferroelectric doping.