Ferroelectric Domain Walls Research Articles

Erasable Ferroelectric Domain Wall Diodes* *Supported by the National Key Basic Research Program of China (Grant No. 2019YFA0308500), the Basic Research Project of Shanghai Science and Technology Innovation Action (Grant No. 17JC1400300), and the National Natural Science Foundation of China (Grant Nos. 61674044 and 61904034).

The unipolar diode-like domain wall currents in LiNbO3 single-crystal nanodevices are not only attractive in terms of their applications in nonvolatile ferroelectric domain wall memory, but also useful in half-wave and full-wave rectifier systems, as well as detector, power protection, and steady voltage circuits. Unlike traditional diodes, where the rectification functionality arises from the contact between n-type and p-type conductors, which are unchanged after off-line production, ferroelectric domain wall diodes can be reversibly created, erased, positioned, and shaped, using electric fields. We demonstrate such functionality using ferroelectric mesa-like cells, formed at the surface of an insulating X-cut LiNbO3 single crystal. Under the application of an in-plane electric field above a coercive field along the polar Z axis, the domain within the cell is reversed to be antiparallel to the unswitched bottom domain via the formation of a conducting domain wall. The wall current was rectified using two interfacial volatile domains in contact with two side Pt electrodes. Unlike the nonvolatile inner domain wall, the interfacial domain walls disappear to turn off the wall current path after the removal of the applied electric field, or under a negative applied voltage, due to the built-in interfacial imprint fields. These novel devices have the potential to facilitate the random definition of diode-like elements in modern large-scale integrated circuits.

Manipulating ferroelectric behaviors via electron-beam induced crystalline defects.

Ferroelectric nanoplates are attractive for applications in nanoelectronic devices. Defect engineering has been an effective way to control and manipulate ferroelectric properties in nanoscale devices. Defects can act as pinning centers for ferroelectric domain wall motion, altering the switching properties and domain dynamics of ferroelectrics. However, there is a lack of detailed investigation on the interactions between defects and domain walls in ferroelectric nanoplates due to the limitation of previous characterization techniques, which impedes the development of defect engineering in ferroelectric nanodevices. In this study, we applied in situ biasing transmission electron microscopy to explore how dislocation loops, which were judiciously introduced into barium titanate nanoplates via electron beam irradiation, affect the motion of ferroelectric domain walls. The results show that the motion was dramatically suppressed by these localized defects, because of the local strain fields induced by the defects. The pinning effect can be further enhanced by multiple domain walls embedded with defect arrays. These results indicate the possibility of manipulating domain switching in ferroelectric nanoplates via the electron beam.

Magnetoelectric Coupling and Decoupling in Multiferroic Hexagonal YbFeO3 Thin Films

The coupling between ferroelectric and magnetic orders in multiferroic materials and the nature of magnetoelectric (ME) effects are enduring experimental challenges. In this work, we have studied the response of magnetization to ferroelectric switching in thin-film hexagonal YbFeO3, a prototypical improper multiferroic. The bulk ME decoupling and potential domain-wall ME coupling were revealed using x-ray magnetic circular dichroism (XMCD) measurements with in-situ ferroelectric polarization switching. Our Landau theory analysis suggests that the bulk ME-coupled ferroelectric switching path has a higher energy barrier than that of the ME-decoupled path; this extra barrier energy is also too high to be reduced by the magneto-static energy in the process of breaking single magnetic domains into multi-domains. In addition, the reduction of magnetization around the ferroelectric domain walls predicted by the Landau theory may induce the domain-wall ME coupling in which the magnetization is correlated with the density of ferroelectric domain walls. These results provide important experimental evidence and theoretical insights into the rich possibilities of ME couplings in hexagonal ferrites, such as manipulating the magnetic states by an electric field.

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.

Large domain-wall current in BiFeO3 epitaxial thin films

Large domain wall conductivity in insulating ferroelectric thin films provides a new idea for non-destructive readout of domain information of a nonvolatile ferroelectric domain wall memory. However, there are several challenges hindering its development, including the excessive leakage current and insufficient on-state read current. A lot of experimental and theoretical data have referred to the accumulation of oxygen vacancies at the domain wall regions of ferroelectric thin films as the extrinsic contribution of the wall conduction. In this work, we found that the wall current varied with the angle between the applied electric field and the initial polarization, and that the wall current could be increased to 1.9 μA for a bismuth ferrite thin-film device with the reduced oxygen vacancies in contrast to the previous value of 2 nA for the device with rich oxygen vacancies. The oxygen vacancy concentration can be controlled through the thermal annealing of the sample at two atmospheres of 5% hydrogen mixed with argon gas and pure oxygen gas. For the former, the enhanced oxygen vacancies within the annealed sample at the reduced atmosphere decreased the domain wall current by over a few orders of magnitude, in contrast to the significant enhancement of the wall current for the sample in the latter annealed at the oxidizing atmosphere. The change of oxygen vacancy concentration was implied from the Fe2+⇔Fe3+ transition by the X-ray photoelectron spectroscopy analysis. This change confirmed the rigid domain wall conduction mechanism as well as the extrinsic adverse defect compensation of the wall current, paving the way to the exploration of high-power domain-wall nanodevices with sufficient output currents.

Nonlinear focal mapping of ferroelectric domain walls in LiNbO3: Analysis of the SHG microscopy contrast mechanism

Second-harmonic (SH) microscopy is a widely used tool for the study of ferroelectric domains, domain walls, and their substructure. Yet, the contrast mechanism, particularly for the commonly used large numerical aperture, is not fully understood. In this work, we examine the contrast mechanism of SH microscopy in periodically poled LiNbO3 for the case of tightly focused laser beams and in the surface-near regime. The results are interpreted along theoretical calculations that include a vectorial field model for excitation and detection. Our model suggests that the characteristic contrasts mainly originate from interference patterns in the signal due to the sign change of the nonlinear susceptibility at the domain boundary. We find that for large numerical apertures, the tight focusing induces polarization components (axial and orthogonal to incident polarization), and the subsequent mixing of differently polarized light fields via off diagonal tensor elements plays an important role for the domain wall contrast. With our model-based analysis, this work represents the foundation for the investigation of the substructure of domain walls with second-harmonic microscopy.

Modeling ferroelectric domain walls motion and nonlinear dielectric response

The dynamics of ferroelectrics domain walls is modeled by the equation of a particle in a force field. It is considered that over the domain walls act the forces due to the interaction with an external electric field the internal mechanical and electric interactions represented by the effective potential and the friction force produced by the interaction of domain walls with phonons and point defects. The effective potential is determined assuming that the dielectric permittivity dependence on the electric field is given by the relation Simulations reproduce several important features of nonlinear dielectric behavior of ferroelectrics in good agreement with experimental data. Defects are included in the model introducing a generalized damping force with which it is possible to explain the high losses observed for subswitching electric fields. This theoretical approach can be useful in domain wall engineering and to develop new applications of ferroelectrics.

High temperature ferroelectric domain wall memory

For automotive and space applications, the nonvolatile ferroelectric domain wall random access memory (DWRAM) is required to work at a wide temperature range of −40 °C to +150 °C. However, the wall current generally decays with the retention time at high temperatures with the uncertain mechanism. Here we observed the peaked domain wall (DW) currents to decay with time at 450 K in LiNbO3 single crystals, which was correlated with the charge injection into domain wall regions in the mobility of 6.4 × 10−9 cm2/V·s. The extrinsic charge injection suppresses the intrinsic diode-like DW current due to the local Li/O vacancy redistribution to compensate the domain boundary charge. But the injected charge can be removed at an opposite poling voltage and can be frozen upon cooling down to 300 K. The traces of frozen wall regions become conducting upon the wall erasure. Once the memory write and read times are far shorter than the charge injection time, the DWRAM shows good retention of written information with a large on/off current ratio of ~104 at 450 K, suitable to the applications in some extreme conditions.

Giant thermoelectric power factor in charged ferroelectric domain walls of GeTe with Van Hove singularities

Increasing the Seebeck coefficient S in thermoelectric materials usually drastically decreases the electrical conductivity σ, making significant enhancement of the thermoelectric power factor σS2 extremelly challenging. Here we predict, using first-principles calculations, that the extraordinary properties of charged ferroelectric domain walls (DWs) in GeTe enable a five-fold increase of σS2 in the DW plane compared to bulk. The key reasons for this enhancement are the confinement of free charge carriers at the DWs and Van Hove singularities in the DW electronic band structure near the Fermi level. These effects lead to an increased energy dependence of the DW electronic transport properties, resulting in more than a two-fold increase of S with respect to bulk, without considerably degrading the in-plane σ. We propose a design of a nano-thermoelectric device that utilizes the exceptional thermoelectric properties of charged ferroelectric DWs. Our findings should inspire further investigation of ferroelectric DWs as efficient thermoelectric materials.

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

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

Characterization of ferroelectric domain walls by scanning electron microscopy

Ferroelectric domain walls are a completely new type of functional interface, which have the potential to revolutionize nanotechnology. In addition to the emergent phenomena at domain walls, they are spatially mobile and can be injected, positioned, and deleted on demand, giving a new degree of flexibility that is not available at conventional interfaces. Progress in the field is closely linked to the development of modern microscopy methods, which are essential for studying their physical properties at the nanoscale. In this article, we discuss scanning electron microscopy (SEM) as a powerful and highly flexible imaging technique for scale-bridging studies on domain walls, continuously covering nano- to mesoscopic length scales. We review seminal SEM experiments on ferroelectric domains and domain walls, provide practical information on how to visualize them in modern SEMs, and provide a comprehensive overview of the models that have been proposed to explain the contrast formation in SEM. Going beyond basic imaging experiments, recent examples for nano-structuring and correlated microscopy work on ferroelectric domain walls are presented. Other techniques, such as 3D atom probe tomography, are particularly promising and may be combined with SEM in the future to investigate individual domain walls, providing new opportunities for tackling the complex nanoscale physics and defect chemistry at ferroelectric domain walls.

Application of a long short-term memory for deconvoluting conductance contributions at charged ferroelectric domain walls

Ferroelectric domain walls are promising quasi-2D structures that can be leveraged for miniaturization of electronics components and new mechanisms to control electronic signals at the nanoscale. Despite the significant progress in experiment and theory, however, most investigations on ferroelectric domain walls are still on a fundamental level, and reliable characterization of emergent transport phenomena remains a challenging task. Here, we apply a neural-network-based approach to regularize local I(V)-spectroscopy measurements and improve the information extraction, using data recorded at charged domain walls in hexagonal (Er0.99,Zr0.01)MnO3 as an instructive example. Using a sparse long short-term memory autoencoder, we disentangle competing conductivity signals both spatially and as a function of voltage, facilitating a less biased, unconstrained and more accurate analysis compared to a standard evaluation of conductance maps. The neural-network-based analysis allows us to isolate extrinsic signals that relate to the tip-sample contact and separating them from the intrinsic transport behavior associated with the ferroelectric domain walls in (Er0.99,Zr0.01)MnO3. Our work expands machine-learning-assisted scanning probe microscopy studies into the realm of local conductance measurements, improving the extraction of physical conduction mechanisms and separation of interfering current signals.

First-principles study of topologically protected vortices and ferroelectric domain walls in hexagonal YGaO3

Ferroelectric behavior on the meso- and macroscopic scale depends on the formation and dynamics of domains and controlling the domain patterns is imperative to device performance. While density functional theory (DFT) calculations have successfully described the basic properties of ferroelectric domain walls, the necessarily small cell sizes used for the calculations hampers DFT studies of complex domain patterns. Here, we simulate large-scale complex ferroelectric domain patterns in ferroelectric ${\mathrm{YGaO}}_{3}$ using multisite local orbitals as implemented in the DFT code conquest. The multisite local orbital basis set gives similar bulk structural and electronic properties, and atomic domain wall structures and energetics as those obtained with conventional plane-wave DFT. With this basis set model, 3600-atom cells are used to simulate topologically protected vortices. The local atomic structure at the vortex cores is subtly different from the domain walls, with a lower electronic band gap suggesting enhanced local conductance at these cores.

Effect of ferroelectric domain walls on the dielectric properties of PbZrO3 thin films

In antiferroelectric PbZrO3 thin films, a weak residual ferroelectric phase is often observed on the double hysteresis loop and it is important to know its impact on the dielectric properties. To study this residual phase, a low and homogeneous electric field can be used because antiferroelectric domain walls are not sensitive to homogeneous fields; thus, contributions of ferroelectric domain wall motions to permittivity and dielectric losses can be isolated. In this paper, the hyperbolic law characterization is used on lead zirconate thin films, which present a residual ferroelectric phase. The study shows that domain wall contributions of the ferroelectric phase are small (less than 2% of the total permittivity), but their impacts are very important in the overall dielectric losses (≈26%). These losses are, however, lower than those obtained in pure ferroelectric materials due to a residual state composed of well distributed ferroelectric clusters of small size with no interactions between domain walls.

Investigation for the Thermodynamic Polarization Degradation of the BaTiO3 Crystal Thin Films

According to the latest development of memory, it is found that the motion of ferroelectric domain wall is very important for the high-density non-volatile devices of ferroelectric materials. This paper mainly studies the microstructure of barium titanate (BaTiO3) crystal films with the molecular dynamics of crystal domain motion and then models the polarization degradation with the conservation of the nucleation energy-focusing and free energy thermodynamic processes in the crystalline domains. First, by the means of XRD diffractometer and JADE software, the crystal axis orientation of BaTiO3 thin film is analysed, and then the crystal axis orientation is determined by referring the published measurement results. Further, for the ferroelectric crystal BaTiO3 thin film, the thermodynamic process is studied and the temperature dependences of the polarization ratio with respect to the spontaneous polarization is modelled. Numerical simulations have shown the much higher temperature dependence on the nucleation energy than on the free energy in a domain wall system, leading to the possibility to reach a high polarization ratio with the optimal deposition conditions to control the local energy and the nucleation energy cost of crystalline domains.

Giant Domain Wall Conductivity in Self‐Assembled BiFeO3 Nanocrystals

AbstractEver‐increasing demand on electronic devices with ultrahigh‐density non‐volatile data storage has attracted great interest in novel ferroelectric memories based on conductive ferroelectric domain walls. Embedded in an insulating material, ferroelectric domain walls have the capability of being (re)created, displaced, erased, and altered in their spatial configurations and electronic characteristics. However, the domain wall conductivities are in most cases not yet sufficiently high to ensure the current density required to drive read‐out circuits operating at high speeds. In this work, a giant domain wall current (>10 µA) of a single charged domain wall is obtained through conductive atomic force microscopy with a bias field of 4 V. This is achieved in self‐assembled BiFeO3 nanocrystals grown by sol‐gel method on Nb‐doped SrTiO3 substrates. Local configurations of domains and domain wall types are studied using vector piezoresponse force microscopy and high‐resolution transmission electronic microscopy. The enhancement of the wall current is shown to be due to the formation of conducting pathways of charged defects accumulated along domain walls and traversing the nanocrystals. The diverse domain walls can be manipulated by electric field in a perpendicular architecture. The perpendicular array structure of BiFeO3 nanocrystals should have great potentials for developing perpendicular nanoelectronic prototypes.

Domain-wall engineering and topological defects in ferroelectric and ferroelastic materials

Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level. When this spatial confinement is combined with observations of emergent functional properties, such as polarity in non-polar systems or electrical conductivity in otherwise insulating materials, it becomes clear that domain walls represent new and exciting objects in matter. In this Review, we discuss the exotic polarization profiles that can arise at domain walls with multiple order parameters and the different mechanisms that lead to domain-wall polarity in non-polar ferroelastic materials. The emergence of energetically degenerate variants of the domain walls themselves suggests the existence of interesting quasi-1D topological defects within such walls. We also provide an overview of the general notions that have been postulated as fundamental mechanisms responsible for domain-wall conduction in ferroelectrics. We then discuss the prospect of combining domain walls with transition regions observed at phase boundaries, homo- and heterointerfaces, and other quasi-2D objects, enabling emergent properties beyond those available in today’s topological systems. Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level and emergent functional properties. This Review discusses the exotic polarization profiles that arise at domain walls and the fundamental mechanisms responsible for domain-wall conduction.

Optoelectronic Ferroelectric Domain‐Wall Memories Made from a Single Van Der Waals Ferroelectric

AbstractDue to the potential applications in optoelectronic memories, optical control of ferroelectric domain walls has emerged as an intriguing and important topic in modern solid‐state physics. However, its device implementation in a single ferroelectric, such as conventional BaTiO3 or PZT ceramics, still presents huge challenges in terms of the poor material conductivity and the energy mismatch between incident photons and ferroelectric switching. Here, using the generation of photocurrent in conductive α‐In2Se3 (a van der Waals ferroelectric) with a two‐terminal planar architecture, the first demonstration of optical‐engineered ferroelectric domain wall in a non‐volatile manner for optoelectronic memory application is reported. The α‐In2Se3 device exhibits a large optical‐writing and electrical‐erasing (on/off) ratio of >104, as well as multilevel current switching upon optical excitation. The narrow direct bandgap of the multilayer α‐In2Se3 ferroelectric endows the device with broadband optical‐writing wavelengths greater than 900 nm. In addition, photonic synapses with approximate linear weight updates for neuromorphic computing are also achieved in the ferroelectric devices. This work represents a breakthrough toward technological applications of ferroelectric nanodomain engineering by light.

Tunable Non-Volatile Memory by Conductive Ferroelectric Domain Walls in Lithium Niobate Thin Films

Ferroelectric domain wall conductance is a rapidly growing field. Thin-film lithium niobate, as in lithium niobate on insulators (LNOI), appears to be an ideal template, which is tuned by the inclination of the domain wall. Thus, the precise tuning of domain wall inclination with the applied voltage can be used in non-volatile memories, which store more than binary information. In this study, we present the realization of this concept for non-volatile memories. We obtain remarkably stable set voltages by the ferroelectric nature of the device as well as a very large increase in the conduction, by at least five orders of magnitude at room temperature. Furthermore, the device conductance can be reproducibly tuned over at least two orders of magnitude. The observed domain wall (DW) conductance tunability by the applied voltage can be correlated with phase-field simulated DW inclination evolution upon poling. Furthermore, evidence for polaron-based conduction is given.

Interface magnetoelectric effect and its sensitivity on interface structures in Fe/AgNbO3 and SrRuO3/AgNbO3 heterostructures: A first-principles investigation

The interface magnetoelectric (ME) effect is promising for novel technological applications as it allows the control of interface magnetization at heterointerfaces by the external electric field. Here in this article, we explore the interface ME effect and its sensitivity on interface atomic structure and bonding in ferromagnetic-ferroelectric oxide hetrostructures within the framework of density functional theory. In particular, we consider Fe/AgNbO3 and SrRuO3/AgNbO3 superlattices with different possible defect-free interfaces. Our results suggest that interface magnetization and thereby interface ME effect can critically depend on interface structure. The interface magnetization is found to depend on atomic bondings which are sensitive to atomic displacements at the interface. This leads to ME coupling due to induced change in the interface magnetization as the electric polarization is reversed in the ferroelectric film. In addition, the contribution to interface ME coupling also comes from the change in exchange-splitting between spin-polarized electrons and consequently change in the interface magnetic moments due to ferroelectric polarization reversal. The change in exchange splitting, in turn, is caused by spin-dependent screening of the bound polarization charges at the interface. In Fe/AgNbO3 system, the interface ME coefficient is found to exhibit opposing trend for Fe|AgO and Fe|NbO2 interfaces. In addition to interface ME effect, the formation of ferroelectric interface domain wall (IDW) in these systems is also explored. The IDW is found to form only in SrRuO3/AgNbO3 system with SrO|NbO2 interface due to oppositely oriented strong dipole moments at the two interfaces. The strong dependence of interface ME effect on interface atomic structure may have implication for its experimental observation in oxide heterostructures. In particular, the interface defects may render the conclusive observation of this phenomenon challenging due to resulting fluctuations in the observed data.