Stationary domain wall contribution to enhanced ferroelectric susceptibility

High Entropy Nonlinear Dielectrics with Superior Thermally Stable Performance

A high configurational entropy, achieved through a proper design of compositions, can minimize the Gibbs free energy and stabilize the quasi-equilibrium phases in a solid-solution form. This leads to the development of high-entropy materials with unique structural characteristics and excellent performance, which otherwise could not be achieved through conventional pathways. This work develops a high-entropy nonlinear dielectric system, based on the expansion of lead magnesium niobate-lead titanate. A dense and uniform distribution of nano-polar regions is observed in the samples owing to the addition of Ba, Hf, and Zr ions, which lead to enhanced performance of nonlinear dielectrics. The fact that no structural phase transformation is detected up to 250 °C, and no noticeable change or a steep drop in structural and electrical characteristics is observed at high temperatures suggests a robust thermal stability of the dielectric systems developed. With these advantages, these materials hold vast potential for applications such as dielectric energy storage, dielectric tunability, and electrocaloric effect. Thus, this work offers a new high-entropy configuration with elemental modulation, with enhanced dielectric material features.

Strain-Driven Mixed-Phase Domain Architectures and Topological Transitions in Pb1- x Srx TiO3 Thin Films

The potential for creating hierarchical domain structures, or mixtures of energetically degenerate phases with distinct patterns that can be modified continually, in ferroelectric thin films offers a pathway to control their mesoscale structure beyond lattice-mismatch strain with a substrate. Here, it is demonstrated that varying the strontium content provides deterministic strain-driven control of hierarchical domain structures in Pb_{1- x} Sr_x TiO₃  solid-solution thin films wherein two types, c/a and a₁ /a₂ , of nanodomains can coexist. Combining phase-field simulations, epitaxial thin-film growth, detailed structural, domain, and physical-property characterization, it is observed that the system undergoes a gradual transformation (with increasing strontium content) from droplet-like a₁ /a₂  domains in a c/a domain matrix, to a connected-labyrinth geometry of c/a domains, to a disconnected labyrinth structure of the same, and, finally, to droplet-like c/a domains in an a₁ /a₂  domain matrix. A relationship between the different mixed-phase modulation patterns and its topological nature is established. Annealing the connected-labyrinth structure leads to domain coarsening forming distinctive regions of parallel c/a and a₁ /a₂  domain stripes, offering additional design flexibility. Finally, it is found that the connected-labyrinth domain patterns exhibit the highest dielectric permittivity.

Thin-Film Ferroelectrics

Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial-thin-film-based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro-, meso-, and macroscopic length scales. This review traces the evolution of ferroelectric thin-film research through the early days developing understanding of the roles of size and strain on ferroelectrics to the present day, where such understanding is used to create complex hierarchical domain structures, novel polar topologies, and controlled chemical and defect profiles. The extension of epitaxial techniques, coupled with advances in high-throughput simulations, now stands to accelerate the discovery and study of new ferroelectric materials. Coming hand-in-hand with these new materials is new understanding and control of ferroelectric functionalities. Today, researchers are actively working to apply these lessons in a number of applications, including novel memory and logic architectures, as well as a host of energy conversion devices.

High-density switchable skyrmion-like polar nanodomains integrated on silicon

Topological domains in ferroelectrics^1-5 have received much attention recently owing to their novel functionalities and potential applications^6,7 in electronic devices. So far, however, such topological polar structures have been observed only in superlattices grown on oxide substrates, which limits their applications in silicon-based electronics. Here we report the realization of room-temperature skyrmion-like polar nanodomains in lead titanate/strontium titanate bilayers transferred onto silicon. Moreover, an external electric field can reversibly switch these nanodomains into the other type of polar texture, which substantially modifies their resistive behaviours. The polar-configuration-modulated resistance is ascribed to the distinct band bending and charge carrier distribution in the core of the two types of polar texture. The integration of high-density (more than 200 gigabits per square inch) switchable skyrmion-like polar nanodomains on silicon may enable non-volatile memory applications using topological polar structures in oxides.

Strain coupling of ferroelastic domains and misfit dislocations in [101]-oriented ferroelectric PbTiO3 films

High-index perovskite ferroelectric thin films possess excellent dielectric permittivity, piezoelectric coefficient, and exotic ferroelectric switching properties. They also exhibit complications in the ferroelastic domains, misfit dislocations, and strain-relaxation behaviors. Exploring the relationship of the ferroelastic domains and misfit dislocations may be of benefit for promoting the high-quality growth of these thin films. Here, the strain field of the ferroelastic domains and misfit dislocations in [101]-oriented PbTiO₃/(La, Sr)(Al, Ta)O₃ epitaxial thin films were investigated by advanced aberration-corrected (scanning) transmission electron microscopy (TEM) combined with geometry phase analysis (GPA). Two types of misfit dislocations with projected Burgers vectors of a[001] or a[100] on the (010) plane were elucidated, whose strain fields included in-plane strain and lattice rotation coupled with the c domains above them. Besides, it was demonstrated that the coupling was kept inside the PbTiO₃ films when the film thickness was increased. Furthermore, the polarization rotation was observed in both narrow c domains and around the misfit dislocation cores, which may be attributed to the flexoelectric effect. These results are expected to provide useful information for understanding the nucleation and propagation mechanism of ferroelastic domains and for further modifying the growth of high-index ferroelectric thin films.

The strain coupling of misfit dislocations and ferroelastic domains is revealed in [101]-oriented PbTiO₃/(La, Sr)(Al, Ta)O₃ films and flexoelectric-induced polarization rotation is observed around the misfit dislocation cores.

A CMOS-compatible morphotropic phase boundary

Morphotropic phase boundaries (MPBs) show substantial piezoelectric and dielectric responses, which have practical applications. The predicted existence of MPB in HfO₂-ZrO₂solid solution thin film has provided a new way to increase the dielectric properties of a silicon-compatible device. Here, we present a new fabrication design by which the density of MPBρMPBand consequently the dielectric constantϵ_rof HfO₂-ZrO₂thin film was considerably increased. TheρMPBwas controlled by fabrication of a 10 nm [1 nm Hf_0.5Zr_0.5O₂(ferroelectric)/1 nm ZrO₂(antiferroelectric)] nanolaminate followed by an appropriate annealing process. The coexistence of orthorhombic and tetragonal structures, which are the origins of ferroelectric (FE) and antiferroelectric (AFE) behaviors, respectively, was structurally confirmed, and a double hysteresis loop that originates from AFE ordering, with some remnant polarization that originates from FE ordering, was observed inP-Ecurve. A remarkable increase inϵ_rcompared to the conventional HfO₂-ZrO₂thin film was achieved by controlling the FE-AFE ratio. The fabrication process was performed at low temperature (250 °C) and the device is compatible with silicon technology, so the new design yields a device that has possible applications in near-future electronics.

Terahertz Reading of Ferroelectric Domain Wall Dielectric Switching

Ferroelectric domain walls (DWs) are important nanoscale interfaces between two domains. It is widely accepted that ferroelectric domain walls work idly at terahertz (THz) frequencies, consequently discouraging efforts to engineer the domain walls to create new applications that utilize THz radiation. However, the present work clearly demonstrates the activity of domain walls at THz frequencies in a lead-free Aurivillius phase ferroelectric ceramic, Ca_0.99Rb_0.005Ce_0.005Bi₂Nb₂O₉, examined using THz-time-domain spectroscopy (THz-TDS). The dynamics of domain walls are different at kHz and THz frequencies. At low frequencies, domain walls work as a group to increase dielectric permittivity. At THz frequencies, the defective nature of domain walls serves to lower the overall dielectric permittivity. This is evidenced by higher dielectric permittivity in the THz band after poling, reflecting decreased domain wall density. An elastic vibrational model has also been used to verify that a single frustrated dipole in a domain wall represents a weaker contribution to the permittivity than its counterpart within a domain. The work represents a fundamental breakthrough in understanding the dielectric contributions of domain walls at THz frequencies. It also demonstrates that THz probing can be used to read domain wall dielectric switching.

Recent Progress on Topological Structures in Ferroic Thin Films and Heterostructures

Topological spin/polarization structures in ferroic materials continue to draw great attention as a result of their fascinating physical behaviors and promising applications in the field of high-density nonvolatile memories as well as future energy-efficient nanoelectronic and spintronic devices. Such developments have been made, in part, based on recent advances in theoretical calculations, the synthesis of high-quality thin films, and the characterization of their emergent phenomena and exotic phases. Herein, progress over the last decade in the study of topological structures in ferroic thin films and heterostructures is explored, including the observation of topological structures and control of their structures and emergent physical phenomena through epitaxial strain, layer thickness, electric, magnetic fields, etc. First, the evolution of topological spin structures (e.g., magnetic skyrmions) and associated functionalities (e.g., topological Hall effect) in magnetic thin films and heterostructures is discussed. Then, the exotic polar topologies (e.g., domain walls, closure domains, polar vortices, bubble domains, and polar skyrmions) and their emergent physical properties in ferroelectric oxide films and heterostructures are explored. Finally, a brief overview and prospectus of how the field may evolve in the coming years is provided.

Modulation of charged a1/a2 domains and piezoresponses of tensile strained PbTiO3 films by the cooling rate

Controlling domain width, orientation, and patterns in oxide ferroelectrics are not only important for fundamental research but also for potential electronic application. Here, a series of PbTiO₃ thin films under various cooling rates were deposited on (110)-oriented NdScO₃ substrates by pulsed laser deposition and investigated by using conventional transmission electron microscopy, Cs-corrected scanning TEM and piezoresponse force microscopy. Contrast analysis and electron diffraction revealed that PbTiO₃ films are a₁/a₂ domain patterns under large tensile strains with different cooling rates. The a₁/a₂ domains distribute periodically and the domain width increases with decrease in the cooling rates. Upon increasing the cooling rate, the domain density increases and the domain configurations become complicated. There are special square frame-like domain patterns with charged domain walls found in the PTO films with the fast cooling rate. PFM measurement shows that the PTO films with high cooling rate exhibit enhanced piezoresponse behavior which is ascribed to the high density domain/domain walls and special domain configurations. The formation mechanism of the different domain configurations is discussed in terms of the effect of cooling rates, defects and thermal kinetics. These results are expected to provide useful information for domain/domain wall control and thus facilitate further modulation of the properties for potential applications.

The cooling rates can induce domain configurations to change, thus realizing the evolution of the domains of the PTO films.

Kinetic control of tunable multi-state switching in ferroelectric thin films

Deterministic creation of multiple ferroelectric states with intermediate values of polarization remains challenging due to the inherent bi-stability of ferroelectric switching. Here we show the ability to select any desired intermediate polarization value via control of the switching pathway in (111)-oriented PbZr_0.2Ti_0.8O₃ films. Such switching phenomena are driven by kinetic control of the volume fraction of two geometrically different domain structures which are generated by two distinct switching pathways: one direct, bipolar-like switching and another multi-step switching process with the formation of a thermodynamically-stable intermediate twinning structure. Such control of switching pathways is enabled by the competition between elastic and electrostatic energies which favors different types of ferroelastic switching that can occur. Overall, our work demonstrates an alternative approach that transcends the inherent bi-stability of ferroelectrics to create non-volatile, deterministic, and repeatedly obtainable multi-state polarization without compromising other important properties, and holds promise for non-volatile multi-state functional applications.

The use of ferroeletric materials for multi-state device applications is still challenging. Here, the authors present a mechanism to stabilize non-volatile polarization states by populating volume fractions of two domain structures in PbZr_0.2Ti_0.8O₃ via kinetic control of switching pathways.

Effects of poling and crystallinity on the dielectric properties of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 at cryogenic temperatures

The mechanisms underlying the anomalously large, room temperature piezoelectric activity of relaxor-PbTiO₃ type single crystals have previously been linked to low temperature relaxations in the piezoelectric and dielectric properties. We investigate the properties of Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ between 10 and 300 K using dielectric permittivity measurements. We compare results on single crystal plates measured in the [001] and [111] directions with a polycrystalline ceramic of the same composition. Poled crystals have very different behaviour to unpoled crystals, whereas the dielectric spectrum of the polycrystalline ceramic changes very little on poling. A large, frequency dependent dielectric relaxation is seen in the poled [001] crystal around 100 K. The relaxation is much less prominent in the [111] cut crystal, and is not present in the polycrystalline ceramic. The unique presence of the large relaxation in poled, [001] oriented crystals indicates that the phenomenon is not due their relaxor nature alone. We propose that heterophase dynamics such as the motion of phase domain boundaries are responsible for both the anomalous electromechanical and dielectric behaviour.

Thickness-Dependent Evolution of Piezoresponses and Stripe 90° Domains in (101)-Oriented Ferroelectric PbTiO3 Thin Films

High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO₃ (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO₃)_0.29(SrTa_1/2Al_1/2O₃)_0.71 (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM). PFM measurements reveal that periodic stripe domains are dominant in 50 nm thick PTO films. Besides stripe domains, a/ c domains appear in films with thickness more than 60 nm. A thickness-dependent evolution of piezoresponse amplitude indicates that the 50 nm thick PTO films demonstrate a superior piezoresponse. Electron diffraction and contrast analysis clarify that all these (101)-oriented PTO films contain periodic stripe ferroelectric 90° domains. The domain periods increase with the film thickness following Kittel's law. Aberration-corrected STEM imaging reveals that the stripe ferroelectric 90° domains have an alternate arrangement of wide and narrow c domains with polarization directions along [100] for c₁ domains and [001̅] for c₂ domains, forming a "head-to-tail" polarization configuration. Further strain analysis reveals that stripe domains have uniform strain distributions and distinct lattice rotations around domain walls. It is proposed that the periodic arrangement of high-density stripe 90° domains in 50 nm thick PTO films is the main contributor to the superior piezoresponse behavior. These results are expected to provide useful information to understand the domain structures in (101)-oriented PTO thin films and thus facilitate further modulation of the properties for potential applications.

Reducing Coercive-Field Scaling in Ferroelectric Thin Films via Orientation Control

The desire for low-power/voltage operation of devices is driving renewed interest in understanding scaling effects in ferroelectric thin films. As the dimensions of ferroelectrics are reduced, the properties can vary dramatically, including the robust scaling relationship between coercive field ( E_c) and thickness ( d), also referred to as the Janovec-Kay-Dunn (JKD) law, wherein E_c ∝ d^-2/3. Here, we report that whereas (001)-oriented heterostructures follow JKD scaling across the thicknesses range of 20-330 nm, (111)-oriented heterostructures of the canonical tetragonal ferroelectric PbZr_0.2Ti_0.8O₃ exhibit a deviation from JKD scaling wherein a smaller scaling exponent for the evolution of E_c is observed in films of thickness ≲ 165 nm. X-ray diffraction reveals that whereas (001)-oriented heterostructures remain tetragonal for all thicknesses, (111)-oriented heterostructures exhibit a transition from tetragonal-to-monoclinic symmetry in films of thickness ≲ 165 nm as a result of the compressive strain. First-principles calculations suggest that this symmetry change contributes to the deviation from the expected scaling, as the monoclinic phase has a lower energy barrier for switching. This structural evolution also gives rise to changes in the c/ a lattice parameter ratio, wherein this ratio increases and decreases in (001)- and (111)-oriented heterostructures, respectively, as the films are made thinner. In (111)-oriented heterostructures, this reduced tetragonality drives a reduction of the remanent polarization and, therefore, a reduction of the domain-wall energy and overall energy barrier to switching, which further exacerbates the deviation from the expected scaling. Overall, this work demonstrates a route toward reducing coercive fields in ferroelectric thin films and provides a possible mechanism to understand the deviation from JKD scaling.

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.

Charge screening strategy for domain pattern control in nano-scale ferroelectric systems

Strain engineering is a widespread strategy used to enhance performance of devices based on semiconductor thin films. In ferroelectrics strain engineering is used to control the domain pattern: When an epitaxial film is biaxially compressed, e.g. due to lattice mismatch with the substrate, the film displays out-of-plane, often strongly enhanced polarization, while stretching the film on the substrate results in in-plane polarization. However, this strategy is of a limited applicability in nanorods because of the small rod/substrate contact area. Here we demonstrate another strategy, in which the polar axis direction is controlled by charge screening. When charge screening is maintained by bottom and top metallization, the nanorods display an almost pure c-domain configuration (polarization perpendicular to the substrate); when the sidewalls of the nanorods are metallized too, a-domain formation prevails (polarization parallel to the substrate). Simulations of the depolarization fields under various boundary conditions support the experimental observations. The employed approach can be expanded to other low-dimensional nano-scale ferroelectric systems.

Stable charged antiparallel domain walls in hyperferroelectrics

Charge-neutral 180° domain walls that separate domains of antiparallel polarization directions are common structural topological defects in ferroelectrics. In normal ferroelectrics, charged 180° domain walls running perpendicular to the polarization directions are highly energetically unfavorable because of the depolarization field and are difficult to stabilize. We explore both neutral and charged 180° domain walls in hyperferroelectrics, a class of proper ferroelectrics with persistent polarization in the presence of a depolarization field, using density functional theory. We obtain zero temperature equilibrium structures of head-to-head and tail-to-tail walls in recently discovered ABC-type hexagonal hyperferroelectrics. Charged domain walls can also be stabilized in canonical ferroelectrics represented by LiNbO₃ without any dopants, defects or mechanical clamping. First-principles electronic structure calculations show that charged domain walls can reduce and even close the band gap of host materials and support quasi-two-dimensional electron(hole) gas with enhanced electrical conductivity.

Enhancing the Domain Wall Conductivity in Lithium Niobate Single Crystals

Domain walls (DWs) in ferroelectric/ferroic materials have been a central research focus for the last 50 years; DWs bear a multitude of extraordinary physical parameters within a unit-cell-sized lateral confinement. Especially, one outstanding feature has recently attracted a lot of attention for room-temperature applications, which is the potential to use DWs as two-dimensional (2D) conducting channels that completely penetrate bulk compounds. Domain wall currents in lithium niobate (LNO) so far lie in the lower pA regime. In this work, we report on an easy-to-use and reliable protocol that allows enhancing domain wall conductivity (DWC) in single-crystalline LNO (sc-LNO) by 3 to 4 orders of magnitude. sc-LNO thus has become one of the most prospective candidates to engineer DWC applications, notably for domain wall transport both with and without photoexcitation. DWs were investigated here for several days to weeks, both before and after DWC enhancement. 2D local-scale inspections were carried out using adequate local-probe techniques, i.e., piezoresponse force microscopy and conductive atomic force microscopy, while Cerenkov second-harmonic generation was applied for mapping the DW constitution in three-dimensional space across the full LNO single crystal. The comparison between these nano- and microscale inspections allows us to unambiguously correlate the DW inclination angle α close to the sample surface to the measured domain wall current distribution. Moreover, ohmic or diode-like electronic transport characteristics along such DWs can be readily interpreted when analyzing the DW inclination profile.

Polydomain structures in ferroelectric and ferroelastic epitaxial films

A review of theoretical models, phase field modeling and experimental studies of domain structures in epitaxial films is presented. The thermodynamic theory of such domain structures is presented within the macroscopic thermo-mechanical framework. The theory allows for the evaluation of the main parameters of the domain structure using the energy minimization approach applied to the energy of elastic interactions. For homophase (polytwin) films, the thermodynamic theory provides a quantitative tool that can be used to estimate domain fractions in the film and the type of domain structure architecture. For heterophase films, the theory describes (a) the conditions under which two-phase structures can be obtained in epitaxial films, and (b) the phase and domain fractions in these films. The thermodynamic theory can also be used to describe the extrinsic contributions from domain structures to the functional properties of epitaxial ferroelectric films. The review of phase field modeling demonstrates that computational results reproduce the predictions of the thermodynamic theory. It is also shown that the phase field modeling that utilizes the energy minimization procedure for elastic and interfacial energies can be used to predict domain morphology for the films with two-phase structures produced either by phase transformation or through the co-deposition of immiscible phases. The experimental data presented in the review validate predictions of the thermodynamic model and the results of phase field modeling.

Enhancement of Local Photovoltaic Current at Ferroelectric Domain Walls in BiFeO3

Domain walls, which are intrinsically two dimensional nano-objects exhibiting nontrivial electronic and magnetic behaviours, have been proven to play a crucial role in photovoltaic properties of ferroelectrics. Despite this recognition, the electronic properties of domain walls under illumination until now have been accessible only to macroscopic studies and their effects upon the conduction of photovoltaic current still remain elusive. The lack of understanding hinders the developing of nanoscale devices based on ferroelectric domain walls. Here, we directly characterize the local photovoltaic and photoconductive properties of 71° domain walls on BiFeO₃ thin films with a nanoscale resolution. Local photovoltaic current, proven to be driven by the bulk photovoltaic effect, has been probed over the whole illuminated surface by using a specially designed photoelectric atomic force microscopy and found to be significantly enhanced at domain walls. Additionally, spatially resolved photoconductive current distribution reveals a higher density of excited carriers at domain walls in comparison with domains. Our measurements demonstrate that domain wall enhanced photovoltaic current originates from its high conduction rather than the internal electric field. This photoconduction facilitated local photovoltaic current is likely to be a universal property of topological defects in ferroelectric semiconductors.

Misfit Strain Relaxation of Ferroelectric PbTiO3/LaAlO3 (111) Thin Film System

Ferroelectric thin films grown on high index substrates show unusual structural and switching dynamics due to their special strain states. Understanding the misfit relaxation behavior is crucial to facilitate the high index thin film growth with improved quality. In this paper, ferroelectric PbTiO₃ thin films were grown on LaAlO₃ (111) substrates by pulsed laser deposition technique. The microstructures were investigated by combinations of conventional and aberration-corrected transmission electron microscopy. Diffraction contrast analysis and high resolution imaging reveal that high density interfacial dislocations were distributed at the interfaces. These dislocations have mixed character with Burgers vectors of a <110> and line directions of <112>. The edge components of the dislocations, with the Burgers vectors parallel to the interface, accommodate the lattice mismatch and are the main contributor to the misfit relaxation of this system. The formation mechanism of these dislocations is proposed and discussed to elucidate the novel mismatch relaxation behavior of <111> oriented perovskite films.

Fabrication of Tetragonal Pb(Zr,Ti)O3 Nanorods by Focused Ion Beam and Characterization of the Domain Structure

It has been widely revealed and discussed that the properties of ferroelectric nanostructures vary with their dimensionality and size. The mechanical substrate clamping and the depolarization field are considered as major factors, which cause their unique properties. In this paper, we fabricated tetragonal {100}-Pb(Zr, Ti)O3 rods with 100 nm - 4 μm widths on Nb-doped SrTiO3 substrates by using focused ion beam, and characterized their domain structure by synchrotron micro X-ray diffraction. It was found that the clapping angle in the a/c-domain structure became larger with decreasing the rod width, which indicates the significant reduction of substrate clamping by fabricating narrow rods.

Néel-like domain walls in ferroelectric Pb(Zr,Ti)O3 single crystals

In contrast to the flexible rotation of magnetization direction in ferromagnets, the spontaneous polarization in ferroelectric materials is highly confined along the symmetry-allowed directions. Accordingly, chirality at ferroelectric domain walls was treated only at the theoretical level and its real appearance is still a mystery. Here we report a Néel-like domain wall imaged by atom-resolved transmission electron microscopy in Ti-rich ferroelectric Pb(Zr_1−xTi_x)O₃ crystals, where nanometre-scale monoclinic order coexists with the tetragonal order. The formation of such domain walls is interpreted in the light of polarization discontinuity and clamping effects at phase boundaries between the nesting domains. Phase-field simulation confirms that the coexistence of both phases as encountered near the morphotropic phase boundary promotes the polarization to rotate in a continuous manner. Our results provide a further insight into the complex domain configuration in ferroelectrics, and establish a foundation towards exploring chiral domain walls in ferroelectrics.

Flexible rotation of spontaneous polarization at ferroelectric domain walls is predicted in theory but lacks evidence from experiment. Here, Wei et al. image a Néel-like domain wall in Ti-rich ferroelectric Pb(Zr_1−xTi_x)O₃ crystals, providing insight in exploring chiral domain walls in ferroelectrics.

Controlled creation and displacement of charged domain walls in ferroelectric thin films

Charged domain walls in ferroelectric materials are of high interest due to their potential use in nanoelectronic devices. While previous approaches have utilized complex scanning probe techniques or frustrative poling here we show the creation of charged domain walls in ferroelectric thin films during simple polarization switching using either a conductive probe tip or patterned top electrodes. We demonstrate that ferroelectric switching is accompanied - without exception - by the appearance of charged domain walls and that these walls can be displaced and erased reliably. We ascertain from a combination of scanning probe microscopy, transmission electron microscopy and phase field simulations that creation of charged domain walls is a by-product of, and as such is always coupled to, ferroelectric switching. This is due to the (110) orientation of the tetragonal (Pb,Sr)TiO₃ thin films and the crucial role played by the limited conduction of the LSMO bottom electrode layer used in this study. This work highlights that charged domain walls, far from being exotic, unstable structures, as might have been assumed previously, can be robust, stable easily-controlled features in ferroelectric thin films.

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.

Highly mobile ferroelastic domain walls in compositionally graded ferroelectric thin films

Domains and domain walls are critical in determining the response of ferroelectrics, and the ability to controllably create, annihilate, or move domains is essential to enable a range of next-generation devices. Whereas electric-field control has been demonstrated for ferroelectric 180° domain walls, similar control of ferroelastic domains has not been achieved. Here, using controlled composition and strain gradients, we demonstrate deterministic control of ferroelastic domains that are rendered highly mobile in a controlled and reversible manner. Through a combination of thin-film growth, transmission-electron-microscopy-based nanobeam diffraction and nanoscale band-excitation switching spectroscopy, we show that strain gradients in compositionally graded PbZr1-xTixO3 heterostructures stabilize needle-like ferroelastic domains that terminate inside the film. These needle-like domains are highly labile in the out-of-plane direction under applied electric fields, producing a locally enhanced piezoresponse. This work demonstrates the efficacy of novel modes of epitaxy in providing new modalities of domain engineering and potential for as-yet-unrealized nanoscale functional devices.

Asymmetric Response of Ferroelastic Domain-Wall Motion under Applied Bias

The switching of domains in ferroelectric and multiferroic materials plays a central role in their application to next-generation computer systems, sensing applications, and memory storage. A detailed understanding of the response to electric fields and the switching behavior in the presence of complex domain structures and extrinsic effects (e.g., defects and dislocations) is crucial for the design of improved ferroelectrics. In this work, in situ transmission electron microscopy is coupled with atomistic molecular dynamics simulations to explore the response of 71° ferroelastic domain walls in BiFeO3 with various orientations under applied electric-field excitation. We observe that 71° domain walls can have intrinsically asymmetric responses to opposing biases. In particular, when the electric field has a component normal to the domain wall, forward and backward domain-wall velocities can be dramatically different for equal and opposite fields. Additionally, the presence of defects and dislocations can strongly affect the local switching behaviors through pinning or nucleation of the domain walls. These results offer insight for controlled ferroelastic domain manipulation via electric-field engineering.

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.

Ferroelectric polarization reversal via successive ferroelastic transitions

Switchable polarization makes ferroelectrics a critical component in memories, actuators and electro-optic devices, and potential candidates for nanoelectronics. Although many studies of ferroelectric switching have been undertaken, much remains to be understood about switching in complex domain structures and in devices. In this work, a combination of thin-film epitaxy, macro- and nanoscale property and switching characterization, and molecular dynamics simulations are used to elucidate the nature of switching in PbZr(0.2)Ti(0.8)O3 thin films. Differences are demonstrated between (001)-/(101)- and (111)-oriented films, with the latter exhibiting complex, nanotwinned ferroelectric domain structures with high densities of 90° domain walls and considerably broadened switching characteristics. Molecular dynamics simulations predict both 180° (for (001)-/(101)-oriented films) and 90° multi-step switching (for (111)-oriented films) and these processes are subsequently observed in stroboscopic piezoresponse force microscopy. These results have implications for our understanding of ferroelectric switching and offer opportunities to change domain reversal speed.