Control of Domain Structures in Multiferroic Thin Films through Defect Engineering

Large Electromechanical Response in a Polycrystalline Alkali-Deficient (K,Na)NbO3 Thin Film on Silicon

The demand for large electromechanical performance in lead-free polycrystalline piezoelectric thin films is driven by the need for compact, high-performance microelectromechanical systems (MEMS) based devices operating at low voltages. Here we significantly enhance the electromechanical response in a polycrystalline lead-free oxide thin film by utilizing lattice-defect-induced structural inhomogeneities. Unlike prior observations in mismatched epitaxial films with limited low-frequency enhancements, we achieve large electromechanical strain in a polycrystalline (K,Na)NbO3 film integrated on silicon. This is achieved by inducing self-assembled Nb-rich planar faults with a nonstoichiometric composition. The film exhibits an effective piezoelectric coefficient of 565 pm V-1 at 1 kHz, surpassing those of lead-based counterparts. Notably, lattice defect growth is substrate-independent, and the large electromechanical response is extended to even higher frequencies in a polycrystalline film. Improved properties arise from unique lattice defect morphology and frequency-dependent relaxation behavior, offering a new route to remarkable electromechanical response in polycrystalline thin films.

Sub-Nanosecond Reconfiguration of Ferroelectric Domains in Bismuth Ferrite

Domain switching is crucial for achieving desired functions in ferroic materials that are used in various applications. Fast control of domains at sub-nanosecond timescales remains a challenge despite its potential for high-speed operation in random-access memories, photonic, and nanoelectronic devices. Here, ultrafast laser excitation is shown to transiently melt and reconfigure ferroelectric stripe domains in multiferroic bismuth ferrite on a timescale faster than 100 picoseconds. This dynamic behavior is visualized by picosecond- and nanometer-resolved X-ray diffraction and time-resolved X-ray diffuse scattering. The disordering of stripe domains is attributed to the screening of depolarization fields by photogenerated carriers resulting in the formation of charged domain walls, as supported by phase-field simulations. Furthermore, the recovery of disordered domains exhibits subdiffusive growth on nanosecond timescales, with a non-equilibrium domain velocity reaching up to 10 m s-1 . These findings present a new approach to image and manipulate ferroelectric domains on sub-nanosecond timescales, which can be further extended into other complex photoferroic systems to modulate their electronic, optical, and magnetic properties beyond gigahertz frequencies. This approach could pave the way for high-speed ferroelectric data storage and computing, and, more broadly, defines new approaches for visualizing the non-equilibrium dynamics of heterogeneous and disordered materials.

Diverse Defects in Alkali Niobate Thin Films: Understanding at Atomic Scales and Their Implications on Properties

Defects in ferroelectric materials have many implications on the material properties which, in most cases, are detrimental. However, engineering these defects can also create opportunities for property enhancement as well as for tailoring novel functionalities. To purposely manipulate these defects, a thorough knowledge of their spatial atomic arrangement, as well as elastic and electrostatic interactions with the surrounding lattice, is highly crucial. In this work, analytical scanning transmission electron microscopy (STEM) is used to reveal a diverse range of multidimensional crystalline defects (point, line, planar, and secondary phase) in (K,Na)NbO₃ (KNN) ferroelectric thin films. The atomic-scale analyses of the defect-lattice interactions suggest strong elastic and electrostatic couplings which vary among the individual defects and correspondingly affect the electric polarization. In particular, the observed polarization orientations are correlated with lattice relaxations as well as strain gradients and can strongly impact the properties of the ferroelectric films. The knowledge and understanding obtained in this study open a new avenue for the improvement of properties as well as the discovery of defect-based functionalities in alkali niobate thin films.

Real-Time Transformation of Flux-Closure Domains with Superhigh Thermal Stability

Polar topologies have received extensive attention due to their exotic configurations and functionalities. Understanding their responsive behaviors to external stimuli, especially thermal excitation, is highly desirable to extend their applications to high temperature, which is still unclear. Here, combining in situ transmission electron microscopy and phase-field simulations, the thermal dynamics of the flux-closure domains were illuminated in PbTiO₃/SrTiO₃ multilayers. In-depth analyses suggested that the topological transition processes from a/c domains to flux-closure quadrants were influenced by the boundary conditions of PbTiO₃ layers. The symmetrical boundary condition stabilized the flux-closure domains at higher temperature than in the asymmetrical case. Furthermore, the reversible thermal responsive behaviors of the flux-closure domains displayed superior thermal stability, which maintained robust up to 450 °C (near the Curie temperature). This work provides new insights into the dynamics of polar topologies under thermal excitation and facilitates their applications as nanoelectronics under extreme conditions.

Anisotropic dislocation-domain wall interactions in ferroelectrics

Dislocations are usually expected to degrade electrical, thermal and optical functionality and to tune mechanical properties of materials. Here, we demonstrate a general framework for the control of dislocation-domain wall interactions in ferroics, employing an imprinted dislocation network. Anisotropic dielectric and electromechanical properties are engineered in barium titanate crystals via well-controlled line-plane relationships, culminating in extraordinary and stable large-signal dielectric permittivity (≈23100) and piezoelectric coefficient (≈2470 pm V^-1). In contrast, a related increase in properties utilizing point-plane relation prompts a dramatic cyclic degradation. Observed dielectric and piezoelectric properties are rationalized using transmission electron microscopy and time- and cycle-dependent nuclear magnetic resonance paired with X-ray diffraction. Succinct mechanistic understanding is provided by phase-field simulations and driving force calculations of the described dislocation-domain wall interactions. Our 1D-2D defect approach offers a fertile ground for tailoring functionality in a wide range of functional material systems.

Recent Progress in Research on Ferromagnetic Rhenium Disulfide

Since long-range magnetic ordering was observed in pristine Cr₂Ge₂Te₆ and monolayer CrCl₃, two-dimensional (2D) magnetic materials have gradually become an emerging field of interest. However, it is challenging to induce and modulate magnetism in non-magnetic (NM) materials such as rhenium disulfide (ReS₂). Theoretical research shows that defects, doping, strain, particular phase, and domain engineering may facilitate the creation of magnetic ordering in the ReS₂ system. These predictions have, to a large extent, stimulated experimental efforts in the field. Herein, we summarize the recent progress on ferromagnetism (FM) in ReS₂. We compare the proposed methods to introduce and modulate magnetism in ReS₂, some of which have made great experimental breakthroughs. Experimentally, only a few ReS₂ materials exhibit room-temperature long-range ferromagnetic order. In addition, the superexchange interaction may cause weak ferromagnetic coupling between neighboring trimers. We also present a few potential research directions for the future, and we finally conclude that a deep and thorough understanding of the origin of FM with and without strain is very important for the development of basic research and practical applications.

Origin of giant electric-field-induced strain in faulted alkali niobate films

A large electromechanical response in ferroelectrics is highly desirable for developing high-performance sensors and actuators. Enhanced electromechanical coupling in ferroelectrics is usually obtained at morphotropic phase boundaries requiring stoichiometric control of complex compositions. Recently it was shown that giant piezoelectricity can be obtained in films with nanopillar structures. Here, we elucidate its origin in terms of atomic structure and demonstrate a different system with a greatly enhanced response. This is in non-stoichiometric potassium sodium niobate epitaxial thin films with a high density of self-assembled planar faults. A giant piezoelectric coefficient of ∼1900 picometer per volt is demonstrated at 1 kHz, which is almost double the highest ever reported effective piezoelectric response in any existing thin films. The large oxygen octahedral distortions and the coupling between the structural distortion and polarization orientation mediated by charge redistribution at the planar faults enable the giant electric-field-induced strain. Our findings demonstrate an important mechanism for realizing the unprecedentedly giant electromechanical coupling and can be extended to many other material functions by engineering lattice faults in non-stoichiometric compositions.

Maximizing the electromechanical response is crucial for developing piezoelectric devices. Here, the authors demonstrate a giant electric-field-induced strain and its origin in alkali niobate epitaxial thin films with self-assembled planar faults.

Evolution from Lead-Based to Lead-Free Piezoelectrics: Engineering of Lattices, Domains, Boundaries, and Defects Leading to Giant Response

Piezoelectric materials are known to mankind for more than a century, with numerous advancements made in both scientific understandings and practical applications. In the last two decades, in particular, the research on piezoelectrics has largely been driven by the constantly changing technological demand, and the drive toward a sustainable society. Hence, environmental-friendly "lead-free piezoelectrics" have emerged in the anticipation of replacing lead-based counterparts with at least comparable performance. However, there are still obstacles to be overcome for realizing this objective, while the efforts in this direction already seem to culminate. Therefore, novel structural strategies need to be designed to address these issues and for further breakthrough in this field. Here, various strategies to enhance piezoelectric properties in lead-free systems with fundamental and historical context, and from atomic to macroscopic scale, are explored. The main challenges currently faced in the transition from lead-based to lead-free piezoelectrics are identified and key milestones for future research in this field are suggested. These include: i) decoding the fundamental mechanisms; ii) large temperature-stable piezoresponse; and iii) fabrication-friendly and tailorable composition. Strategic insights and general guidelines for the synergistic design of new piezoelectric materials for obtaining a large piezoelectric response are also provided.

Large-Scale Epitaxial Growth of Ultralong Stripe BiFeO3 Films and Anisotropic Optical Properties

The controlled synthesis of large-scale ferroelectric domains with high uniformity is crucial for practical applications in next-generation nanoelectronics on the basis of their intriguing properties. Here, ultralong and highly uniform stripe domains in (110)-oriented BiFeO₃ thin films are large-area synthesized through a pulsed laser deposition technique. Utilizing scanning transmission electron microscopy and piezoresponse force microscopy, we verified that the ferroelectric domains have one-dimensional 109° domains and the length of a domain is up to centimeter scale. More importantly, the ferroelectric displacement is directly determined on atomic-scale precision, further confirming the domain structure. We find that the unique one-dimensional ferroelectric domain significantly enhances the optical anisotropy. Furthermore, we demonstrate that the purely parallel domain patterns can be used to control photovoltaic current. These ultralong ferroelectric domains can be patterned into various functional devices, which may inspire research efforts to explore their properties and various applications.

Fabrication of Bimetallic Oxides (MCo2O4: M=Cu, Mn) on Ordered Microchannel Electro-Conductive Plate for High-Performance Hybrid Supercapacitors

AB2O4-type binary-transition metal oxides (BTMOs) of CuCo2O4 and MnCo2O4 were successfully prepared on ordered macroporous electrode plates (OMEP) for supercapacitors. Under the current density of 5 mA cm−2, the CuCo2O4/OMEP electrode achieved a specific capacitance of 1199 F g−1. The asymmetric supercapacitor device prepared using CuCo2O4/OMEP as the positive electrode and carbon-based materials as the negative electrode (CuCo2O4/OMEP//AC) achieved the power density of 14.58 kW kg−1 under the energy density of 11.7 Wh kg−1. After 10,000 GCD cycles, the loss capacitance of CuCo2O4/OMEP//AC is only 7.5% (the retention is 92.5%). The MnCo2O4/OMEP electrode shows the specific and area capacitance of 843 F g−1 and 5.39 F cm−2 at 5 mA cm−2. The MnCo2O4/OMEP-based supercapacitor device (MnCo2O4/OMEP//AC) has a power density of 8.33 kW kg−1 under the energy density of 11.6 Wh kg−1 and the cycle stability was 90.2% after 10,000 cycles. The excellent power density and cycle stability prove that the prepared hybrid supercapacitor fabricated under silicon process has a good prospect as the power buffer device for solar cells.

Defect engineering in perovskite oxide thin films

Perovskite oxide thin films are a category of multifunctional materials that have intriguing electrical, magnetic, and photovoltaic properties that can be harnessed combinatorially in future microelectronic devices. However, the inevitable existence of defects in perovskites, regardless of the materials' processing conditions, plays a significant role in their functional properties, which could be either detrimental or beneficial, depending on the exact chemical nature of these defects. As such, defect engineering is an important research area in perovskite thin films that aims at understanding the chemical nature of the defects, from which the physical properties of materials can be more precisely manipulated. Here, we review the common defects in perovskite oxide thin films, which include point defects, dopants, domains and domain walls. The factors that impact the appearance and existence of defects and the corresponding mechanisms are also discussed. While summarizing our previous work, the state-of-the-art in the field from other groups has also been discussed. Most of the defects exist as defect dipoles that affect the oxidation states of relevant ions and induce anomalous behaviors, such as ferroelectricity in otherwise non-ferroelectric thin films, as well as enhanced electrical conductivity in insulators. Furthermore, the couplings between defect dipoles and other degrees of freedom including epitaxial strains and interfaces also provide new strategies to modulate the functional properties of perovskite thin films. Particularly, the coupling between defects and domain wall motion can be regarded as a universal tool to modulate the electric and magnetic properties of thin films of perovskite oxides. It is our hope that this review could promote defect engineering as a general regulation strategy to embellish the functional properties of perovskite oxide thin films.

Multiferroic heterostructures for spintronics

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

Unexpected Giant Microwave Conductivity in a Nominally Silent BiFeO3 Domain Wall

Nanoelectronic devices based on ferroelectric domain walls (DWs), such as memories, transistors, and rectifiers, have been demonstrated in recent years. Practical high-speed electronics, on the other hand, usually demand operation frequencies in the gigahertz (GHz) regime, where the effect of dipolar oscillation is important. Herein, an unexpected giant GHz conductivity on the order of 10³ S m^-1 is observed in certain BiFeO₃ DWs, which is about 100 000 times greater than the carrier-induced direct current (dc) conductivity of the same walls. Surprisingly, the nominal configuration of the DWs precludes the alternating current (ac) conduction under an excitation electric field perpendicular to the surface. Theoretical analysis shows that the inclined DWs are stressed asymmetrically near the film surface, whereas the vertical walls in a control sample are not. The resultant imbalanced polarization profile can then couple to the out-of-plane microwave fields and induce power dissipation, which is confirmed by the phase-field modeling. Since the contributions from mobile-carrier conduction and bound-charge oscillation to the ac conductivity are equivalent in a microwave circuit, the research on local structural dynamics may open a new avenue to implement DW nano-devices for radio-frequency applications.

Real-time studies of ferroelectric domain switching: a review

Ferroelectric materials have been utilized in a broad range of electronic, optical, and electromechanical applications and hold the promise for the design of future high-density nonvolatile memories and multifunctional nano-devices. The applications of ferroelectric materials stem from the ability to switch polarized domains by applying an electric field, and therefore a fundamental understanding of the switching dynamics is critical for design of practical devices. In this review, we summarize the progress in the study of the microscopic process of ferroelectric domain switching using recently developed in situ transmission electron microscopy (TEM). We first briefly introduce the instrumentation, experimental procedures, imaging mechanisms, and analytical methods of the state-of-the-art in situ TEM techniques. The application of these techniques to studying a wide range of complex switching phenomena, including domain nucleation, domain wall motion, domain relaxation, domain-defect interaction, and the interplay between different types of domains, is demonstrated. The underlying physics of these dynamic processes are discussed.

Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures

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

Structures and electronic properties of domain walls in BiFeO3 thin films

Domain walls (DWs) in ferroelectrics are atomically sharp and can be created, erased, and reconfigured within the same physical volume of ferroelectric matrix by external electric fields. They possess a myriad of novel properties and functionalities that are absent in the bulk of the domains, and thus could become an essential element in next-generation nanodevices based on ferroelectrics. The knowledge about the structure and properties of ferroelectric DWs not only advances the fundamental understanding of ferroelectrics, but also provides guidance for the design of ferroelectric-based devices. In this article, we provide a review of structures and properties of DWs in one of the most widely studied ferroelectric systems, BiFeO₃ thin films. We correlate their conductivity and photovoltaic properties to the atomic-scale structure and dynamic behaviors of DWs.

Controllable defect driven symmetry change and domain structure evolution in BiFeO3 with enhanced tetragonality

Defect engineering has been a powerful tool to enable the creation of exotic phases and the discovery of intriguing phenomena in ferroelectric oxides. However, the accurate control of the concentration of defects remains a big challenge. In this work, ion implantation, which can provide controllable point defects, allows us to produce a controlled defect driven true super-tetragonal (T) phase with a single-domain-state in ferroelectric BiFeO3 thin films. This point-defect engineering is found to drive the phase transition from the as-grown mixed rhombohedral-like (R) and tetragonal-like (MC) phase to true tetragonal (T) symmetry and induce the stripe multi-nanodomains to a single domain state. By further increasing the injected dose of the He ion, we demonstrate an enhanced tetragonality super-tetragonal (super-T) phase with the largest c/a ratio of ∼1.3 that has ever been experimentally achieved in BiFeO3. A combination of the morphology change and domain evolution further confirms that the mixed R/MC phase structure transforms to the single-domain-state true tetragonal phase. Moreover, the re-emergence of the R phase and in-plane nanoscale multi-domains after heat treatment reveal the memory effect and reversible phase transition and domain evolution. Our findings demonstrate the reversible control of R-Mc-T-super T symmetry changes (leading to the creation of true T phase BiFeO3 with enhanced tetragonality) and multidomain-single domain structure evolution through controllable defect engineering. This work also provides a pathway to generate large tetragonality (or c/a ratio) that could be extended to other ferroelectric material systems (such as PbTiO3, BaTiO3 and HfO2) which might lead to strong polarization enhancement.

Distorted Monolayer ReS2 with Low-Magnetic-Field Controlled Magnetoelectricity

Two dimensional (2D) materials possessing ferroelectric/ferromagnetic orders and especially low-magnetic-field controlled magnetoelectricity have great promise in spintronics and multistate data storage. However, ferroelectric and magnetoelectric (ME) dipoles in the atom-thick 2D materials are difficult to be realized due to structural inversion symmetry, thermal actuation, and depolarized field. To overcome these difficulties, the monolayer structure must possess an in-plane inversion asymmetry in order to provide out-of-plane ferroelectric polarization. Herein, crystal chemistry is adopted to engineer specific atomic displacement in monolayer ReS₂ to change the crystal symmetry to induce out-of-plane ferroelectric polarization at room temperature. The cationic Re vacancy in the atom-displaced ReS₂ monolayer causes spin polarization of two immediate neighbor sulfur atoms to generate magnetic ordering, and the ferroelectric distortion near the Re vacancy locally tunes the ferromagnetic order thereby triggering low-magnetic-field controlled ME polarization at about 28 K. As a result, 2D ME coupling multiferroics is achieved. Our results not only reveal a design methodology to attain coexistence of ferroelectric and ferromagnetic orders in 2D materials but also provide insights into magnetoelectricity in 2D materials.