Large polarization gradients and temperature-stable responses in compositionally-graded ferroelectrics

Large Built-in Fields and Tunable Ferroelectricity in Composition-Graded ScAlN Thin Films Deposited by Reactive Sputtering

The control of the coercive field and growth of abnormally oriented grains (AOGs) in wurtzite ferroelectric ScxAl1-xN thin films is crucial for piezoelectric and ferroelectric applications. However, elevated Sc concentrations generally result in significant AOG growth and high leakage current, degrading piezoelectric and ferroelectric properties. Here, compositionally graded ScxAl1-xN layered structures grown by sputtering are explored to effectively limit the AOG growth, compared to the 8-10% typically observed in conventional films, without requiring process optimization. Notably, a large built-in electric field of ≈0.45 MV cm-1, nearly 2.5 times larger than in graded PbZrxTi1-xO3 (PZT) and BaSrxTi1-xO3 (BST) systems, is achieved. This built-in field induces unique features, including horizontal shifts in polarization-electric field hysteresis loops, bi-stable states in capacitance-voltage characteristics, and highly asymmetric electrostrain behavior. Unlike other graded systems, this large built-in field in the graded ScxAl1-xN films arises primarily from polarization and chemical gradients. These findings offer a simplified, scalable and CMOS-compatible strategy to overcome the AOG challenges and tune ferroelectric properties of ScxAl1-xN thin films, providing the path for potential applications in advanced photonic and MEMS devices with improved performance and low-power consumption.

Compositionally-graded ferroelectric thin films by solution epitaxy produce excellent dielectric stability

The composition in ferroelectric oxide films is decisive for optimizing properties and device performances. Controlling a composition distribution in these films by a facile approach is thus highly desired. In this work, we report a solution epitaxy of PbZrxTi1-xO3 films with a continuous gradient of Zr concentration, realized by a competitive growth at ~220 °C. These intriguing films demonstrate a frequency-independent of dielectric permittivity below 100 kHz from room-temperature to 280 °C. In particular, the permittivity of the films can be largely regulated from 100 to 50 by slightly varying Zr compositional gradient. These results were revealed to arise from a built-in electric field within the films due to a coupling between the composition gradient and unidirectional spontaneous polarization. Our findings may pave a way to prepare compositionally-graded ferroelectric films by a solution approach, which is promising for practical dielectric, pyroelectric and photoelectric technical applications.

Advanced Materials for Energy Harvesting and Soft Robotics: Emerging Frontiers to Enhance Piezoelectric Performance and Functionality

Piezoelectric energy harvesting captures mechanical energy from a number of sources, such as vibrations, the movement of objects and bodies, impact events, and fluid flow to generate electric power. Such power can be employed to support wireless communication, electronic components, ocean monitoring, tissue engineering, and biomedical devices. A variety of self-powered piezoelectric sensors, transducers, and actuators have been produced for these applications, however approaches to enhance the piezoelectric properties of materials to increase device performance remain a challenging frontier of materials research. In this regard, the intrinsic polarization and properties of materials can be designed or deliberately engineered to enhance the piezo-generated power. This review provides insights into the mechanisms of piezoelectricity in advanced materials, including perovskites, active polymers, and natural biomaterials, with a focus on the chemical and physical strategies employed to enhance the piezo-response and facilitate their integration into complex electronic systems. Applications in energy harvesting and soft robotics are overviewed by highlighting the primary performance figures of merits, the actuation mechanisms, and relevant applications. Key breakthroughs and valuable strategies to further improve both materials and device performance are discussed, together with a critical assessment of the requirements of next-generation piezoelectric systems, and future scientific and technological solutions.

SrTiO3 and (Ba,Sr)TiO3 Films Epitaxially Grown on SrRuO3 Using Atomic Layer Deposition

High dielectric constant (k) materials have been investigated to improve the performance of dynamic random access memory (DRAM) capacitors. However, the conventional binary oxides have reached their fundamental limit of k < 100. In this study, we investigated alternative ternary oxides, SrTiO3 (STO) and (Ba,Sr)TiO3 (BSTO), which were epitaxially grown on SrRuO3 (SRO) using atomic layer deposition (ALD). The structural compatibility between SRO and STO enables the in situ crystallization of STO during ALD at a low temperature of 300 °C. Consequently, STO on SRO exhibited no film deformation, a common issue during high temperature postdeposition annealing, and maintained superior crystallinity at a thin thickness down to 50 Å. Furthermore, the dielectric constant of STO can be adjusted by modulating its tunable ferroelectric and dielectric properties through Ba doping. BSTO, with a high dielectric constant (kmax:527) achieved at a Ba doping concentration of approximately 50%, displayed a low leakage current density (3.9 × 10-8 A cm-2 @ 1 V) and demonstrated excellent reliability of 1012 cycles in the metal-insulator-metal capacitors. This study proposes a promising alternative to satisfy the extreme EOT required for next-generation DRAM capacitors.

Antiferroelectric Heterostructures Memristors with Unique Resistive Switching Mechanisms and Properties

A novel antiferroelectric material, PbSnO3 (PSO), was introduced into a resistive random access memory (RRAM) to reveal its resistive switching (RS) properties. It exhibits outstanding electrical performance with a large memory window (>104), narrow switching voltage distribution (±2 V), and low power consumption. Using high-resolution transmission electron microscopy, we observed the antiferroelectric properties and remanent polarization of the PSO thin films. The in-plane shear strains in the monoclinic PSO layer are attributed to oxygen octahedral tilts, resulting in misfit dislocations and grain boundaries at the PSO/SRO interface. Furthermore, the incoherent grain boundaries between the orthorhombic and monoclinic phases are assumed to be the primary paths of Ag+ filaments. Therefore, the RS behavior is primarily dominated by antiferroelectric polarization and defect mechanisms for the PSO structures. The RS behavior of antiferroelectric heterostructures controlled by switching spontaneous polarization and strain, defects, and surface chemistry reactions can facilitate the development of new antiferroelectric device systems.

Auxetic piezoelectric effect in heterostructures

Inherent symmetry breaking at the interface has been fundamental to a myriad of physical effects and functionalities, such as efficient spin-charge interconversion, exotic magnetic structures and an emergent bulk photovoltaic effect. It has recently been demonstrated that interface asymmetry can induce sizable piezoelectric effects in heterostructures, even those consisting of centrosymmetric semiconductors, which provides flexibility to develop and optimize electromechanical coupling phenomena. Here, by targeted engineering of the interface symmetry, we achieve piezoelectric phenomena behaving as the electrical analogue of the negative Poisson's ratio. This effect, termed the auxetic piezoelectric effect, exhibits the same sign for the longitudinal (d33) and transverse (d31, d32) piezoelectric coefficients, enabling a simultaneous contraction or expansion in all directions under an external electrical stimulus. The signs of the transverse coefficients can be further tuned via in-plane symmetry anisotropy. The effects exist in a wide range of material systems and exhibit substantial coefficients, indicating potential implications for all-semiconductor actuator, sensor and filter applications.

Designing a Periodic Structure with a Composition Gradient Stack Unit to Realize a Good Comprehensive Dielectric Property of Yttrium Iron Garnet Multilayer Film

Y3Fe5O12 (YIG) thin films are highly needed in microwave devices, but the low saturation magnetization and low dielectric constant greatly limit the application of YIG thin films. It was reported that the ion substitution, for example, Pr3+, could increase the dielectric constant of Y3-xPrxFe5O12 (YPrxIG). Unfortunately, the dielectric loss would also be significantly increased. In this work, [YPr0.20IG/YPr0.15IG/YPr0.10IG]N multilayer films were fabricated via the chemical solution deposition method, by designing a periodic structure with the [YPr0.20IG/YPr0.15IG/YPr0.10IG] composition gradient stack. In comparison to the average composition of YPr0.15IG, high saturation magnetization, high dielectric constant, and low loss were successfully simultaneously achieved in the multilayer structure. The N = 6 film exhibited a higher saturation magnetization of 252.8 emu/cm3 than the value (213.1) of the YPr0.15IG (average composition) film. The dielectric constant of the N = 6 film reached 25.6 in contrast to the value of 18.3 for the YPr0.15IG film at 12.4 GHz, which was the contribution of the rapid flip of the electric dipole of a single-unit dielectric material and the accumulation of interface charge. Furthermore, the dielectric loss of the film with N = 6 decreased to 0.0036 compared with the value (0.0102) of the average composition film. This work demonstrated a strategy of designing a periodic structure with a composition gradient stack unit to realize a good comprehensive dielectric property through taking advantage of the multiple effects of "coherent growth, component matching, and interface accumulation".

Exchange-Interaction-Like Behavior in Ferroelectric Bilayers

Interlayer coupling in materials, such as exchange interactions at the interface between an antiferromagnet and a ferromagnet, can produce exotic phenomena not present in the parent materials. While such interfacial coupling in magnetic systems is widely studied, there is considerably less work on analogous electric counterparts (i.e., akin to electric "exchange-bias-like" or "exchange-spring-like" interactions between two polar materials) despite the likelihood that such effects can also engender new features associated with anisotropic electric dipole alignment. Here, electric analogs of such exchange interactions are reported, and their physical origins are explained for bilayers of in-plane polarized Pb1-x Srx TiO3 ferroelectrics. Variation of the strontium content and thickness of the layers provides for deterministic control over the switching properties of the bilayer system resulting in phenomena analogous to an exchange-spring interaction and, leveraging added control of these interactions with an electric field, the ability to realize multistate-memory function. Such observations not only hold technological promise for ferroelectrics and multiferroics but also extend the similarities between ferromagnetic and ferroelectric materials to include the manifestation of exchange-interaction-like phenomena.

Influence of Light Atoms on Quantification of Atomic Column Positions in Distorted Perovskites with HAADF-STEM

Measurement of picometer-scale atomic displacements by aberration-corrected STEM has become invaluable in the study of crystalline materials, where it can elucidate ordering mechanisms and local heterogeneities. HAADF-STEM imaging, often used for such measurements due to its atomic number contrast, is generally considered insensitive to light atoms such as oxygen. Light atoms, however, still affect the propagation of the electron beam in the sample and, therefore, the collected signal. Here, we demonstrate experimentally and through simulations that cation sites in distorted perovskites can appear to be displaced by several picometers from their true positions in shared cation-anion columns. The effect can be decreased through careful choice of sample thickness and beam voltage or can be entirely avoided if the experiment allows reorientation of the crystal along a more favorable zone axis. Therefore, it is crucial to consider the possible effects of light atoms and crystal symmetry and orientation when measuring atomic positions.

Solution epitaxy of polarization-gradient ferroelectric oxide films with colossal photovoltaic current

Solution growth of single-crystal ferroelectric oxide films has long been pursued for the low-cost development of high-performance electronic and optoelectronic devices. However, the established principles of vapor-phase epitaxy cannot be directly applied to solution epitaxy, as the interactions between the substrates and the grown materials in solution are quite different. Here, we report the successful epitaxy of single-domain ferroelectric oxide films on Nb-doped SrTiO3 single-crystal substrates by solution reaction at a low temperature of ~200 oC. The epitaxy is mainly driven by an electronic polarization screening effect at the interface between the substrates and the as-grown ferroelectric oxide films, which is realized by the electrons from the doped substrates. Atomic-level characterization reveals a nontrivial polarization gradient throughout the films in a long range up to ~500 nm because of a possible structural transition from the monoclinic phase to the tetragonal phase. This polarization gradient generates an extremely high photovoltaic short-circuit current density of ~2.153 mA/cm2 and open-circuit voltage of ~1.15 V under 375 nm light illumination with power intensity of 500 mW/cm2, corresponding to the highest photoresponsivity of ~4.306×10-3 A/W among all known ferroelectrics. Our results establish a general low-temperature solution route to produce single-crystal gradient films of ferroelectric oxides and thus open the avenue for their broad applications in self-powered photo-detectors, photovoltaic and optoelectronic devices.

B-Site Nanoscale-Ordered Structure Enables Ultra-High Tunable Performance

Tunable devices constructed by ferroelectric thin films are often desired to possess a low dielectric loss while maintainging a high dielectric tunability over a broad operating temperature range in applications, for example, resonators, filters, or phase shifters. However, it is difficult to simultaneously achieve these characteristics by traditional strategies, such as doping and strain modifying. Here, we demonstrate that the dielectric tunability of the sol-gel-prepared Pb(Sc1/2Nb1/2)0.9(Mg1/3Nb2/3)0.1O3 (PSNMN) thin film can be almost doubled from ~47% to ~80.0% (at 10 kHz) at a low electric field (~530 kV/cm), and the dielectric loss can be sharply reduced by more than an order of magnitude, from ~0.50 to ~0.037 (at 1 kHz) when the thin film was annealed in air at 650°C for 15 h under the help of an atmosphere-compensating-block (ACB) made from the proto-PSNMN gel. Moreover, the PSNMN thin film annealed with ACB also exhibited an extremely high thermally-stable dielectric tunability in an ultrabroad temperature range (>130 K), which could be attributed to the Maxwell-Wagner (MW) effect generated by the interface between the PSNMN disordered matrix and the B-site nanoscale-ordered structure formed during the long-term annealing process. The reduced dielectric loss is mainly benefited from the reduced concentration of oxygen vacancy and the possible MW effects, and the enhanced dielectric tunability could be ascribed to the weaker domain-pinning effect by oxygen vacancy. The breakthrough provides a new universal strategy to achieve utrahigh tunable performance in A(B'1/2B"1/2)O3 ferroelectric thin films with a B-site nanoscale-ordered structure, meanwhile it paves the way for ultraintergrated tunable thin-film-devices with great phase shifter performance in practical applications.

Reversibly Controlled Ternary Polar States and Ferroelectric Bias Promoted by Boosting Square-Tensile-Strain

Interaction between dipoles often emerges intriguing physical phenomena, such as exchange bias in the magnetic heterostructures and magnetoelectric effect in multiferroics, which lead to advances in multifunctional heterostructures. However, the defect-dipole tends to be considered the undesired to deteriorate the electronic functionality. Here, deterministic switching between the ferroelectric and the pinched states by exploiting a new substrate of cubic perovskite, BaZrO₃ is reported, which boosts the square-tensile-strain to BaTiO₃ and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation. First-principles calculations propose a complex of an oxygen vacancy and two Ti³⁺ ions coins a charge-neutral defect-dipole. Cooperative control of the defect-dipole and the spontaneous polarization reveals ternary in-plane polar states characterized by biased/pinched hysteresis loops. Furthermore, it is experimentally demonstrated that three electrically controlled polar-ordering states lead to switchable and nonvolatile dielectric states for application of nondestructive electro-dielectric memory. This discovery opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates.

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.

Freestanding complex-oxide membranes

Complex oxides show a vast range of functional responses, unparalleled within the inorganic solids realm, making them promising materials for applications as varied as next-generation field-effect transistors, spintronic devices, electro-optic modulators, pyroelectric detectors, or oxygen reduction catalysts. Their stability in ambient conditions, chemical versatility, and large susceptibility to minute structural and electronic modifications make them ideal subjects of study to discover emergent phenomena and to generate novel functionalities for next-generation devices. Recent advances in the synthesis of single-crystal, freestanding complex oxide membranes provide an unprecedented opportunity to study these materials in a nearly-ideal system (e.g. free of mechanical/thermal interaction with substrates) as well as expanding the range of tools for tweaking their order parameters (i.e. (anti-)ferromagnetic, (anti-)ferroelectric, ferroelastic), and increasing the possibility of achieving novel heterointegration approaches (including interfacing dissimilar materials) by avoiding the chemical, structural, or thermal constraints in synthesis processes. Here, we review the recent developments in the fabrication and characterization of complex-oxide membranes and discuss their potential for unraveling novel physicochemical phenomena at the nanoscale and for further exploiting their functionalities in technologically relevant devices.

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.

Local negative permittivity and topological phase transition in polar skyrmions

Topological solitons such as magnetic skyrmions have drawn attention as stable quasi-particle-like objects. The recent discovery of polar vortices and skyrmions in ferroelectric oxide superlattices has opened up new vistas to explore topology, emergent phenomena and approaches for manipulating such features with electric fields. Using macroscopic dielectric measurements, coupled with direct scanning convergent beam electron diffraction imaging on the atomic scale, theoretical phase-field simulations and second-principles calculations, we demonstrate that polar skyrmions in (PbTiO₃)_n/(SrTiO₃)_n superlattices are distinguished by a sheath of negative permittivity at the periphery of each skyrmion. This enhances the effective dielectric permittivity compared with the individual SrTiO₃ and PbTiO₃ layers. Moreover, the response of these topologically protected structures to electric field and temperature shows a reversible phase transition from the skyrmion state to a trivial uniform ferroelectric state, accompanied by large tunability of the dielectric permittivity. Pulsed switching measurements show a time-dependent evolution and recovery of the skyrmion state (and macroscopic dielectric response). The interrelationship between topological and dielectric properties presents an opportunity to simultaneously manipulate both by a single, and easily controlled, stimulus, the applied electric field.

Analysis on the electric field distribution in a relaxor ferroelectric KTN crystal near field-induced phase transition using optical deflection measurements

Spatially analyzing non-uniform distributions of electric phenomena such as electric field and permittivity in ferroelectric devices is very challenging. In this study, we apply an optical beam deflection method to map the non-uniform electric phenomena in relaxor ferroelectric potassium tantalate niobate (KTN) crystals. To adequately correlate the physical parameters and their spatial distributions in KTN crystals, a general model that describes the giant electro-optic response and associated beam deflection is derived. The proposed model is in good agreement with the experimental results and is envisioned to be useful for analyzing electric field-induced phenomena in non-linear dielectric materials and devices.

Beyond Substrates: Strain Engineering of Ferroelectric Membranes

Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high-quality substrates. Here, using the ferroelectric BaTiO₃ , production of precisely strain-engineered, substrate-released nanoscale membranes is demonstrated via an epitaxial lift-off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide-metal/ferroelectric/oxide-metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm^-1 and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100-nm-thick film). In devices integrated on flexible polymers, enhanced room-temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS-compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth.

Terahertz field-induced ferroelectricity in quantum paraelectric SrTiO3

"Hidden phases" are metastable collective states of matter that are typically not accessible on equilibrium phase diagrams. These phases can host exotic properties in otherwise conventional materials and hence may enable novel functionality and applications, but their discovery and access are still in early stages. Using intense terahertz electric field excitation, we found that an ultrafast phase transition into a hidden ferroelectric phase can be dynamically induced in quantum paraelectric strontium titanate (SrTiO₃). The induced lowering in crystal symmetry yields substantial changes in the phonon excitation spectra. Our results demonstrate collective coherent control over material structure, in which a single-cycle field drives ions along the microscopic pathway leading directly to their locations in a new crystalline phase on an ultrafast time scale.

Probing local order in multiferroics by transmission electron microscopy

The ongoing trend toward miniaturization has led to an increased interest in the magnetoelectric effect, which could yield entirely new device concepts, such as electric field-controlled magnetic data storage. As a result, much work is being devoted to developing new robust room temperature (RT) multiferroic materials that combine ferromagnetism and ferroelectricity. However, the development of new multiferroic devices has proved unexpectedly challenging. Thus, a better understanding of the properties of multiferroic thin films and the relation with their microstructure is required to help drive multiferroic devices toward technological application. This review covers in a concise manner advanced analytical imaging methods based on (scanning) transmission electron microscopy which can potentially be used to characterize complex multiferroic materials. It consists of a first broad introduction to the topic followed by a section describing the so-called phase-contrast methods, which can be used to map the polar and magnetic order in magnetoelectric multiferroics at different spatial length scales down to atomic resolution. Section 3 is devoted to electron nanodiffraction methods. These methods allow measuring local strains, identifying crystal defects and determining crystal structures, and thus offer important possibilities for the detailed structural characterization of multiferroics in the ultrathin regime or inserted in multilayers or superlattice architectures. Thereafter, in Section 4, methods are discussed which allow for analyzing local strain, whereas in Section 5 methods are addressed which allow for measuring local polarization effects on a length scale of individual unit cells. Here, it is shown that the ferroelectric polarization can be indirectly determined from the atomic displacements measured in atomic resolution images. Finally, a brief outlook is given on newly established methods to probe the behavior of ferroelectric and magnetic domains and nanostructures during in situ heating/electrical biasing experiments. These in situ methods are just about at the launch of becoming increasingly popular, particularly in the field of magnetoelectric multiferroics, and shall contribute significantly to understanding the relationship between the domain dynamics of multiferroics and the specific microstructure of the films providing important guidance to design new devices and to predict and mitigate failures.

Pulsed laser deposition with rapid beam deflection by a galvanometer mirror scanner

A pulsed laser deposition system with rapid beam deflection (RBD-PLD) by a galvanometer mirror scanner has been developed for alternating ablation of multiple targets with a single laser instrument. In this system, the alternating deposition of different target materials is carried out by scanning the laser beam between the positionally fixed targets with a galvanometer mirror instead of mechanically switching the target positions on a fixed optical path of the laser beam as is done in conventional pulsed laser deposition (PLD) systems. Thus, the "wait" time required for switching target materials to be deposited, which typically takes several seconds in a conventional system, can be made as short as a few milliseconds. We demonstrate some of the advantages of this PLD system in several technologically important aspects of thin film synthesis: (1) fast fabrication of binary alloy films, (2) preparation of natural composition spread libraries, (3) effect of the target switching time on the deposition of volatile compounds, (4) control of the degree of mixing of two different materials in a film, and (5) efficient growth of compositionally graded thin films.

On the mechanisms of tip-force induced switching in ferroelectric thin films: the crossover of depolarization, shear strain and flexoelectricity

The recent observation of mechanical switching of ferroelectric polarization has placed the mechanical manipulation of ferroelectrics on an equal footing with the conventional electrical manipulation. However, discussions on the exact switching mechanisms due to mechanical loads are ongoing for the complexity in experimental situations. In this work, based on continuum mechanical and thermodynamic modeling and simulation, we analyze the mechanisms of tip-force induced switching in ferroelectric thin films. The roles of depolarization, shear strain and flexoelectricity in mechanical switching, both in normal and sliding loading modes, are separated out and the switching characteristics are analyzed. The depolarization field in the film is demonstrated to enable bidirectional switching. The coupling between shear strain and polarization components is shown to be important in the sliding loading mode. A great influence of flexoelectricity-modified polarization boundary condition on the switching process is revealed. The previous speculation that the switching process experiences an intermediate paraelectric phase is proved. The regulation of loading force, misfit strain, temperature and film thickness on the switching are further given for each mechanism. Taking all of the three mechanisms into account, we present the phase diagrams of mechanical switching for films in an initial upward or downward polarization state. The revealed characteristics of various switching mechanisms should provide useful guidelines for their verification in experiments, and the tunability of the switching by various influencing factors is instructive for the design and optimization of ferroelectric devices via mechanical engineering.

Formation of polarization needle-like domain and its unusual switching in compositionally graded ferroelectric thin films: an improved phase field model

Compositionally graded ferroelectrics (cgFEs), which possess a spatial variation in material composition, exhibit anomalous or even unprecedented properties. Despite several breakthroughs having been achieved in experiments, there surprisingly is a lack of an effective simulation approach for cgFEs, thereby greatly hindering a deep understanding about underlying mechanisms hidden behind the observed phenomena. In this study, an improved phase field model is proposed for a cgFE made of PbZr_(1-x)Ti _x O₃ based on the Ginzburg-Landau theory. The improved approach enables us to capture several key phenomena occurring in the cgFE PbZr_0.8Ti_0.2O₃ ⇔ PbZr_0.2Ti_0.8O₃ thin film that are observed from experiments, such as the formation of needle-like domains with curved domain walls, ferroelastic switching under an electric field, and the voltage offset of the hysteresis loop. Results obtained from the improved approach indicate that the high elastic energy near the needle-tip of an a-domain gives rise to a deflection of the domain wall from the regular a/c domain wall, while the high concentration of the depolarization field shrinks a part of the a-domain near the needle-tip and terminates the a-domain within the cgFE thin film. These facilitate the stabilization of needle-like domains with curved domain walls in the cgFE thin film. Furthermore, the flexoelectricity is proven to play an important role in the voltage offset of the hysteresis loop. On the other hand, the ferroelectric switching process in the cgFE thin film exhibits a vastly different response in comparison to that in homogeneous ferroelectric thin films, including local switching initiation and formation and annihilation of vortex-antivortex pairs during the switching. The present study, therefore, provides an incisive approach for investigations on cgFEs, which may bring new understanding and unique insights into these complex materials, as well as novel potential applications.

Understanding the Role of Ferroelastic Domains on the Pyroelectric and Electrocaloric Effects in Ferroelectric Thin Films

Temperature- and electric-field-induced structural transitions in a polydomain ferroelectric can have profound effects on its electrothermal susceptibilities. Here, the role of such ferroelastic domains on the pyroelectric and electrocaloric response is experimentally investigated in thin films of the tetragonal ferroelectric PbZr_0.2 Ti_0.8 O₃ . By utilizing epitaxial strain, a rich set of ferroelastic polydomain states spanning a broad thermodynamic phase space are stabilized. Using temperature-dependent scanning-probe microscopy, X-ray diffraction, and high-frequency phase-sensitive pyroelectric measurements, the propensity of domains to reconfigure under a temperature perturbation is quantitatively studied. In turn, the "extrinsic" contributions to pyroelectricity exclusively due to changes between the ferroelastic domain population is elucidated as a function of epitaxial strain. Further, using highly sensitive thin-film resistive thermometry, direct electrocaloric temperature changes are measured on these polydomain thin films for the first time. The results demonstrate that temperature- and electric-field-driven domain interconversion under compressive strain diminish both the pyroelectric and the electrocaloric effects, while both these susceptibilities are enhanced due to the exact-opposite effect from the extrinsic contributions under tensile strain.

Periodic Giant Polarization Gradients in Doped BiFeO3 Thin Films

The ultimate challenge for the development of new multiferroics with enhanced properties lies in achieving nanoscale control of the coupling between different ordering parameters. In oxide-based multiferroics, substitutional cation dopants offer the unparalleled possibility to modify both the electric and magnetic properties at a local scale. Herein it is demonstrated the formation of a dopant-controlled polar pattern in BiFeO₃ leading to the spontaneous instauration of periodic polarization waves. In particular, nonpolar Ca-doped rich regions act as spacers between consecutive dopant-depleted regions displaying coupled ferroelectric states. This alternation of layers with different ferroelectric state creates a novel vertical polar structure exhibiting giant polarization gradients as large as 70 μC cm^-2 across 30 Å thick domains. The drastic change in the polar state of the film is visualized using high-resolution differential phase-contrast imaging able to map changes in ferroelectric polarization at atomic scale. Furthermore, a periodic distortion in the Fe-O-Fe bonding angle suggests a local variation in the magnetic ordering. The findings provide a new insight into the role of doping and reveal hitherto unexplored means to tailor the functional properties of multiferroics by doping engineering.

Compositionally Graded Multilayer Ceramic Capacitors

Multilayer ceramic capacitors (MLCC) are widely used in consumer electronics. Here, we provide a transformative method for achieving high dielectric response and tunability over a wide temperature range through design of compositionally graded multilayer (CGML) architecture. Compositionally graded MLCCs were found to exhibit enhanced dielectric tunability (70%) along with small dielectric losses (<2.5%) over the required temperature ranges specified in the standard industrial classifications. The compositional grading resulted in generation of internal bias field which enhanced the tunability due to increased nonlinearity. The electric field tunability of MLCCs provides an important avenue for design of miniature filters and power converters.