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A Landau–Devonshire thermodynamic energy density function for ferroelectric wurtzite aluminum scandium nitride (Al1−xScxN) solid solution is developed. It is parametrized using available experimental and theoretical data, enabling the accurate reproduction of composition-dependent ferroelectric properties, such as spontaneous polarization, dielectric permittivity, and piezoelectric constants, for both bulk and thin films. The maximum concentration of Sc for the wurtzite structure to remain ferroelectric is found to be 61 at. %. A detailed analysis of Al1−xScxN thin films reveals that the ferroelectric phase transition and properties are insensitive to substrate strain. This study lays the foundation for quantitative modeling of novel ferroelectric wurtzite solid solutions. [ABSTRACT FROM AUTHOR]

The electrical boundary condition plays an important role in the manipulation of domain structures in low-dimensional ferroelectric materials, especially ferroelectric nanowires. Here, using phase-field simulations, we systematically investigate the influence of electrical boundary conditions on the domain structure in porous PbTiO3 ferroelectric nanowires. Our results demonstrate the formation of four types of domain structures via varying electrical boundary conditions, which possess distinguished local polarization and energy configurations. We further show that the domain structures are also dependent on the nanowire radius, including the breakdown of a metastable concentric toroidal domain structure upon reducing the radius to 14 nm. The present work provides guidance for further experimental studies on the control of polar domain structures through manipulating the electrical boundary condition and the ferroelectric size, which paves the way for developing multi-functions of low-dimensional ferroelectric systems. [ABSTRACT FROM AUTHOR]

Polymer blends are regarded as a straightforward and effective method to enhance the energy storage performance of dielectric film capacitors. However, how the components and structures within the blend systems affect the energy density and efficiency remains insufficiently explored in-depth. In this discourse, employing a polymer blend of ferroelectric and linear dielectric phases as a paradigm, we perform phase-field simulations to elucidate the effects of ferroelectric phase volume fractions, geometrical dimensions, and the dielectric constant of the linear phase on the energy storage capabilities. Concurrently, we have devised six divergent blending microstructures to probe the ramifications of structural variances on the overarching performance metrics. We also analyze the domain configurations and switching dynamics under varying electric fields to understand the performance variations and delineate the determinants conducive to superior energy density and efficiency. This paper theoretically establishes the component–content–structure–performance relationships of different polymer blend systems, which is expected to better guide the innovative design of new polymer blend dielectrics. [ABSTRACT FROM AUTHOR]

Spontaneous polarization as large as ∼ 28 μ C / cm 2 was recently observed around the dislocation cores in non-polar SrTiO 3 bulk crystals, and its origin was attributed to the flexoelectric effect, i.e., polarization induced by strain gradients. However, the roles of flexoelectricity, relative to other electromechanical contributions, and the nature of dislocations, i.e., edge vs screw dislocations in the induced polarization, are not well understood. In this work, we study the role of flexoelectricity in inducing polarization around three types of dislocation cores in SrTiO 3 : b = a (100) edge dislocation, b = a (110) edge dislocation, and b = a (010) screw dislocation, where b is the Burgers vector. For the edge dislocations, polarization can be induced by electrostriction alone, while flexoelectricity is essential for stabilizing the symmetric polarization pattern. The shear component of the flexoelectric tensor has a dominant effect on the magnitude and spatial distribution of the flexoelectric polarization. In contrast, no polarization is induced around the b = a (010) screw dislocation through either electrostriction or flexoelectricity. Our findings provide an in-depth understanding of the role of flexoelectricity in inducing polarization around dislocation cores and offer insights into the defect engineering of dielectric/ferroelectric materials. [ABSTRACT FROM AUTHOR]

The morphotropic phase boundary (MPB) has been recently extended from ferroelectrics to ferromagnets, especially for giant magnetostrictive materials. The giant magnetostrictive system in turn presents abundant research opportunities on MPB investigation. In this work, we demonstrate that a magnetic-ordering tricritical type MPB near room temperature can be constructed in the Tb x Ho 1 − x (Fe 0.9 Co 0.1 ) 2 magnetostrictive system. An ultrahigh magnetostrictive sensitivity is obtained that achieves the performance of the commercial Tb 0.27 Dy 0.73 Fe 2 (Terfenol-D) polycrystal in the Tb 0.2 Ho 0.8 (Fe 0.9 Co 0.1 ) 2 alloy. The detailed domain microstructures near the magnetic-ordering tricritical region are determined using phase-field simulations. Our results provide the underlying insights of magnetic-ordering tricritical MPB needed to guide the design of ultrasensitive magnetostrictive materials. [ABSTRACT FROM AUTHOR]

The three-dimensional arrangement and orientation of domain walls in ferroelectric K0.9Na0.1NbO3/(110)NdScO3 epitaxial thin films were investigated at different temperatures both experimentally by means of piezoresponse force microscopy and three-dimensional x-ray diffraction and theoretically by three-dimensional phase-field simulations. At room temperature, a well-ordered herringbone-like domain pattern appears in which there is a periodic arrangement of a1a2/MC monoclinic phases. Four different types of domain walls are observed, which can be characterized by out-of-plane tilt angles of ±45° and in-plane twist angles of ±21°. For the orthorhombic high-temperature phase, a periodic a1/a2 stripe domain pattern with exclusive in-plane polarization is formed. Here, two different types of domain walls are observed, both of them having a fixed out-of-plane domain wall angle of 90° but distinguished by different in-plane twist angles of ±45°. The experimental results are fully consistent with three-dimensional phase-field simulations using anisotropic misfit strains. The qualitative agreement between the experiment and the theory applies, in particular, to the wide phase transition range between about 180 °C and 260 °C. In this temperature range, a complex interplay of coexisting monoclinic a1a2/MC and orthorhombic a1/a2 phases takes place. [ABSTRACT FROM AUTHOR]

The concept of topological quantum number, or topological charge, has been used extensively to describe topological defects or solitons. Nematic liquid crystals contain both integer and half-integer topological defects, making them useful models for testing the rules that govern topological defects. Here, we investigated topological defects in nematic liquid crystals using the phase-field method. If there are no defects along a loop path, the total charge number is described by an encircled loop integral. We found that the total charge number is conserved, and the conservation of defects number is determined by a boundary during the generation and annihilation of positive–negative topological defects when the loop integral is confined. These rules can be extended to other two-dimensional systems with topological defects. [ABSTRACT FROM AUTHOR]

The (PbTiO3)n/(SrTiO3)n (PTO/STO) superlattice system has been shown to exhibit interesting topological phases (e.g., vortices and skyrmions) in addition to normal ferroelectric domain states. Existing studies are mostly focused on the dependence of topological polar distributions and properties of PTO/STO superlattice on its periodicity. Here, we study the strain effect on the topological phase transitions and ferroelectric domain structures employing phase-field simulations. We summarized in an isotropic strain (in-plane misfit strain along the x direction is equal to that along the y direction) periodicity phase diagram displaying the stability regions of different polar topological states, including normal ferroelectric twins, vortices, skyrmions, and mixtures of vortices and twins. We also analyzed the polarization configurations under anisotropic in-plane strains (in-plane misfit strain along the x direction is not equal to that along the y direction) and demonstrated that the strain anisotropy can be used to tune the directions of vortex arrays along either the [100] pc or [010] pc directions or labyrinth vortex arrays. This work offers guidance to manipulating polar structures in the PTO/STO superlattices via strain engineering. [ABSTRACT FROM AUTHOR]

In ferroelectric heterostructures, the interaction between intrinsic polarization and the electric field generates a rich set of localized electrical properties. The local electric field is determined by several connected factors, including the charge distribution of individual unit cells, the interfacial electromechanical boundary conditions, and chemical composition of the interfaces. However, especially in ferroelectric perovskites, a complete description of the local electric field across micro-, nano-, and atomic-length scales is missing. Here, by applying four-dimensional scanning transmission electron microscopy (4D STEM) with multiple probe sizes matching the size of structural features, we directly image the electric field of polarization vortices in (PbTiO3)16/(SrTiO3)16 superlattices and reveal different electric field configurations corresponding to the atomic scale electronic ordering and the nanoscale boundary conditions. The separability of two different fields probed by 4D STEM offers the possibility to reveal how each contributes to the electronic properties of the film. [ABSTRACT FROM AUTHOR]

Under electrical bias, mixed ionic conductors such as SrTiO3 are characterized by oxygen vacancy migration which leads to resistance degradation. The defect chemistry to describe the relationship between conductivity and oxygen vacancies is usually obtained by high temperature conductivity data or quenching experiments. These techniques can investigate the equilibrated state only. Here, we introduce a new approach using in-situ impedance studies with applied dc voltage to analyze the temperature dependent electrical properties of degraded SrTiO3 single crystals. This procedure is most beneficial since it includes electric field driven effects. The benefits of the approach are highlighted by comparing acceptor doped and undoped SrTiO3. This approach allows the determination of the temperature activation of both anodic and cathodic conductivity of Fe-doped SrTiO3 in the degraded state. The anodic activation energy matches well with the published results, while the activation energy of the degraded cathode region reported here is not in agreement with earlier assumptions. The specific discrepancies of the experimental data and the published defect chemistry are discussed, and a defect chemistry model that includes the strong temperature dependence of the electron conductivity in the cathode region is proposed. [ABSTRACT FROM AUTHOR]

The past decade has witnessed the tremendous scientific and technological potential of nanoscale flexoelectricity in solids. The flexoelectric effect describes the universal generation of electric polarization in response to strain gradients and could be inversely enhanced at reduced nanoscale dimensions. Based on this unique scaling effect, nanoscale flexoelectricity has shown exciting physical phenomena, promising novel electronic, electromechanical, and photovoltaic applications. One of the most powerful ways to harness nanoscale flexoelectricity is to press the surface of a material through an atomic force microscope (AFM) tip to generate large strain gradients. This so-called AFM tip pressing allows us to locally break the inversion symmetry in any materials and study all the fascinating physical phenomena associated with inversion asymmetry. Although this technique has recently facilitated many important studies on nanoscale flexoelectricity, its effective use still requires a more solid foundation. In this review, we provide a comprehensive guideline to exploring nanoscale flexoelectricity via AFM tip pressing. We also discuss recent progress and the future research direction of AFM tip pressing-driven nanoscale flexoelectricity. [ABSTRACT FROM AUTHOR]

A series of LaAlO3 thin films with different thicknesses were deposited by pulsed laser deposition at temperatures from 720 °C to 800 °C. The results from grazing incidence x-ray diffraction and reciprocal space mapping indicate that a thin layer of LaAlO3 adjacent to the SrTiO3 substrate remains almost coherently strained to the substrate, while the top layer starts to relax quickly above a certain critical thickness, followed by a gradual relaxation at larger film thickness when they are grown at lower temperatures. The atomic force microscopy results show that the fast relaxation is accompanied by the formation of cracks on the film surface. This can be ascribed to the larger energy release rate when compared with the resistance of LaAlO3 to cracking, according to calculations from the Griffith fracture theory. For films grown at 720 °C, a drop in sheet resistance by two orders of magnitude is observed when the top layer starts to relax, indicating a relationship between the strain and the conductivity of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface. The strain engineered by growth temperature provides a useful tool for the manipulation of the electronic properties of oxide heterointerfaces. [ABSTRACT FROM AUTHOR]

A phase field model is developed to examine microstructural evolution of an infiltrated solid oxide fuel cell cathode. It is employed to generate the three-phase backbone microstructures and morphology of infiltrate nano-particles [La1-xSrxMnO3 (LSM)]. Two-phase Y2O3+ZrO2 and LSM backbones composed of 0.5-1 µm particles are first generated and then seeded with infiltrate, and evolution is compared for starting infiltrate particle diameters of 5 nm and 10nm. The computed lifetime triple phase boundary (3PB) density of the infiltrated cathode is then compared to the cathode backbone. Results indicate that initial coarsening of infiltrate nano-particles is the primary evolution process, and infiltrate coarsening is the majority contributor to 3PB reduction. However, at all times, the infiltrated cathode possesses significantly greater 3PB length than even the uncoarsened backbone. Infiltrate particle size effects indicate that the smaller particle size produces greater 3PB length for the same infiltration amount, consistent with intuition. A maximum 3PB enhancement is reached when increasing infiltrate particle loading, and the maximum enhancement depends on infiltrate particle size. It is found that architectural degradation modes will insignificantly affect the lifetime performance of infiltrated cathodes. This work suggests that lifetime optimized particle size/loading combinations are identifiable, and can be precise if additional fundamental data become available. [ABSTRACT FROM AUTHOR]

The static magnetic solutions in magnetic composites with arbitrary susceptibility inhomogeneity and anisotropy are accurately computed using an efficient numerical algorithm based on a proposed Fourier spectral iterative perturbation method for 3-dimensional systems. An advantage of this method is that the interphase boundary conditions are automatically considered without explicitly tracking interphase interfaces in the composites. This method can be conveniently implemented in phase-field modeling of microstructure evolution in systems with inhomogeneous susceptibility as well as inhomogeneous spontaneous magnetization distributions. Based on the proposed method, the effects of microstructures including the susceptibility mismatch between the inclusions and matrix, inclusions volume fraction, and inclusions arrangement on the effective susceptibility and local static magnetic field distribution of the composite are investigated. It is found that the interactions among the inclusions embedded in the matrix play critical roles in determining the composite properties. [ABSTRACT FROM AUTHOR]

There has been significant progress on the fundamental science and technological applications of complex oxides and multiferroics. Among complex oxide thin films, barium strontium titanate (BST) has become the material of choice for room-temperature-based voltage-tunable dielectric thin films, due to its large dielectric tunability and low microwave loss at room temperature. BST thin film varactor technology based reconfigurable radio frequency (RF)/microwave components have been demonstrated with the potential to lower the size, weight, and power needs of a future generation of communication and radar systems. Low-power multiferroic devices have also been recently demonstrated. Strong magneto-electric coupling has also been demonstrated in different multiferroic heterostructures, which show giant voltage control of the ferromagnetic resonance frequency of more than two octaves. This manuscript reviews recent advances in the processing, and application development for the complex oxides and multiferroics, with the focus on voltage tunable RF/microwave components. The over-arching goal of this review is to provide a synopsis of the current state-of the-art of complex oxide and multiferroic thin film materials and devices, identify technical issues and technical challenges that need to be overcome for successful insertion of the technology for both military and commercial applications, and provide mitigation strategies to address these technical challenges. [ABSTRACT FROM AUTHOR]

PbTiO3/BaTiO3 ferroelectric superlattices commensurately constrained by a SrTiO3 substrate are studied using both phenomenological theory and phase field simulations. It is found that the spontaneous polarization of the superlattice does not vary significantly with volume fraction until PbTiO3 becomes the majority of the superlattice. A minimum tetragonality exists at room temperature when the PbTiO3 volume fraction is around 90%, resulted from its different sensitivity to epitaxial strains and the electrostatic coupling of the two polarized layers in this PbTiO3/BaTiO3 superlattice. [ABSTRACT FROM AUTHOR]

Lead-free ferroelectrics K1−xNaxNbO3 are promising candidates for sensors and electromechanical and cooling devices. Here, we study the thermodynamics of phase transitions and electrocaloric (EC) properties of K0.5Na0.5NbO3 single crystals subject to electric, thermal, and mechanical stimuli, from which we establish the temperature–pressure and temperature-electric field phase diagrams. The focus is on understanding the pressure effect on the ferroelectric properties of K1−xNaxNbO3, which is critical to develop chemical doping strategies to achieve targeted performances via chemical strain engineering. Under pressure-free conditions, the K0.5Na0.5NbO3 single crystal has a Curie temperature of around 673 K, which is the optimal operating temperature for the EC effect. We show that a hydrostatic pressure of around 7 GPa can reduce the optimal operating temperature to near room temperature, 300 K, at the expense of decreasing the EC temperature change ΔTEC from 5.4 K to 2.4 K when the applied electric field is 10 MV/m. The present work provides a fundamental understanding of the pressure-induced phase transitions and ferroelectric properties and can be employed to guide the future design of K1−xNaxNbO3-based materials. [ABSTRACT FROM AUTHOR]

The dielectric and piezoelectric properties of a KNbO3 single crystal under applied hydrostatic pressure and positive bias electric field are investigated using phenomenological Landau-Ginzburg-Devonshire thermodynamic theory. It is shown that the hydrostatic pressure effect on the dielectric and piezoelectric properties is similar to temperature, suggesting a common underlying mechanism for the piezoelectric anisotropy and its enhancement. The stable phase diagram of KNbO3 as a function of temperature and positive bias electric field is constructed. The maximum piezoelectric coefficient d33o* varying with temperature and electric field is calculated. [ABSTRACT FROM AUTHOR]

Frequency dependent dynamic electromechanical response of the mixed ionic-electronic conductor film to a periodic electric bias is analyzed for different electronic and ionic boundary conditions. Dynamic effects of mobile ions concentration (stoichiometry contribution), charge state of acceptors (donors), electron concentration (electron-phonon coupling via the deformation potential), and flexoelectric effect contribution are discussed. A variety of possible nonlinear dynamic electromechanical responses of mixed electronic ionic conductors (MIEC) films including quasi-elliptic curves, asymmetric hysteresis-like loops with pronounced memory window, and butterfly-like curves are calculated. The electromechanical response of ionic semiconductor is predicted to be a powerful descriptor of local valence states, band structure and electron-phonon correlations that can be readily measured in the nanoscale volumes and in the presence of strong electronic conductivity. [ABSTRACT FROM AUTHOR]

The effect of misfit strain and deadlayer on the polarization switching of epitaxial ferroelectric thin film is investigated using a phase field model. Simulation results show that a compressive misfit strain increases the coercive field and remanent polarization of ferroelectric thin film with deadlayers, whereas a tensile misfit strain decreases these factors. The presence of a deadlayer between the ferroelectric thin film and the electrode prevents charge compensation on the ferroelectric surface and reduces the coercive field and remanent polarization of the film. A periodic a/c/a/c multiple domain structure is found in ferroelectric thin film with tensile misfit strain when there is no deadlayer. However, when a deadlayer is present in the film, the c domains vanish and the out-of-plane component of polarization degrades. The degradation of the out-of-plane polarization makes ferroelectric thin films with tensile misfit strain and deadlayers lose their ferroelectric property in the thickness direction, which can be attributed to the combined effects of the deadlayer and tensile strain. The coercive field and remanent polarization of ferroelectric thin film decrease with the thickness of the deadlayer. [ABSTRACT FROM AUTHOR]

Orientation dependence of the longitudinal piezoelectric coefficients (d33*) of a KNbO3 single crystal has been investigated as a function of temperature by using the Landau-Ginzburg-Devonshire phenomenological theory. It is shown that the maximum of d33* is not always along the polarization direction of the ferroelectric phase. The enhancement of d33* along a nonpolar direction is attributed to a ferroelectric phase transition at which a polarization changes its direction. In the tetragonal phase, the maximum of d33t* at high temperatures is along the tetragonal polar direction and then changes its direction toward the polar direction of the orthorhombic phase when the temperature is close to the tetragonal-orthorhombic phase transition. The maximum of d33o* of the orthorhombic phase depends on both the high-temperature and low temperature ferroelectric phase transitions. In the rhombohedral phase, the maximum of d33r* is relatively insensitive to temperature due to the absence of any further phase transitions in the low temperature regime. These results can be generalized to the phase transitions induced by external electric field, pressure, and composition variations. [ABSTRACT FROM AUTHOR]

The Landau–Ginzburg–Devonshire phenomenological theory is employed to model and predict the ferroelectric phase transitions and properties of single-domain potassium niobate (KNbO3). Based on the LGD theory and the experimental data of KNbO3 single crystal, an eighth-order polynomial of free energy function is proposed. The fitted coefficients are validated by comparing to a set of experimental measured values including phase transition temperatures, spontaneous polarization, dielectric constants, and lattice constants. The effects of hydrostatic pressure and external electric field on phase transition temperatures and piezoelectric coefficients are investigated. The free energy function may be used to predict ferroelectric domain structures and properties of KNbO3 bulk and films by phase-field approach. [ABSTRACT FROM AUTHOR]

We study the effect of the depolarization field on a domain wall structure near the surface of a ferroelectric. Since in real situation bound and screening charges form an electric double layer, the breaking of this layer by the domain wall induces stray depolarization field, which in turn changes the domain wall structure. Power law decay of the stray field results in the power law of polarization saturation near the surface, as compared to exponential saturation in the bulk. Obtained results predict that the surface broadening of ferroelectric domain walls appeared near Curie temperature as well as describe domain wall depth profile in weak ferroelectrics. We qualitatively describe extra-broad domain walls near LiNbO3 and LiTaO3 surfaces observed experimentally at room temperature, which probably originate at high temperatures but did not fully relax their width with temperature decrease allowing for lattice pinning and defect centers. Thus results have broad implication for fundamental issues such as maximal information storage density in ferroelectric data storage, domain wall pinning mechanisms at surfaces and interfaces, and nucleation dynamics. [ABSTRACT FROM AUTHOR]

Unlike ideal 180° ferroelectric walls that are a unit cell wide (∼0.5 nm), real walls in ferroelectrics have been reported to be many nanometers wide (1–10 nm). Using scanning nonlinear dielectric microscopy of lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) ferroelectrics, we show that the wall width at surfaces can vary considerably and even reach ∼100 nm in places where polar defects adjoin a wall. The consequence of such variable wall widths is investigated on the specific property of threshold field required for wall motion. Using microscopic phase-field modeling, we show that the threshold field for moving an antiparallel ferroelectric domain wall dramatically drops by two to three orders of magnitude if the wall was diffuse by only ∼1–2 nm, which agrees with experimental wall widths and threshold fields for these materials. Modeling also shows that wall broadening due to its intersection with a surface will influence the threshold field for wall motion only for very thin films (1–10 nm) where the surface broadening influences the bulk wall width. Such pre-existing and slightly diffuse domain walls with low threshold fields for wall motion may offer a general mechanism to explain significantly lower experimental coercive fields for domain reversal in ferroelectrics as compared to the thermodynamic predictions. [ABSTRACT FROM AUTHOR]

The grain size effects on the phase coexistence and magnetostrictive response of Tb1−xDyxFe2 polycrystals near the ferromagnetic morphotropic phase boundary (MPB) are revealed through phase-field modeling. It shows that phase coexistence is a universal phenomenon in polycrystals for both ferromagnetic and ferroelectric MPB and that the range of compositions for phase coexistence increases with decreasing grain sizes. A large, reversible, and anhysteretic magnetostrictive response at low external fields is also found in the fine-grained polycrystals around the ferromagnetic MPB, which offers us a route to developing nanocrystalline magnetostrictive materials. [ABSTRACT FROM AUTHOR]

Potassium-sodium niobate K1-xNaxNbO3 (KNN) is one of the most promising lead-free piezoelectric materials. While there have been many studies on the microstructures and properties of KNN ceramics, the phase transitions and ferroelectric domain structures of KNN thin films are not well understood. In this work, we employ three-dimensional (3D) phase-field simulations to obtain the ferroelectric domain structures of KNN (0 ≤ x ≤ 0.5) thin films under a range of temperatures (0 K to 1300 K) and equiaxial misfit strains (–1.5% to 1.5%), based on which we establish the misfit strain-temperature phase diagrams of KNbO3 and K0.5Na0.5NbO3 thin films. We identify a wide variety of complex domain structures with coexisting ferroelectric phases, implying enhanced dielectric and piezoelectric properties. We expect this work to provide guidance for the strain engineering of domain structures and properties of KNN thin films. [ABSTRACT FROM AUTHOR]

It has been known that ferroelectric charged domain walls (CDWs), which break the polarization continuity, may be electrically active with an elevated conductivity. However, the bound charge at CDWs may render them energetically unstable, and thus, forming CDWs naturally and manipulating them electrically is still challenging. Here, we theoretically utilize phase-field simulations to design spontaneously generated CDWs with center-type quad-domains in a single square-shaped BiFeO3 nanoisland. It is shown that the stability of the spontaneously emerging head-to-head domain walls with center-convergent quad-domains is mainly determined by three contributions, namely, the geometrical constraint from approximately 45°-tilted bottom edges, the electric boundary condition, and the necessary screening free charges to compensate head-to-head domain walls. It is demonstrated that the center-convergent quad-domains with head-to-head CDWs can be electrically switched to the center-divergent one with tail-to-tail CDWs, providing guidance for achieving ferroelectric domain-wall-based nanodevices with low-power dissipation. [ABSTRACT FROM AUTHOR]

Adding high-κ ceramic nanoparticles into polymers is a general strategy to improve the performances in energy storage. Classic effective medium theories may fail to predict the effective permittivity in polymer nanocomposites wherein the space charge effects are important. In this work, a computational model is developed to understand the space charge effects on the frequency-dependent dielectric properties including the real permittivity and the loss for polymer nanocomposites with both randomly distributed and aggregated nanoparticle fillers. It is found that the real permittivity of the SrTiO3/polyethylene (12% SrTiO3 in volume fraction) nanocomposite can be increased to as high as 60 when there is nanoparticle aggregation and the ion concentration in the bulk polymer is around 1016 cm-3. This model can be employed to quantitatively predict the frequency-dependent dielectric properties for polymer nanocomposites with arbitrary microstructures. [ABSTRACT FROM AUTHOR]

Understanding electric field effects on precession dynamics is crucial to the design of spin transfer torque devices for improving the performance in nano-oscillator. In this letter, the precession dynamics of a CoFeB/MgO multi-layer structured nano-oscillator under externally applied electric field is predicted using a micromagnetic simulation. It is revealed that the electric field can modify the range of oscillation spectra in single frequency mode. With the increase in electric field, there is a red-shift of the resonant frequency. When a positive electric field pulse is applied, a phase lag of the spin precession is induced, which is proportional to the pulse amplitude and duration. The present work is expected to stimulate future experimental efforts on designing devices with electric-field modulated spin transfer torque nano-oscillators. [ABSTRACT FROM AUTHOR]

First-principles calculations were employed to explore magnetocrystalline anisotropy energy (MAE) of Ni2X(X=Fe, Co)Ga alloys. TheMAEof Ni2FeGa is found to show a concave behavior as a function of tetragonal distortion and easy-axis of magnetization in martensitic phase is along long axis, which have been interpreted by the shift of Fe dxy+dyz peak in minority spin channel near Fermi level. The substitution of Ni by Co in Ni2FeGa alloys rotates magnetic easy axis from long axis to short axis in non-modulated phase while substitution of Fe by Co did not, which is in agreement with experiment. Magnetic anisotropy constant and magnetic stress have been estimated with calculated MAE of martensite phases. By comparing first-principles estimated values of magnetic and twinning stresses, we confirmed the condition, whether large magnetic field-induced strains in FSMAs could be obtained or not. This information can provide theoretical guidance in searching new types of FSMAs with large magnetic field induced strain. [ABSTRACT FROM AUTHOR]

Ferroelectric and ferroelastic domain walls play important roles in ferroelectric properties. However, their couplings with flexoelectricity have been less understood. In this work, we applied phase-field simulation to investigate the flexoelectric coupling with ferroelectric a/c twin structures in lead ziconate titanate thin films. Local stress gradients were found to exist near twin walls that created both lateral and vertical electric fields through the flexoelectric effect, resulting in polarization inclinations from either horizontal or normal orientation, polarization rotation angles deviated from 90°, and consequently highly asymmetric a/c twin walls. By tuning the flexoelectric strengths in a reasonable range from first-principles calculations, we found that the transverse flexoelectric coefficient has a larger influence on the polarization rotation than longitudinal and shear coefficients. As polar rotations that commonly occur at compositional morphotropic phase boundaries contribute to the piezoelectric enhancement, this work calls for further exploration of alternative strain-engineered polar rotations via flexoelectricity in ferroelectric thin films. [ABSTRACT FROM AUTHOR]

An introduction to a special issue on strain engineering in functional materials is presented including topics on advancements in conventional strain engineering, developments in functional and strongly correlated oxides, and strain engineering of thermal properties in both conventional thermoelectrics and novel semimetals.

Pr(Fe0.75Co0.15Cu0.01Nb0.04B0.05)1.93 alloys were synthesized by annealing its melt-spinning ribbons under different pressures and temperatures. It was demonstrated that the average grain size decreases with increasing pressure from 3 GPa to 6 GPa under the same annealing temperature of 853 K but increases with increasing temperature from 823 K to 923 K under the same annealing pressure of 6 GPa. A negative correlation between the coercivity and average grain size was found in the present investigated system. Grain refinement without losing the advantage of volume fraction of magnetostrictive phase offers the sample annealed under 6 GPa and 853 K the optimized magnetostrictive property, which might make it potential material for magnetostrictive application. [ABSTRACT FROM AUTHOR]

Phase-field method micromagnetic microelastic modeling is employed to simulate the thermal domain stability and enhanced magnetostrictive responses around the ferromagnetic morphotropic phase boundary (MPB) in giant magnetostrictive Tb1–xDyxFe2 (x ≈ 0.27) single crystal. The simulation shows that the rhombohedral and tetragonal phases coexist in equilibrium in the vicinity of MPB region due to the balance of weak magnetocrystalline anisotropy and strong exchange, magnetostatic and ferroelastic interaction. Enhanced magnetostrictive response is found in the vicinity of MPB, which could be attributed to the low-energy rotating pathways of local magnetization vectors in the phase coexisting region. [ABSTRACT FROM AUTHOR]

In general, a dielectric material will eject (or absorb) heat when an electric field is applied and absorb (or eject) heat when the field is removed, under isothermal condition, which is known as the normal (or negative) electrocaloric (EC) effect. For some applications, it is highly desired that an EC material will absorb heat (cooling the surrounding) without subsequent heating under an electric pulse. Here, we show that such an EC material can be realized in a properly designed hybrid normal ferroelectric/relaxor ferroelectric polymer blend in which the normal ferroelectric component induces dipole ordering in the relaxor polymer in the poled state, which can be switched to a de-poled state by an external field. More importantly, the de-poled state can be maintained by the relaxor component when the de-poling field is removed. Consequently, the hybrid blend exhibits a large cooling (an isothermal entropy change ΔS=11.5 J kg–1K–1) without the subsequent heating upon the application of an electric pulse. [ABSTRACT FROM AUTHOR]

Systematic first-principles calculations were performed to study the lattice dynamics of Yb14MnSb11 and hence to obtain a wide range of its thermodynamic properties at high temperatures. The calculated results were analyzed in terms of the lattice contribution and the electronic contribution, together with a comparison with a collection of experimental thermochemical data. At 0 K, the electronic density of states showed the typical feature of a p-type semiconductor—a small amount of unoccupied electronic states exclusively made of the major spin by a range of ∼0.6 eV above the Fermi energy. It showed that the Mn atom had a ferromagnetic spin moment of ∼4 μB. As a semiconductor, it was found that the electronic contribution to the heat capacity was substantial, with an electronic heat capacity coefficient of ∼0.0006 J/mole-atom/K2. [ABSTRACT FROM AUTHOR]

While electric field induced ferroelectric switching has been extensively studied and broadly utilized, pure mechanical switching via flexoelectric effect has recently opened up an alternative method for domain writing due to their highly localized, electrically erasable and electric damage free characteristics. Thus far, few studies have been made on the coupling effect of electro-mechanical switching in ferroelectric materials, likely due to the experimental difficulty in the accurate definition of the tip-surface contact area and in the identification of mechanical contribution from electrical effect. Here, we employed self-consistent phase-field modeling to investigate the bi-polar switching behavior of (001) oriented Pb(Zr0.2Ti0.8)O3 thin film under concurrent electric and strain field created via a piezoresponse force microscope probe. By separating the effects from electric field, homogeneous strain and strain gradient, we revealed that the homogeneous strain suppresses the spontaneous polarization and accordingly increases the coercive field, and the strain gradient favors unipolar switching and inhibit it in the reverse direction, thus causing lateral offset of the hysteresis loop. The uncertainty of flexoelectric coefficients and the influence of flexocoupling coefficients on switching have also been discussed. Our study could necessitate further understanding of the electric, piezoelectric, and flexoelectric contribution to the switching behavior in nanoscale ferroelectric oxides. [ABSTRACT FROM AUTHOR]

The switching behavior of a (BaTiO3)8/(SrTiO3)4 superlattice grown on a SrTiO3 substrate was simulated utilizing the phase field method. To investigate the effect of the mechanical constraint of the substrate on switching, three types of superlattice/substrate interface mechanical relaxation conditions were considered: (1) fully commensurate, (2) partially relaxed, and (3) fully relaxed. Our simulation results demonstrate that the hysteresis loops under the three types of constraints are very different. The interfacial coherency dramatically affects the coercive field and remanent polarization of the superlattices. The mechanism underlying the hysteresis loop variation with interfacial coherency was investigated by analyzing the ferroelectric domain configuration and its evolution during the switching process. The simulated hysteresis loop of the fully relaxed superlattice exhibits a shape that is potentially relevant to the application of ferroelectrics for energy storage materials. [ABSTRACT FROM AUTHOR]

Conducting characteristics of topological defects in ferroelectric materials, such as charged domain walls, engendered a broad interest on their scientific merit and the possibility of novel applications utilizing domain engineering. At the same time, the problem of electron transport in ferroelectrics still remains full of unanswered questions and becomes yet more relevant over the growing interest in ferroelectric semiconductors and new improper ferroelectric materials. We have employed selfconsistent phase-field modeling to investigate the physical properties of a local metal-ferroelectric (Pb(Zr0.2Ti0.8)O3) junction in applied electric field. We revealed an up to 10-fold local enhancement of electric field realized by large polarization gradient and over-polarization effects due to inherent non-linear dielectric properties of Pb(Zr0.2Ti0.8)O3. The effect is independent of bias polarity and maintains its strength prior, during and after ferroelectric switching. The observed field enhancement can be considered on similar grounds as increased doping level, giving rise to reduced switching bias and threshold voltages for charge injection, electrochemical and photoelectrochemical processes. [ABSTRACT FROM AUTHOR]

We report that when (La,Sr)MnO3 films grown on orthorhombic (110)0 DyScO3 (DSO) substrates once reach a critical thickness, anisotropic patterned cracks are introduced which propagate parallel to [1-10]0 direction. The typical surface morphology and cross-sectional TEM are examined in order to clarify the formation of these cracks. The cause of crack propagation through the film and into the substrate is identified as the underlying large anisotropic strain and relatively low stiffness of DSO substrates. The electrical conduction in the crack region is found to be several orders of magnitude higher than that in the uncracked areas of the film. The energy dispersive x-ray-verified La0.7Sr0.3MnO3 deposited into cracks may be responsible for the anomalous conducting behaviors. Finally, a simple model is constructed for estimating substrate fracture toughness and calculating an upper bound of film fracture toughness. [ABSTRACT FROM AUTHOR]

Epitaxial strain is a powerful tool to induce functional properties such as ferroelectricity in thin films of materials that do not possess ferroelectricity in bulk form. In this work, a ferroelectric state was stabilized in thin films of the incipient ferroelectric, CaTiO3, through the careful control of the biaxial strain state and TiO6 octahedral rotations. Detailed structural characterization was carried out by synchrotron x-ray diffraction and scanning transmission electron microscopy. CaTiO3 films grown on La0.18Sr0.82Al0.59Ta0.41O3 (LSAT) and NdGaO3 (NGO) substrates experienced a 1.1% biaxial strain state but differed in their octahedral tilt structures. A suppression of the out-of-plane rotations of the TiO6 octahedral in films grown on LSAT substrates resulted in a robust ferroelectric I4mm phase with remnant polarization ~5 μC/cm² at 10K and Tc near 140 K. In contrast, films grown on NGO substrates with significant octahedral tilting showed reduced polarization and Tc. These results highlight the key role played by symmetry in controlling the ferroelectric properties of perovskite oxide thin films. [ABSTRACT FROM AUTHOR]

Based on phase field modeling and thermodynamic analysis, purely electric-field-driven magnetization reversal was shown to be possible in a multiferroic heterostructure of a square-shaped amorphous Co40Fe40B20 nanomagnet on top of a ferroelectric layer through electrostrain. The reversal is made possible by engineering the mutual interactions among the built-in uniaxial magnetic anisotropy, the geometry-dependent magnetic configuration anisotropy, and the magnetoelastic anisotropy. Particularly, the incorporation of the built-in uniaxial anisotropy made it possible to reverse magnetization with one single unipolar electrostrain pulse, which is simpler than previous designs involving the use of bipolar electrostrains and may alleviate ferroelectric fatigue. Critical conditions for triggering the magnetization reversal are identified. [ABSTRACT FROM AUTHOR]

Polycrystalline Pt thin films of different thicknesses (0-75 nm) were introduced using magnetron sputtering in Pb(Zr0.52Ti0.48)O3 (PZT, 400 nm in thickness)/Pt/Ni multiferroic film heterostructures, aimed at optimizing the transfer efficiency of magnetostrictive strain from the bottom Ni foil to the top PZT film αE31 was directly measured, while the strain transfer efficiency k was obtained by combined experimental and theoretical analysis. At the optimum Pt-thickness of 30 nm, the polycrystalline film heterostructure shows the largest αE31 of 772mVcm-1 Oe-1 at a low dc magnetic bias field of 86 Oe, as well as the highest k of 83% that is comparable to that in epitaxial quasi-2-2 film heterostructures. [ABSTRACT FROM AUTHOR]

Flexoelectric coefficient is a fourth-rank tensor arising from the coupling between strain gradient and electric polarization and thus exists in all crystals. It is generally ignored for macroscopic crystals due to its small magnitude. However, at the nanoscale, flexoelectric contributions may become significant and can potentially be utilized for device applications. Using the phase-field method, we study the mechanical switching of electric polarization in ferroelectric thin films by a strain gradient created via an atomic force microscope tip. Our simulation results show good agreement with existing experimental observations. We examine the competition between the piezoelectric and flexoelectric effects and provide an understanding of the role of flexoelectricity in the polarization switching. Also, by changing the pressure and film thickness, we reveal that the flexoelectric field at the film bottom can be used as a criterion to determine whether domain switching may happen under a mechanical force. [ABSTRACT FROM AUTHOR]

A three-dimensional phenomenological model is proposed to describe both ferroelectricity and antiferroelectricity based on the Ginzburg-Landau-Devonshire theory. Its application to the multiferroic Sm-doped BiFeO3 system describes the temperature-, pressure-, and composition-induced ferroelectric to antiferroelectric phase transitions. The constructed temperature-composition and temperature-pressure phase diagrams show that compressive hydrostatic pressure and Sm doping have similar effects on the ferroelectric and antiferroelectric phase transitions. It is also indicated from the temperature-pressure phase diagram that the experimentally observed phase of BiFeO3 under the hydrostatic pressure from 3GPa to 10GPa is a PbZrO3-like antiferroelectric phase. [ABSTRACT FROM AUTHOR]

A nonlinear phase-field model has been developed for describing the electrodeposition process in electrochemical systems that are highly out of equilibrium. Main thermodynamic driving forces for the electrode-electrolyte interface (EEI) evolution are limited to local variations of overpotential and ion concentration. Application of the model to Li-ion batteries describes the electrode interface motion and morphology change caused by charge mass transfer in the electrolyte, an electrochemical reaction at the EEI and cation deposition on the electrode surface during the charging operation. The Li electrodeposition rate follows the classical Butler-Volmer kinetics with exponentially and linearly depending on local overpotential and cation concentration at the electrode surface, respectively. Simulation results show that the Li deposit forms a fiber-like shape and grows parallel to the electric field direction. The longer and thicker deposits are observed both for higher current density and larger rate constant where the surface reaction rate is expected to be high. The proposed diffuse interface model well captures the metal electrodeposition phenomena in plenty of non-equilibrium electrochemical systems. [ABSTRACT FROM AUTHOR]

A phase-field model is developed for studying the cation interdiffusion across electrolyte-electrode interfaces in solid oxide fuel cell (SOFC) that can be contributing to long timescale performance degradation. Demonstrated on an interface between an 8%molY2O3-stabilized ZrO2 and a La0.65Sr0.3MnO3-x typically used in SOFC, time-dependent evolution of the cation interdiffusion profiles are predicted by linking the phase-field model to a diffusion equation. The simulated interdiffusion profiles agree with independent experimental data in both time and space domains at different temperatures. [ABSTRACT FROM AUTHOR]

Strain effects on domain structures and thermal stability in single-domain magnetic thin films were studied using thermodynamic analysis. The strain-domain structure and stability diagrams were established and compared to several existing experimental results. The structure diagram displays various stable single-domain states under in-plane normal and/or in-plane shear strains by minimizing the free energy density whereas the stability diagram takes into account possible thermal excitations and hence illustrate the thermally stable magnetic single-domain states. The results improve the understanding of strain-magnetization correlation in magnetic thin films and provide useful insight for the development of strain-engineered magnetic nanostructures with novel functionalities. [ABSTRACT FROM AUTHOR]

Ferroelectric superlattice heterostructures have recently been explored for potential applications in electronic devices. In this letter, we employed the phase-field approach to simulate the domain structure and switching of a (BaTiO3)8/(SrTiO3)3 superlattice film constrained by a GdScO3 substrate. A constricted ferroelectric hysteresis loop was observed with a high saturation polarization but a small coercive field. The shape of the hysteresis loop is understood by analyzing the ferroelectric polarization distributions during switching. It is demonstrated that the multilayers stack behaves as dipole spring ferroelectric, named in analogy to exchange spring magnets in magnetic multilayers that show similar loops. [ABSTRACT FROM AUTHOR]

A thermodynamic potential as a function of the ferromagnetic, antiferromagnetic, and martensite order parameters is developed using existing experimental data. It successfully reproduces the single domain properties of Ni45Co5Mn36.7In13.3 bulk crystal, including the Curie temperature, the transition temperature from ferromagnetic austenite phase to antiferromagnetic martensite phase as well as its response to an externally applied magnetic field, the saturated magnetization, the martensite strain, the entropy change, and the magnetic permeability. It can be applied to explore the stress-strain and magnetic field-strain behaviors and implemented in phase field simulations to study the ferromagnetic, antiferromagnetic, and martensite domain stability and evolution. [ABSTRACT FROM AUTHOR]