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Strain-mediated magnetoelectric effect can be utilized as an energy-efficient approach for spin manipulation. However, over 90° magnetization rotation is still challenging in un-patterned magnetic films, as the piezo-strain driven by ferroelectric domain switching is generally uniaxial rather than unidirectional, which limits the developments of non-volatile magnetic memory and logic devices. Here we demonstrate the rotation of local magnetization with a large angle of 136° by applying strains with a shear component at a fixed magnetic field of 45 Oe in FeCoSiB/PMN-PT (011) multiferroic heterostructures, revealed by a vector-resolved quantitative magneto-optical Kerr effect (MOKE) microscopy. Phase-field simulations confirm that the approximate 140° rotation of magnetization vectors is a consequence of the shear strain associated with ferroelectric/ferroelastic switching of PMN-PT (011) substrates. The visualization of over 90° magnetization rotation induced by the strain with a shear component paves the way for deterministic magnetization switching that has important implications in the energy-efficient spintronic devices.

Porous tetragonal PbTiO3 nanowires, synthesized through an intermediate pre-perovskite structure, exhibit distinct behaviors from those of the corresponding bulk PbTiO3. Here we investigate the role of nanopores in the ferroelectric, dielectric, and piezoelectric properties of ferroelectric PbTiO3 nanowires employing phase-field simulations. It is found that the presence of pores gives rise to large enhancements in both dielectric constant and piezoelectric coefficient by ~50% and 30%, respectively, compared with those of the bulk PbTiO3. It is shown that the smaller the pore size is, the higher the dielectric and piezoelectric responses of the nanowire are. A charge compensation mechanism is proposed to explain the experimentally measured change of oxygen ions concentration at the pore surfaces. The findings provide in-depth insights into modulation of material properties through nanopores.

While extensive experimental activities have been carried out to study the phase transitions and ferroelectric properties of BaZrxTi1-xO3 solid solutions, the corresponding theoretical understanding is largely lacking due to the unavailability of thermodynamic potentials for this system. In this work, an eighth-order polynomial of thermodynamic potential for BaZrxTi1-xO3 (x≤0.3) solid solutions is established based on the existing potential coefficients of BaTiO3 and theexperimentally measured phase diagram of BaZrxTi1-xO3 solid solutions. It is then employed to predict and understand the domain structures and switching for BaZrxTi1-xO3 (x ≤0.3) single crystals using phase-field simulations. The simulated domain structures and switching are consistent with thermodynamic analysis and available experimental measurements, validating the established thermodynamic potential. It is expected that this thermodynamic potential will find wide applications to studying the phase transitions and ferroelectric properties of BaZrxTi1-xO3 (x ≤0.3) bulk and nanoscale materials.

Understanding magnetic domain structures and their responses to electric fields in multiferroic heterostructures is critical to the design of electric-field-driven spintronic devices. High-throughput finite-element and phase-field simulations are performed to probe the piezoelectric strain anisotropy and its relaxation, magnetic domain structures and their responses to applied voltages as function of the in-plane dimensions and thickness of the magnetic Ni nanoislands grown on a Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) membrane. The piezoelectric strain anisotropy is found to increase with the in-plane aspect ratio, but it can be significantly relaxed, as large as >80%, in nanoislands of thickness larger than >15 nm. Magnetic domain diagrams are established to identify the domain structures for Ni nanoislands of different lengths of in-plane major and minor axis, as well as thickness. When a voltage is applied to the multiferroic heterostructure, the single-domain magnetic domain can be switched by the piezoelectric strain, whereas the vortex domain is not switched. However, for a multiferroic heterostructure with a thick nanoisland wherein most of the piezoelectric strain is relaxed, the single-domain magnetic domain shows a weak response to the voltage and cannot be switched. The effect of thickness variation in real magnets on the switching is also discussed. The present results are expected to provide guidance to the understanding and design of multiferroic nanostructures for achieving electric-field-modulated magnetic properties.

Light-rare-earth-based Pr ( Fe 1 - x Si x ) 1.93 alloys ( 0 ⩽ x ⩽ 0.1 ) with pure cubic Laves phase were synthesized by high-pressure annealing method. Si substitution effects on magnetic, electrical and magnetostrictive properties of light-rare-earth-based Pr ( Fe 1 - x Si x ) 1.93 alloys were systematically investigated. It was found that the lattice parameter a and Curie temperature T C of the cubic Laves phase in the alloys increase with the increasing Si content up to x = 0.05 , which might be ascribed to the preferential occupation of Si in the Laves phase interstitial sites. The magnetization at the maximum available field of 15 kOe, σ 15 k , decreases monotonically with the increasing x. A significant increase of 67% in electrical resistivity was observed in Pr ( Fe 0.9 Si 0.1 ) 1.93 alloy at room temperature. The magnetostriction at the field of 3 kOe of Pr ( Fe 0.95 Si 0.05 ) 1.93 is about 542 ppm, which is even larger than the saturation magnetostriction of heavy-rare-earth-based Tb 0.2 Dy 0.22 Ho 0.58 Fe 2 single crystal ( λ s = 530 ppm). The attractive price, lower eddy current loss, together with high magnetostrictive response suggest that the Pr ( Fe 0.95 Si 0.05 ) 1.93 alloy might be a good candidate material for the potential magnetostrictive applications.

The collective dynamics of topological structures1-6 are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions3,4 have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage7. Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices5,6, and these are promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures. Here, using terahertz-field excitation and femtosecond X-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders-of-magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices3. A previously unseen tunable mode, hereafter referred to as a vortexon, emerges in the form of transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond timescales. Its frequency is considerably reduced (softened) at a critical strain, indicating a condensation (freezing) of structural dynamics. We use first-principles-based atomistic calculations and phase-field modelling to reveal the microscopic atomic arrangements and corroborate the frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.

Short-range ordered polar nanoregions are key to the giant electromechanical properties exhibited by relaxor ferroelectrics. Stabilization of the long-range ferroelectric order in relaxor systems has typically been achieved by applying external fields. In this work, spontaneous (zero-field) ferroelectric order is demonstrated in the composites constituting of nonergodic relaxor matrix phase $0.91\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{Ti}{\mathrm{O}}_{3}\text{\ensuremath{-}}0.09\mathrm{BaTi}{\mathrm{O}}_{3}$ with ZnO inclusions. Direct structural evidence is provided for the long-range ferroelectric order in the composites using in situ electric-field-dependent synchrotron investigations and $^{23}\mathrm{Na}$ nuclear magnetic resonance spectroscopy. Thermodynamic analysis incorporating microelasticity reveals the role of spatial residual stress in stabilizing the ferroelectric order. The work provides a direct correlation between the stabilized ferroelectric order and enhanced thermal stability, which can be utilized to guide the design of spontaneous long-range order in other relaxor systems.

Fossil fuel internal combustion engines generate and release a huge amount of nitrogen dioxide, leading to respiratory and allergic diseases such as asthma, pneumonia, and possibly tuberculosis. Here we develop an alveolus-inspired membrane sensor (AIMS) for self-powered wearable nitrogen dioxide detection and personal physiological assessment. The bionic AIMS exhibits an excellent sensitivity up to 452.44%, a good linearity of 0.976, and superior selectivity under a NO

In conventional light-emitting diodes the epitaxial strain and related piezoelectric polarization arising along the polar [0001] growth direction of the InGaN/GaN quantum wells (QWs) induce internal fields which adversely affect the radiative recombination of electron-hole pairs therein. Growing the quantum wells along a nonpolar orientation can, in principle, avoid this problem but seems to face with another problem associated with indium clustering. In this study, we present experimental evidence that supports the inhomogeneous distribution of indium in non-polar a-plane InGaN QWs by using dark-field inline electron holography as well as atom probe tomography measurements and discuss the possible origin by density functional theory calculation. A model non-polar a-plane QW structure with 10 nm-thick In0.1Ga0.9N double QWs was investigated and compared with the polar c-plane QWs with the same QW structure. Unlike the random distribution in the polar QWs, the indium atoms in the non-polar QW exhibit inhomogeneous distribution and show a tendency of periodic clustering. We suggest the dipole interaction energy and the strain energy associated with indium substitution could have a substantial influence on the local composition of strained InGaN QWs and, particularly, triggers In clustering in the non-polar a-plane QW structure. Accompanying phase field modeling rationalizes that In clustering can also modify the in-plane polarization through piezoelectric effects, preventing the electrostatic potential from diverging along the in-plane polar direction.

The effective piezoelectric properties of [001]c fiber textured Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ceramics were studied by phase-field modeling. The effects of microstructures such as texture, grain shape, grain boundaries, residual pores and heterogeneous growth templates were investigated. It was found that the degree of texture plays a dominant role in determining the properties. The pores, heterogeneous templates and grain boundaries reduce the properties significantly at high degrees of texture with the effect diminishing at decreasing degrees of texture. The presence of heterogeneous templates leads to a more significant reduction in the properties than pores although the piezoelectric coefficients of pores are zero. The shape of grains has a weak effect at all degrees of texture. By utilizing the experimentally measured microstructural parameters in the calculations and comparing the computed properties with the corresponding measurements, we showed that the low performance of sintered textured PMN-PT ceramics ( d 33 ∼1000 pC/N) relative to single crystals ( d 33 ∼2800 pC/N) is mainly due to the insufficiently high degree of texture even with Lotgering factors up to 0.9, while the influences of other microstructures are weak.

We report the sea sponge-inspired design and preparation of piezoelectric composite generators (PCGs) based on a three-dimensional electroceramic skeleton. The remarkable improvements in the piezopotential of the bioinspired structure have been theoretically analyzed using numerical simulations based on a phase-field simulation. The open-circuit voltage, short-circuit current density and instantaneous power density of the bioinspired PCG reach up to ∼25 V, ∼550 nA cm−2 and ∼2.6 μW cm−2, respectively, corresponding to about 16 times higher power than that of conventional particle based piezoelectric polymer composites. Moreover, the bioinspired PCG displays 30 times higher strain–voltage efficiency under stretching than the state-of-the-art performance of the flexible piezoelectric energy harvesters reported so far.

Electrostatic capacitors with high charge/discharge speed and long cycle lifetime play an essential role in advanced electronic and electrical power systems. However, their low energy density (usually less than 1 J cm−3) has limited their development. In this study, core–shell structured Ba0.4Sr0.6TiO3@SiO2 nanoparticles were synthesized by a wet-chemical method, and dense ceramics with enhanced energy storage density were fabricated by spark plasma sintering (SPS). In situ TEM and DSC techniques were used to investigate the interface reaction between Ba0.4Sr0.6TiO3 and SiO2. The results revealed that the secondary phase ((Ba,Sr)2TiSi2O8) was unavoidable, but it could be suppressed due to the low sintering temperature and short sintering period used in the SPS technique. With the increasing SiO2 coating amount, the polarization decreased monotonously, whereas the dielectric breakdown strength increased to a maximum of 400 kV cm−1 and then decreased slightly. The enhancement in the dielectric breakdown strength was ascribed to the formation of nanosized (Ba,Sr)2TiSi2O8 coating on Ba0.4Sr0.6TiO3 grains, whereas the subsequent degradation of performance might have been caused by the sub-micrometric secondary phase precipitated at the grain boundaries. The effect of microstructure on the breakdown strength was further confirmed by numerical simulation using COMSOL. As a result, the Ba0.4Sr0.6TiO3 ceramics with 8 mol% SiO2 showed a maximum energy storage density of 1.60 J cm−3 at 400 kV cm−1 with an ultrahigh energy efficiency of 90.9%. This study opens an effective way for the design of high-performance dielectric ceramics.

Nanofillers in polymer composites will induce interface regions in the polymer with non-uniform nanostructures which can be approximated as multilayer shells surrounding nanofillers. This paper studies topological nanostructures in polymers that interface with one-dimensional (1-D) and zero-dimensional (0-D) nanofillers. This new class of dielectric polymer nanocomposites involves nanofillers at ultra-low volume loading (

Modeling the effective ion conductivities of heterogeneous solid electrolytes typically involves the use of a computer-generated microstructure consisting of randomly or uniformly oriented fillers in a matrix. However, the structural features of the filler/matrix interface, which critically determine the interface ion conductivity and the microstructure morphology, have not been considered during the microstructure generation. Using nanoporous β-Li3PS4 electrolyte as an example, we develop a phase-field model that enables generating nanoporous microstructures of different porosities and connectivity patterns based on the depth and the energy of the surface (pore/electrolyte interface), both of which are predicted through density functional theory (DFT) calculations. Room-temperature effective ion conductivities of the generated microstructures are then calculated numerically, using DFT-estimated surface Li-ion conductivity (3.14 × 10–3 S/cm) and experimentally measured bulk Li-ion conductivity (8.93 × 10–...

This paper systematically compares the numerical implementation and computational cost between the Fourier spectral iterative perturbation method (FSIPM) and the finite element method (FEM) in solving partial differential equilibrium equations with inhomogeneous material coefficients and eigen-fields (e.g., stress-free strain and spontaneous electric polarization) involved in phase-field models. Four benchmark numerical examples, including inhomogeneous elastic, electrostatic, and steady-state heat conduction problems demonstrate that (1) the FSIPM rigorously requires uniform hexahedral (3D) and quadrilateral (2D) mesh and periodic boundary conditions for numerical implementation while the FEM permits arbitrary mesh and boundary conditions; (2) the FSIPM solutions are comparable to their FEM counterparts, and both of them agree with the analytic solutions, (3) the FSIPM is much faster in solving equilibrium equations than the FEM to achieve the accurate solutions, thus exhibiting a greater potential for large-scale 3D computations.

In order to increase the dielectric constants of polymer-based dielectrics, composite approaches, in which inorganic fillers with much higher dielectric constants are added to the polar polymer matrix, have been investigated. However, high dielectric constant fillers cause high local electric fields in the polymer, resulting in a large reduction of the electric breakdown strength. We show that a significant increase in the dielectric constant can be achieved in polyetherimide nanocomposites with nanofillers whose dielectric constant can be similar to that of the matrix. The presence of nanofillers reduces the constraints on the dipole response to the applied electric field, thus enhancing the dielectric constant. Our results demonstrate that through nanostructure engineering, the dielectric constant of nanocomposites can be enhanced markedly without using high dielectric constant nanofillers.

The discovery of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution single crystals is a breakthrough in ferroelectric materials. A key signature of relaxor-ferroelectric solid solutions is the existence of polar nanoregions, a nanoscale inhomogeneity, that coexist with normal ferroelectric domains. Despite two decades of extensive studies, the contribution of polar nanoregions to the underlying piezoelectric properties of relaxor ferroelectrics has yet to be established. Here we quantitatively characterize the contribution of polar nanoregions to the dielectric/piezoelectric responses of relaxor-ferroelectric crystals using a combination of cryogenic experiments and phase-field simulations. The contribution of polar nanoregions to the room-temperature dielectric and piezoelectric properties is in the range of 50–80%. A mesoscale mechanism is proposed to reveal the origin of the high piezoelectricity in relaxor ferroelectrics, where the polar nanoregions aligned in a ferroelectric matrix can facilitate polarization rotation. This mechanism emphasizes the critical role of local structure on the macroscopic properties of ferroelectric materials., Combining a perovskite ferroelectric with moderate piezoelectric properties and a nonpiezoelectric pervoskite relaxor can create a highly piezoelectric material. Here, the authors help explain this unusual result by quantifying how polar nanoregions in the material contribute to its piezoelectric response.

Cycloidal spin orders are common in multiferroics. One of the prototypical examples is BiFeO3 (BFO) which shows a large polarization and a cycloidal antiferromagnetic order at room temperature. Here we employ Landau theory and phase-field simulations to analyze the coupled switching dynamics of polarization and cycloidal antiferromagnetic orders in BFO. We are able to identify 14 types of transitional spin structures between two cycloids and 9 electric-field-induced spin switching paths. We demonstrate the electric-field-induced rotation of wave vectors of the cycloidal spins and discover 2 types of cycloidal spin switching dynamics: fast local spin flips and slow rotation of wave vectors. Also, we construct road maps to achieve the switching between any two spin cycloids through multi-step applications of electric fields. The work provides a theoretical framework for the phenomenological description of spin cycloids and a fundamental understanding of the switching mechanisms to achieve electrical control of magnetic orders., 23 pages, 8 figures, with supplementary materials

FeRh is a unique alloy which shows temperature dependent phase transition magnetic properties. The lattice parameter (a) of this CsCl-type (B2) structure is 4.1712 &Aring, It undergoes a first order transition from antiferromagnetic (AFM) to ferromagnetic (FM) order at around 370K and hysteretic behavior while cooling and heating. This meta-magnetic transition of FeRh is accompanied by an isotropic expansion in the unit cell volume, which indicates strong coupling between magnetic and structural properties of FeRh. Consequently, the magnetic and transport properties, such as magnetoresistance (MR), are changed during the transition. Due to its unique thermo-magnetic behaviors, FeRh is very important for future spintronic devices. The structure could be applicable for MR devices such as memory, sensors, and many other applications. It is critical to understand how to systematically influence phase transition of FeRh from naturally applied strains. Here, we investigate magnetic properties of FeRh in different strain environments induced by the substrates with different lattice parameters. The study was performed using synchrotron radiation, temperature dependent magnetometry, and magnetic scanning probe microscopy in addition to Landau theory calculations. We found that the naturally induced strains could modulate the magnetic phase locally and globally. The presence of the segments from the nucleation of the ferromagnetic domains, with a very thin layer in the antiferromagnetic matrix and the domain growth, were observed gradually. Using the systematic phenomena, it could be used for immediate applications in the future generation of phase change random access memory (PC-RAM) devices.

Understanding the switching mechanism of magnetic skyrmions is critical for realizing their potential applications in future spintronic devices. Here we study the thermodynamic stability and dynamics of a Neel skyrmion in an ultrathin magnetic nanodisk under biaxial in-plane strains using a combination of phase-field simulations and analytical theory. We demonstrated the switching of a circular skyrmion to a variety of magnetic configurations, including an out-of-plane monodomain or an in-plane vortex under isotropic strains and to an elliptical skyrmion or a stripe domain under anisotropic strains. We successfully formulated a Lagrangian-mechanics-based model to analytically describe the switching dynamics of a skyrmion. Both our simulations and analytical model revealed that the strain-mediated breathing dynamics of skyrmions lead to a counter-intuitive phenomenon in which a lager strain may lead to slower skyrmion-to-monodomain switching.

The morphotropic phase boundary (MPB), separating two ferroic phases with rhombohedral and tetragonal crystal symmetries, has been utilized extensively in ferroelectrics because it can lead to high-performance piezoelectricity. Recently, a parallel ferromagnetic MPB was experimentally reported and was suggested that the optimal point for magneto-mechanical applications might lies on the rhombohedral side. However, the insight of the domain structures and switching mechanism near ferromagnetic MPB is still unclear. In this work, phase-field micromagnetic microelastic modeling was employed to simulate the domain formation and magnetization switching of (Tb0.27Dy0.73)Fe2, whose composition is around the rhombohedral side of ferromagnetic MPB. The results show that four kinds of domains of the rhombohedral phase automatically form twins of {110} or {100} boundaries with 71° and 109° domain walls after a process of nucleation and growth. The rhombohedral domain evolution and phase volume fraction under the external field of 120 kA/m along different directions are investigated. In ferromagnetics subject to an alternating magnetic field, domain magnetization switches to cause a magnetization hysteresis loop and an associated butterfly magnetostriction loop with the alternating magnetic field.

Non-volatile electronic devices based on magnetoelectric multiferroics have triggered new possibilities of outperforming conventional devices for applications. However, ferroelectric reliability issues, such as imprint, retention and fatigue, must be solved before the realization of practical devices. In this study, everlasting ferroelectric retention in the heteroepitaxially constrained multiferroic mesocrystal is reported, suggesting a new approach to overcome the failure of ferroelectric retention. Studied by scanning probe microscopy and transmission electron microscopy, and supported via the phase-field simulations, the key to the success of ferroelectric retention is to prevent the crystal from ferroelastic deformation during the relaxation of the spontaneous polarization in a ferroelectric nanocrystal.

The demand for a new generation of high-temperature dielectric materials toward capacitive energy storage has been driven by the rise of high-power applications such as electric vehicles, aircraft, and pulsed power systems where the power electronics are exposed to elevated temperatures. Polymer dielectrics are characterized by being lightweight, and their scalability, mechanical flexibility, high dielectric strength, and great reliability, but they are limited to relatively low operating temperatures. The existing polymer nanocomposite-based dielectrics with a limited energy density at high temperatures also present a major barrier to achieving significant reductions in size and weight of energy devices. Here we report the sandwich structures as an efficient route to high-temperature dielectric polymer nanocomposites that simultaneously possess high dielectric constant and low dielectric loss. In contrast to the conventional single-layer configuration, the rationally designed sandwich-structured polymer nanocomposites are capable of integrating the complementary properties of spatially organized multicomponents in a synergistic fashion to raise dielectric constant, and subsequently greatly improve discharged energy densities while retaining low loss and high charge-discharge efficiency at elevated temperatures. At 150 °C and 200 MV m(-1), an operating condition toward electric vehicle applications, the sandwich-structured polymer nanocomposites outperform the state-of-the-art polymer-based dielectrics in terms of energy density, power density, charge-discharge efficiency, and cyclability. The excellent dielectric and capacitive properties of the polymer nanocomposites may pave a way for widespread applications in modern electronics and power modules where harsh operating conditions are present.

Magnetic domain-wall motion driven by a voltage dissipates much less heat than by a current, but none of the existing reports have achieved speeds exceeding 100 m/s. Here phase-field and finite-element simulations were combined to study the dynamics of strain-mediated voltage-driven magnetic domain-wall motion in curved nanowires. Using a ring-shaped, rough-edged magnetic nanowire on top of a piezoelectric disk, we demonstrate a fast voltage-driven magnetic domain-wall motion with average velocity up to 550 m/s, which is comparable to current-driven wall velocity. An analytical theory is derived to describe the strain dependence of average magnetic domain-wall velocity. Moreover, one 180° domain-wall cycle around the ring dissipates an ultrasmall amount of heat, as small as 0.2 fJ, approximately 3 orders of magnitude smaller than those in current-driven cases. These findings suggest a new route toward developing high-speed, low-power-dissipation domain-wall spintronics.

Both experimental results and theoretical models suggest the decisive role of the filler-matrix interfaces on the dielectric, piezoelectric, pyroelectric, and electrocaloric properties of ferroelectric polymer nanocomposites. However, there remains a lack of direct structural evidence to support the so-called interfacial effect in dielectric nanocomposites. Here, a chemical mapping of the interfacial coupling between the nanofiller and the polymer matrix in ferroelectric polymer nanocomposites by combining atomic force microscopy-infrared spectroscopy (AFM-IR) with first-principles calculations and phase-field simulations is provided. The addition of ceramic fillers into a ferroelectric polymer leads to augmentation of the local conformational disorder in the vicinity of the interface, resulting in the local stabilization of the all-trans conformation (i.e., the polar β phase). The formation of highly polar and inhomogeneous interfacial regions, which is further enhanced with a decrease of the filler size, has been identified experimentally and verified by phase-field simulations and density functional theory (DFT) calculations. This work offers unprecedented structural insights into the configurational disorder-induced interfacial effect and will enable rational design and molecular engineering of the filler-matrix interfaces of electroactive polymer nanocomposites to boost their collective properties.

Approximately 22% of the 86 million people in the United States living with prediabetes are unaware of their condition. Detection of acetone in human respiration offers an effective and painless approach for the diagnosis of prediabetes. In this work, a wearable active acetone biosensor employing chitosan and reduced graphene oxide (RGO) as sensitive materials was developed to non-invasively diagnose prediabetes. When operated under 97.3% relative humidity at room temperature, the prepared chitosan and RGO composite film-based sensor exhibited a good sensing response of 27.89% under 10 ppm acetone in respiratory gases, which is about 5 times higher than the sensing response of pure chitosan film-based devices. In addition, finite element analysis and phase-field simulation were conducted to provide theoretical support for the active sensing mechanism. This work not only presents a wirelessly powered wearable active acetone biosensor, but also paves the way for a new method of non-invasive prediabetes diagnosis.

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.

Magnetic skyrmions are swirling spin structures stabilized typically by the Dyzaloshinskii-Moriya interaction. The existing control of magnetic skyrmions has often relied on the use of an electric current, which may cause overheating in densely packed devices. Here we demonstrate, using phase-field simulations, that an isolated Neel skyrmion in a magnetic nanodisk can be repeatedly created and deleted by voltage-induced strains from a juxtaposed piezoelectric. Such a skyrmion switching is non-volatile, and consumes only ~0.5 fJ per switching which is about five orders of magnitude smaller than that by current-induced spin-transfer-torques. It is found that the strain-mediated skyrmion creation occurs through an intermediate vortex-like spin structure, and that the skyrmion deletion occurs though a homogenous shrinkage during which the Neel wall is temporarily transformed to a vortex-wall. These findings are expected to stimulate experimental research into strain-mediated voltage control of skyrmions, as well as other chiral spin structures for low-power spintronics. Isolated magnetic skyrmions can be created and deleted in a multilayer magnetic nanostructure by voltage-induced strains from the underlying piezoelectric layer. A team led by Long-Qing Chen from the Pennsylvania State University carried out phase-field simulations on an integrated multilayer, where a magnetic ultrathin nanodisk is sandwiched by a capping layer and a heavy-metal layer. They consider a net Dyzaloshinskii-Moriya interaction arising only from the magnet/heavy-metal interface, so that a Neel skyrmion is more favorable than a Bloch skyrmion. They demonstrate that the voltage-induced strain alone can drive non-volatile switching from a ferromagnetic state to a skyrmion and vice versa, without using magnetic fields. These results may stimulate experimental efforts in realizing strain-mediated voltage control of skyrmions, as well as other spin structures for low-power spintronics.

The electrocaloric effect (ECE) refers to reversible thermal changes of a polarizable material upon the application or removal of electric fields. Without a compressor or cooling agents, all-solid-state electrocaloric (EC) refrigeration systems are environmentally benign, highly compact, and of very high energy efficiency. Relaxor ferroelectric ceramics and polymers are promising candidates as EC materials. Here, synergistic efforts are made by composing relaxor Ba(Zr0.21 Ti0.79 )O3 nanofibers with P(VDF-TrFE-CFE) to make relaxor-relaxor-type polymer nanocomposites. The ECEs of the nanocomposites are directly measured and these relaxor nanocomposites exhibit, so far, the highest EC temperature change at a modest electric field, along with high thermal stability within a broad temperature range span to room temperature. The superior EC performance is attributed to the interfacial coupling between dipoles across the filler/polymer interfaces. The thermodynamics and kinetics of interfacial coupling are investigated in situ by piezoresponse force microscopy while the real-time evolution of interfacial coupling is simulated and visualized by phase-field modeling.

Using time- and spatially resolved hard x-ray diffraction microscopy, the striking structural and electrical dynamics upon optical excitation of a single crystal of ${\mathrm{BaTiO}}_{3}$ are simultaneously captured on subnanoseconds and nanoscale within individual ferroelectric domains and across walls. A large emergent photoinduced electric field of up to $20\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }\mathrm{V}/\mathrm{m}$ is discovered in a surface layer of the crystal, which then drives polarization and lattice dynamics that are dramatically distinct in a surface layer versus bulk regions. A dynamical phase-field modeling method is developed that reveals the microscopic origin of these dynamics, leading to gigahertz polarization and elastic waves traveling in the crystal with sonic speeds and spatially varying frequencies. The advances in spatiotemporal imaging and dynamical modeling tools open up opportunities for disentangling ultrafast processes in complex mesoscale structures such as ferroelectric domains.

Deterministic switching of a magnetic vortex with an electric field is challenging because electric fields cannot break time-reversal symmetry. Here we demonstrate, using phase-field simulations, a deterministic switching of the vortex chirality in a triangle-shaped nanomagnet by applying an electric field to its underlying ferroelectric layer. The nanomagnet is juxtaposed with an overlying antiferromagnetic layer to acquire an exchange bias from their interface. The simulations show that such deterministic electrically-driven magnetic vortex chirality switching is enabled by a synergistic effect of the electric-field-induced strain from the ferroelectric, the three-fold in-plane shape anisotropy of the nanomagnet, and the exchange-bias field. An electric-field-controlled deterministic switching of magnetic vortex chirality has been computationally demonstrated in a novel magnetoelectric architecture, offering a viable route to designing low-power vortex-based spintronic devices.

Magnetoelectric nanocomposites have been a topic of intense research due to their profound potential in the applications of electronic devices based on spintronic technology. Nevertheless, in spite of significant progress made in the growth of high-quality nanocomposite thin films, the substrate clamping effect still remains a major hurdle in realizing the ultimate magnetoelectric coupling. To overcome this obstacle, an alternative strategy of fabricating a self-assembled ferroelectric–ferrimagnetic bulk heterojunction on a flexible muscovite via van der Waals epitaxy is adopted. In this study, we investigated the magnetoelectric coupling in a self-assembled BiFeO3 (BFO)–CoFe2O4 (CFO) bulk heterojunction epitaxially grown on a flexible muscovite substrate. The obtained heterojunction is composed of vertically aligned multiferroic BFO nanopillars embedded in a ferrimagnetic CFO matrix. Moreover, due to the weak interaction between the flexible substrate and bulk heterojunction, the interface is incoherent ...

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.

The tuning of functional properties in thick oxide films via nanoscaffolds induced large vertical lattice strain., Strain is a novel approach to manipulating functionalities in correlated complex oxides. However, significant epitaxial strain can only be achieved in ultrathin layers. We show that, under direct lattice matching framework, large and uniform vertical strain up to 2% can be achieved to significantly modify the magnetic anisotropy, magnetism, and magnetotransport properties in heteroepitaxial nanoscaffold films, over a few hundred nanometers in thickness. Comprehensive designing principles of large vertical strain have been proposed. Phase-field simulations not only reveal the strain distribution but also suggest that the ultimate strain is related to the vertical interfacial area and interfacial dislocation density. By changing the nanoscaffold density and dimension, the strain and the magnetic properties can be tuned. The established correlation among the vertical interface—strain—properties in nanoscaffold films can consequently be used to tune other functionalities in a broad range of complex oxide films far beyond critical thickness.

Purely voltage-driven, repeatable magnetization reversal provides a tantalizing potential for the development of spintronic devices with a minimum amount of power consumption. Substantial progress has been made in this subject especially on magnetic/ferroelectric heterostructures. Here, we report the in situ observation of such phenomenon in a NiFe thin film grown directly on a rhombohedral Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT) ferroelectric crystal. Under a cyclic voltage applied perpendicular to the PMN-PT without a magnetic field, the local magnetization of NiFe can be repetitively reversed through an out-of-plane excursion and then back into the plane. Using phase field simulations we interpret magnetization reversal as a synergistic effect of the metastable ferroelastic switching in the PMN-PT and an electrically rotatable local exchange bias field arising from the heterogeneously distributed NiO clusters at the interface.

A thermodynamic potential for Ba1-xSrxTiO3 solid solutions is developed, and the corresponding thermodynamic properties of Ba1-xSrxTiO3 single crystals are calculated. The predicted temperature-composition phase diagram from the thermodynamic potential agrees well with the experimental measurements. Based on this potential, the energy storage performances and electrocaloric effects of Ba1-xSrxTiO3 single crystals are obtained using the phase-field method. It is found that there is an optimal Sr concentration which maximizes the discharged energy density of a Ba1-xSrxTiO3 single crystal under an applied electric field. The electrocaloric effects of Ba0.8Sr0.2TiO3, Ba0.7Sr0.3TiO3, Ba0.6Sr0.4TiO3, and Ba0.5Sr0.5TiO3 single crystals are also predicted, from which the corresponding optimal temperatures are identified.

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.