Search

Electric field-induced antiferroelectric-ferroelectric phase transition and the associated nonlinear dielectric behavior in particulate composites are investigated for achieving a high dielectric capacity. A phenomenological thermodynamic model based on the Landau theory is first developed to discuss the generic phenomena of a temperature-electric field phase diagram, coexistence of antiferroelectric and ferroelectric phases, field-induced antiferroelectric-ferroelectric phase transition, and nonlinear dielectric behavior. The model is then used to carry out the phase field simulation of particulate nonlinear dielectric composites. It is found that the composites exhibit nonlinear dielectric behaviors, and the depolarization field in the composites helps reduce the dielectric hysteresis and enhance the reversibility of antiferroelectric-ferroelectric phase transitions, which are desired for energy storage applications. The simulations also reveal the underlying domain-level mechanisms for nucleation and growth processes of the phase transitions. It is shown that the macroscopic properties of the composites sensitively depend on the directional alignment of the antiferroelectric filler particles, and thus the filler morphology is an effective control variable in designing nonlinear dielectric composites. [ABSTRACT FROM AUTHOR]

Piezoelectric materials enable the conversion of mechanical energy into electrical energy and vice-versa. Ultrahigh piezoelectricity has been only observed in single crystals. Realization of piezoelectric ceramics with longitudinal piezoelectric constant (d33) close to 2000 pC N-1, which combines single crystal-like high properties and ceramic-like cost effectiveness, large-scale manufacturing, and machinability will be a milestone in advancement of piezoelectric ceramic materials. Here, guided by phenomenological models and phase-field simulations that provide conditions for flattening the energy landscape of polarization, a synergistic design strategy is demonstrated that exploits compositionally driven local structural heterogeneity and microstructural grain orientation/texturing to provide record piezoelectricity in ceramics. This strategy is demonstrated on [001]PC-textured and Eu3+-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ceramics that exhibit the highest piezoelectric coefficient (small-signal d33 of up to 1950 pC N-1 and large-signal d33* of ≈2100 pm V-1) among all the reported piezoelectric ceramics. Extensive characterization conducted using high-resolution microscopy and diffraction techniques in conjunction with the computational models reveals the underlying mechanisms governing the piezoelectric performance. Further, the impact of losses on the electromechanical coupling is identified, which plays major role in suppressing the percentage of piezoelectricity enhancement, and the fundamental understanding of loss in this study sheds light on further enhancement of piezoelectricity. These results on cost-effective and record performance piezoelectric ceramics will launch a new generation of piezoelectric applications. [ABSTRACT FROM AUTHOR]

Magnetoelectric voltage tunable inductors (VTIs) offer a new paradigm for power electronics circuit design. Here, air-gapped VTIs are demonstrated exhibiting a large inductance tunability of 220% with applied fields under 20 kVcm−1 and operational stability up to 5 MHz that covers the full frequency range of the state-of-the-art power electronics. The design of air-gapped VTI comprises of two C-shaped ferrite cores, one permeability-variable magnetic flux valve (MFV) in between C-shaped cores, and slotted air gap between cores and MFV. The tunability of VTIs is achieved through the electric field modulation of permeability in the MFV based on the magnetoelectric effect. There is a tradeoff between the tunability and saturation current. The introduction of air gap significantly reduces the tunability but provides methodology toward increasing the saturation current and power handling capability due to the increased reluctance. The inductance and tunability reduces by 50% with increase in the air-gap width from 0.07 mm to infinity (condition with no C-shaped cores). Phase field simulations demonstrate that the air gap affects the magnetization, permeability, and tunability of VTIs by tailoring the demagnetization field that further influences the magnetic domain rotation process. VTIs with increased frequency range and saturation current will strengthen the continued development of tunable power electronics. [ABSTRACT FROM AUTHOR]

Interfaces often exhibit unique electronic and magnetic properties that are not present in their bulk constituents. Understanding the atomic-level structure and properties of the interface is crucial for their technological applications. In this article, we report a first-principles study of the γ-FeSi2/Si(001) interface to unravel the atomic-level structure property relationship. An external electric field is included in our model to tune the properties of the interface. Based on our calculations, we found a modest application of an electric field (> 0.15 eV/Å) could stabilize a sixfold and sevenfold coordinated, spin-active interface over a nonmagnetic (eightfold coordinated) interface-providing direct evidence of electric field control of magnetism at the interface. The sixfold as well as sevenfold coordinated structures are shown to favor antiferromagnetic spin ordering arising from the Fe(d)-Si(p)-Fe(d) super-exchange interaction. The distinct non-linear response of the interface structure to the applied electric field can be attributed to the different electronic and magnetic structures at the interface; the sixfold exhibits the highest polarizability over the other coordinated structures. [ABSTRACT FROM AUTHOR]

360° domain walls in planar magnetic nanowires are studied by micromagnetic simulations. It is shown that a nanowire loop with a shape-isotropic wall generator at one end and a shape-anisotropic wall stopper at the other end functions like a data storage stack: 360° domain walls are generated and pushed into stack under rotating field before overflow while popped out and annihilated when field rotating direction is inverted until underflow. The stack capacity is determined by total nanowire loop length. This simple nanowire structure can be integrated into magnetic circuits as an operation unit for 360° domain wall generation and storage. [ABSTRACT FROM AUTHOR]

The physical properties (dielectric, ferroelectric, piezoelectric, etc.) of complex perovskite ferroelectrics depend on the degree of order/disorder and the scale of the ordered domains. In this study, the electrocaloric (EC) properties of three representative complex perovskite ferroelectrics, Pb(Mg 1/3 Nb 2/3)O 3 –8PbTiO 3 (PMN-8PT), 1mol% Sm-doped Pb(Mg 1/3 Nb 2/3)O 3 –8PbTiO 3 (1S-PMN-8PT) and Pb(Sc 1/2 Ta 1/2)O 3 (PST) are evaluated. Multi-layer ceramic capacitors (MLCCs) with identical structural configurations were fabricated for these three compounds, and their EC properties were characterized by direct measurement using a thermocouple. The EC temperature change Δ T of PMN-8PT, 1S-PMN-8PT and PST MLCCs under 20 V μm−1 at room temperature were found to be 1.42 K, 1.54 K, and 3.10 K, respectively. X-ray diffraction and high-resolution transmission electron microscopy data suggests that the high EC performance of PST is related to the ordering of B-site cations (Sc3+ and Ta5+) with the ordering parameter S = 0.82 and a long coherence length of ∼100 nm, such that the sample transitioned from a relaxor ferroelectric to a normal ferroelectric. These results provide pathway towards design of high performance EC materials required for solid state refrigeration and air-conditioning technologies. Multi-layer ceramic capacitors for three representative complex perovskite ferroelectrics, Pb(Mg 1/3 Nb 2/3)O 3 –8PbTiO 3 (PMN-8PT), 1mol% Sm-doped Pb(Mg 1/3 Nb 2/3)O 3 –8PbTiO 3 (1S-PMN-8PT) and Pb(Sc 1/2 Ta 1/2)O 3 (PST), were fabricated. Their EC temperature change Δ T under 20 V μm−1 at room temperature were 1.42 K, 1.54 K, and 3.10 K, respectively. The high EC performance of PST is related to the long-range ordering of B-site cations (Sc3+ and Ta5+). [Display omitted] [ABSTRACT FROM AUTHOR]

The increasing demands of device applications in harsh environments have led to higher expectations for temperature and pressure resistance in high-temperature piezoelectric materials. In order to understand the performance characteristics of BiScO3-PbTiO3 (BS-PT) high-temperature piezoelectric materials in practical device applications, this study focuses on analyzing the dielectric, piezoelectric, and ferroelectric properties of BS-64PT ceramics under uniaxial stress up to 150 MPa, as well as its electromechanical performance under a hydrostatic pressure of up to 400 MPa. As the uniaxial pressure increases, both the bias-field and large-signal piezoelectric coefficients exhibit a pattern of initially increasing and subsequently decreasing. Furthermore, the bias-field piezoelectric coefficient exceeds 450 pC/N and the large-signal piezoelectric coefficient surpasses 630 pC/N under uniaxial pressures below 100 MPa. This highlights their exceptional resistance to depolarization caused by uniaxial stress. To obtain more precise piezoelectric properties for BS-64PT ceramics in the 31 and 33 modes under hydrostatic pressure, the admittance fitting method was utilized. This method takes into account the significant losses at higher pressures. Within the pressure range of 0–400 MPa, the values of d33 and d31 for BS-64PT ceramics exhibited a minor change of 7.6% and 8%, respectively. These findings indicate that BS-64PT ceramics exhibit more stable piezoelectric properties under both uniaxial and hydrostatic pressures when compared to the majority of Pb-based perovskite-structured materials. The exceptional stability of piezoelectric properties in BS-PT ceramics can be primarily attributed to their elevated anisotropy energy or coercivity field compared to other perovskite-structured Pb(Mg1/3Nb2/3)O3 (PMN) / Pb(Zr,Ti)O3 (PZT)-based ceramics reported. [ABSTRACT FROM AUTHOR]

[001]-textured 0.4P(Mg1/3Nb2/3)O3-0.25PbZrO3-0.35PbTiO3-0.5%MnO2 (Mn-PMN-PZT) ceramics were fabricated by templated grain growth using 2 vol. % BaTiO3 in this paper. Full matrices of dielectric (ɛij), elastic (sij, cij), and piezoelectric (dij) parameters were obtained by the resonance–antiresonance method. The dielectric constant ɛ33T of textured ceramics reaches 2600, which is four times that of random ceramics. Textured Mn-PMN-PZT ceramics exhibit high d33 = 984 pC/N and high k33 = 0.89, which is much larger than d33 = 223 pC/N and k33 = 0.70 of random ceramics. However, ɛ11T of ceramics decreases by about 30% after texturing, and the corresponding shear coupling coefficient k15 also decreases from 0.66 to 0.44, which may be due to the reduction in the angle between spontaneous polarization and transverse direction. Furthermore, the temperature stability of the textured ceramics was evaluated as well. The phase transition temperature TR−T was determined by the impedance method to be 120 °C. The textured Mn-PMN-PZT ceramic shows high temperature stability, which is better than PMN-PT. [ABSTRACT FROM AUTHOR]

Abstract Control of magnetic permeability through voltage in magnetoelectric materials promises to create novel voltage tunable inductors. Domain-level phase field modeling and computer simulation are employed to study magnetoelectric composites of Co-modified NiCuZn ferrite and PZT, where ferrite outperforms metallic magnetostrictive materials at high frequencies important for inductor applications. The simulations focus on the interplay between magnetocrystalline anisotropy and stress-induced anisotropy and reveal different regimes of permeability and tunability behaviors. It is shown that the permeability and its tunability can be significantly increased by reducing the intrinsic magnetocrystalline anisotropy through doping NiCuZn-ferrite with Co2+ to form solid solution (1- x)(Ni 0.6 Cu 0.2 Zn 0.2)Fe 2 O 4 - x CoFe 2 O 4 , and the optimal performance is achieved at vanishing magnetocrystalline anisotropy constant K 1 = 0 corresponding to composition x ∼0.02. The findings are confirmed by complementary experiments with good agreements. The effect of internal bias stress is further investigated, which is shown to shift the permeability regimes and thus provides an effective engineering technique to optimize the voltage tunable inductor performance. A tunability as high as ∼500% is obtained under the optimal condition, which is much higher than that obtained in conventional high-frequency voltage tunable inductors. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

180° transverse domain walls (TDWs) in planar magnetic nanowires find important applications in advanced logic and memory devices. Controlling DW structure is important because it determines DW behaviors and is used to store digital information. This paper presents, using micromagnetic simulations, simple designs of nanowire unit that enables reliable generation of TDWs and effective control of their structures under rotating magnetic field. It is shown that a series of TDWs can be created, two per loading cycle, and external magnetic field can be programmed to control the TDW structures in terms of their polarity and chirality. These nanowire units and the corresponding magnetic field control schemes can be integrated into magnetic circuits to generate TDWs of controlled structures, facilitating the development of logic and memory devices based on DW functionalities. [ABSTRACT FROM AUTHOR]

Control of magnetic permeability through voltage promises to create novel electronic devices, such as voltage tunable inductors. The relationship between the structure and property of voltage tunable inductors comprising of magnetoelectric Metglas/PZT composites and the underlying domain-level mechanisms are investigated using theoretical analysis, computer simulation, and complementary experiments. A theoretical model is developed to analyze the roles of material anisotropy, inductor shape, and stress in controlling the Metglas permeability and its tunability. The analysis reveals key roles played by stress-induced anisotropy and the resultant ground magnetization state, and predicts two stress-dependent regimes of inductance tunability. The theory is validated using systematic experiments. The experimental results are used to determine the material and physical parameters. To further elucidate the underlying domain-level mechanisms responsible for controlling the behavior of voltage tunable inductor, phase field modeling is employed to simulate domain microstructures and magnetic permeability of Metglas/PZT composites under varying voltage. The computational results confirm the two regimes of inductance tunability and the controlling role of stress-induced anisotropy. The findings suggest engineering of internal bias stress as an effective means to optimize the inductance tunability of magnetoelectric Metglas/PZT composites. [ABSTRACT FROM AUTHOR]

We report a new type of racetrack memory based on current-controlled movement of magnetic vortices in magnetic nanowires with rectangular cross-section and weak perpendicular anisotropy. Data are stored through the core polarity of vortices and each vortex carries a data bit. Besides high density, non-volatility, fast data access, and low power as offered by domain wall racetrack memory, magnetic vortex racetrack memory has additional advantages of no need for constrictions to define data bits, changeable information density, adjustable current magnitude for data propagation, and versatile means of ultrafast vortex core switching. By using micromagnetic simulations, current-controlled motion of magnetic vortices in cobalt nanowire is demonstrated for racetrack memory applications. [ABSTRACT FROM AUTHOR]

An important role of domain wall pile-up in current-driven domain wall depinning in magnetic nanowires is revealed using micromagnetic simulations. It is found that the critical current for domain wall depinning can be substantially reduced and conveniently tuned by controlling domain wall number in the pile-up at pinning site, in analogy to dislocation pile-up responsible for Hall–Petch effect in mechanical strength. Domain wall pinning and depinning at an s-shape bend is considered, and the effects of curvature and current crowding in magnetic circuit on domain wall behaviors are discussed. [ABSTRACT FROM AUTHOR]

To complement recent experimental findings [Vetter et al., APL Mater. 4, 096103 (2016)] on strong exchange coupling effects in Co-Pt nanochessboards featuring a single peak in the first-order reversal curve density plot and associated magnetic hardening as the characteristic structural dimension is reduced below the critical correlation length, the effects of the strong exchange coupling on the underlying magnetic domains and their evolutions are investigated using micromagnetic simulations. The nanochessboard consists of regularly distributed aligned nanorods of a magnetically hard tetragonal L10 phase and a magnetically soft cubic L12 phase, representing a unique type of exchange-coupled magnetic nanocomposites. The simulations reveal detailed magnetic domain processes specific to the nanochessboard structure and uncover an unusual dependence of the coercivity on the length scale, where an expected magnetic hardening is followed by an unexpected magnetic softening with decreasing characteristic dimension. The length scale-dependent exchange coupling effects, effective magnetic anisotropy, and magnetic domain structures and evolution pathways are analyzed to provide insights into the peculiar magnetic behaviors of Co-Pt nanochessboards. [ABSTRACT FROM AUTHOR]

Magnetism has recently been observed in nominally nonmagnetic iron disilicide in the form of epitaxial γ-FeSi2 nanostructures on Si(111) substrate. To explore the origin of the magnetism in γ-FeSi2/Si(111) nanostructures, we performed a systematic first-principles study based on density functional theory. Several possible factors, such as epitaxial strain, free surface, interface, and edge, were examined. The calculations show that among these factors, only the edge can lead to the magnetism in γ-FeSi2/Si(111) nanostructures. It is shown that magnetism exhibits a strong dependency on the local atomic structure of the edge. Furthermore, magnetism can be enhanced by creating multiple-step edges. In addition, the results also reveal that edge orientation can have a significant effect on magnetism. These findings, thus, provide insights into a strategy to tune the magnetic properties of γ-FeSi2/Si(111) nanostructures through controlling the structure, population, and orientation of the edges. [ABSTRACT FROM AUTHOR]

Dielectric, ferroelectric, and piezoelectric properties of both random and textured Mn--PMN--PZT ceramics were characterized under uniaxial stress. Textured ceramics exhibit a large piezoelectric response under uniaxial pressure; the bias field piezoelectric constant is higher than 700 pC/N when pressure is below 50 MPa. Moreover, textured ceramics also show better depolarization resistance under uniaxial stress fields; overall, 30% of the origin performance will remain when stress approaches 200 MPa, but for random ceramics, it is only 10%. The ferroelectricity of both random and textured ceramics is suppressed by uniaxial compression. Ec, Pr, Ei, and dissipation energy all decrease with increasing uniaxial stress. In addition, phase-field simulation was used to better understand the polarization-changing effects on piezoelectric and dielectric performance. The uniaxial stress causes polarization rotation and increases the angle between polarization and the electric field, which is an important factor leading to the increase of the dielectric constant. [ABSTRACT FROM AUTHOR]

Design of a projecting‐receiving dual‐purpose transducer is challenging due to the difficulty of synthesizing piezoelectric materials with combinatory "soft" properties (high piezoelectric coefficient d) and "hard" properties (low dielectric loss and high mechanical quality factor Qm). In this study, we provide a different perspective to address this challenge via exploiting single‐crystal‐like electrostriction coefficient Q33 through the fabrication of grain oriented or textured "hard" piezoelectric ceramics. Mn‐doped 0.27Pb(In1/2Nb1/2)O3–0.41Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 (Mn:PIN–PMN–PT) piezoelectric ceramics with a high [0 0 1]c texture fraction of 99% were synthesized, which exhibit three times greater piezoelectric properties (d33 ∼ 828 pC/N) than random counterparts (d33 ∼ 251 pC/N), while maintaining low loss (tan δ ∼ 0.5%, Qm = 443). According to the formula d33=2Q33Prε33${d}_{{\mathrm{33}}}{\mathrm{\ = \ 2}}{Q}_{{\mathrm{33}}}\ {P}_{\mathrm{r}}\ {\varepsilon }_{{\mathrm{33}}}$, the large improvement of d33 in textured ceramics is mainly due to a doubling increase in dielectric constant ε33 and Q33 (∼0.057 m4/C2). Notably, the Q33 exhibits remarkable similarity to that of PMN–PT single crystals, further contributing to the enhanced piezoelectric performance of textured ceramics. Phase field model of ferroelectrics was performed to understand the texturing on Q33 and elucidate the underlying mechanism at the domain level. The textured ceramics exhibit excellent combinatory "soft" and "hard" properties, which are the promising materials for developing projecting‐receiving dual‐purpose transducers with high efficiency and high sensitivity. [ABSTRACT FROM AUTHOR]

In FePt polytwin crystals with large magnetocrystalline anisotropy, the boundaries may play a crucial role in the magnetization processes occurring under an external magnetic field. In this study, we employed phase-field modeling and computer simulations to systematically investigate the effect of three types of polytwin boundaries—namely, symmetric (Type I), asymmetric (Type II), and mixed (Type III) boundaries—on magnetization processes as well as coercive fields under an external magnetic field along various directions. Because of the large anisotropy of FePt, the domain wall motion mechanism is usually dominant in the domain switching processes, while the magnetization rotation mechanism only becomes important at the late magnetization stage under a high external magnetic field. Among these three types of polytwin boundaries, the low coercivity is mainly due to the domain wall motion process, which starts from the intersection point at the polytwin boundary. The coercive field for the mixed polytwin boundary (Type III) is always in between the values of Type I and II. [ABSTRACT FROM AUTHOR]

The Co-Pt binary system can form a two-phase nanochessboard structure comprising regularly aligned nanorods of magnetically hard tetragonal L10 phase and magnetically soft cubic L12 phase. This Co-Pt nanochessboard, being an exchange-coupled magnetic nanocomposite, exhibits a strong effect on magnetic domains and coercivity. While the ideal nanochessboard structure has tiles with equal edge lengths (a = b), the non-ideal or nonstandard nanochessboard structure has tiles with unequal edge lengths (a ≠ b). In this study, we employed phase-field modeling and computer simulation to systematically investigate the exchange coupling effect on magnetic properties in nonstandard nanochessboards. The simulations reveal that coercivity is dependent on the length scale, with magnetic hardening occurring below the critical exchange length, followed by magnetic softening above the critical exchange length, similar to the standard nanochessboards. Moreover, the presence of unequal edge lengths induces an anisotropic exchange coupling and shifts the coercivity peak with the length scale. [ABSTRACT FROM AUTHOR]

Surface science techniques (STM, STS, and XPS) were combined with ab initio simulations to detect the local crystal structure and chemistry. Solid phase epitaxy of iron on vicinal Si(111) substrate resulted in the formation of 3 × 3 R30° nanoislands and (2×2) films of γ -FeSi 2 (111). We identify these structures by comparing experimental normalized derivative conductance curves with tomographic simulated local density of states (LDOS). The thermodynamic tendency of γ -FeSi 2 (111) towards Si rich surfaces is manifested in Si rich termination layers and reconstructions. We show that a weighted average of the LDOS from the Fe layer and the reconstruction layer can explain the main states in the normalized derivative conductance curves, enabling in-situ identification of crystal structure and composition of epitaxial deposits. • γ -FeSi 2 nanostructures were grown on Si(111) with (2 × 2) and 3 × 3 R30° surfaces. • STS acquired DOS were interpreted by DFT simulations, to identify the grown structures. • The topmost (reconstruction) and third (iron) layers impact the acquired DOS the most. • Surface effects dominate the DOS in γ -FeSi 2 /Si(111) films with 30 layers or less. [ABSTRACT FROM AUTHOR]