Xingqiao Ma

Current-induced magnetization dynamics in Co/Cu/Co nanopillars

We studied current-induced magnetization dynamics in Co∕Cu∕Co nanopillars using the Landau-Lifshitz-Gilbert equation incorporating the spin transfer torque effect. We show that the magnetization dynamics can be grouped into four types according to its characteristics and the current density value under zero external field. It is found that an external field can significantly affect the magnetization dynamics, either favoring or impeding the magnetization switching depending on its direction.

Elastoplastic phase field model for microstructure evolution

Success has been obtained in predicting the dynamic evolution of microstructures during phase transformation or cracking propagation by using the time-dependent phase field methodology (PFM). However, most efforts of PFM were made in the elastic regime. In this letter, stress distributions around defects such as a hole and a crack in an externally loaded two-dimensional representative volume element were investigated by a proposed phase field model that took both the elastic and plastic deformations into consideration. Good agreement was found for static cases compared to the use of finite element analysis. Therefore, the proposed phase field model provides an opportunity to study the dynamic evolution of microstructures under plastic deformation.

Micromagnetic simulations of current-induced magnetization switching in Co∕Cu∕Co nanopillars

We studied the current-induced magnetic switching in Co∕Cu∕Co nanopillars with an in-plane magnetization traversed by a perpendicular-to-plane spin-polarized current. The Landau-Lifshitz-Gilbert equation incorporating the spin transfer torque (STT) effect was employed. Magnetization switching was found to take place when the current density exceeds a threshold. It is accompanied by drastic oscillations near the magnetic reversal point. The switching time depends on the applied current density. The magnetization can also be switched by a sufficiently long square pulsed current. The roles of anisotropy, exchange, and demagnetization energies in the magnetization switching process of nanopillars are discussed. It is shown that the switching is mainly determined by the competition between STT and the Gilbert damping torque.

Phase-field simulation of hydride precipitation in bi-crystalline zirconium

The morphological evolution of c-hydride precipitation and growth in a zirconium bi-crystal is simulated using a phase field kinetic model. The result shows the heterogeneous nucleation at the grain boundary. The likelihood of hydride growth across the grain boundary is discussed. � 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Dipole spring ferroelectrics in superlattice SrTiO3/BaTiO3 thin films exhibiting constricted hysteresis loops

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.

Magnetodielectric effect of Bi6Fe2Ti3O18 film under an ultra-low magnetic field

Good-quality and fine-grain Bi6Fe2Ti3O18 magnetic ferroelectric films with single-phase layered perovskite structure have been prepared successfully via the metal organic decomposition (MOD) method. The results of low-temperature magnetocapacitance measurements reveal that an ultra-low magnetic field of 10 Oe can produce a non-trivial magnetodielectric response in zero-field cooling conditions, and the relative variation of dielectric constants in a magnetic field is positive, i.e. [er(H)− er(0)]/er(0) = 0.05, when T<55 K, but negative with a maximum of [er(H)− er(0)]/er(0) = −0.14 when 55 K<T<190 K. The magnetodielectric effect shows a sign change at 55 K, which is due to a transition from an antiferromagnetic to a weak ferromagnetic, and vanishes abruptly at around 190 K, which is thought to be associated with an order–disorder transition of iron ions at the B site of perovskite structures. Our results allow the expectation of low-cost applications of detectors and switches for extremely weak magnetic fields over a wide temperature range of 55–190 K.

Influence of interfacial coherency on ferroelectric switching of superlattice BaTiO3/SrTiO3

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.

Effects of unequally biaxial misfit strains on polarization phase diagrams in embedded ferroelectric thin layers: Phase field simulations

Phase field simulations were conducted to investigate the effects of unequally biaxial misfit strains on domain stability diagrams and equilibrium domain structures in an epitaxial ferroelectric PbTiO3 thin layer, which is sandwiched in a nonferroelectric medium. The simulations reveal a multidomain structure in the layer and allow constructing “misfit strain-misfit strain” and “temperature-misfit strain” phase diagrams. It is found that unequally biaxial misfit strains may lead to the presence of a single tetragonal variant, either a-domains or b-domains, which do not exist if the misfit strains are equally biaxial.

Magnetically actuated functional gradient nanocomposites for strong and ultra-durable biomimetic interfaces/surfaces

Magnetically-actuated functional gradient nanocomposites can be locally modulated to generate unprecedented mechanical gradients applied to various interfaces and surfaces following the design principles of natural biological materials. Several thus-far-inaccessible biomimics including a strong and ultra-durable interface mimicking the tooth dentin–enamel junction, a wear-resistant and long-lasting surface coating mimicking biological skins, and flexible yet structurally-stable micropillars mimicking the adhesive setae of insects are demonstrated.

Role of d-d and p-d hybridization in CoTi-based magnetic semiconductors with 21 and 26 valence electrons

We have found that CoTiFeP, CoTiFeAs and CoTiFeSb with 26 valence electrons are magnetic semiconductors by first-principles calculations. The electronic structure, magnetic propeties and origin of the band gap are investigated and compared with the magnetic semiconductor CoTiVAl with 21 valence electrons. It has been found that the magnetic moment in CoTiVAl mainly originates from the large exchange splitting effect of the V atom, which carries the largest magnetic moment of 2.14 μ B, parallel to those of Co and Ti, while in CoTiFeZ (Z = P, As, Sb) compounds, the moments mainly originate from Co and Fe atoms. The atomic moments of Co and Fe are in an antiparallel arrangement with the moments of their nearest neighbors Ti atoms at the B site due to the strong hybridization between Co-3d, Fe-3d and Ti-3d electrons. Based on the classical molecular orbital hybridization theory, different origins of the gap in 21 and 26 valence electrons are analyzed. It is confirmed that p-d hybridization is significant for opening up the band gap, for adjusting the position of the Fermi level and the width of the band gap in our magnetic semiconductors. This will provide practical guidance for searching for new magnetic semiconducting materials.