Domain Wall Migration Research Articles

Micromagnetics and multiscale hysteresis simulations of permanent magnets

PurposeConfronting the unveiled sophisticated structural and physical characteristics of permanent magnets, notably the samarium–cobalt (Sm-Co) alloy, This work aims to introduce a simulation scheme that can link physics-based micromagnetics on the nanostructures and magnetostatic homogenization on the mesoscale polycrystalline structures.Design/methodology/approachThe simulation scheme is arranged in a multiscale fashion. The magnetization behaviors on the nanostructures examined with various orientations are surrogated as the micromagnetic-informed hysterons. The hysteresis behavior of the mesoscale polycrystalline structures with micromagnetic-informed hysterons is then evaluated by computational magnetostatic homogenization.FindingsThe micromagnetic-informed hysterons can emulate the magnetization reversal of the parameterized Sm-Co nanostructures as the local hysteresis behavior on the mesostructures. The simulation results of the mesoscale polycrystal demonstrate that the demagnetization process starts from the grain with the largest orientation angle (a) and then propagates to the surrounding grains.Research limitations/implicationsThe presented scheme depicts the demand for integrating data-driven methods, as the parameters of the surrogate hysteron intrinsically depend on the nanostructure and its orientation. Further hysteron parameters that help the surrogate hysteron emulate the micromagnetic-simulated magnetization reversal should be examined.Originality/valueThis work provides a novel multiscale scheme for simulating the polycrystalline permanent magnets’ hysteresis while recapitulating the nanoscale mechanisms, such as the nucleation of domains, and domain wall migration and pinning. This scheme can be further extended to simulate the part-level hysteresis considering the mesoscale features.

Magnetic Interacted Interaction Effect in MXene Skeleton: Enhanced Thermal-Generation for Electromagnetic Interference Shielding.

With the rapid advancements of portable and wearable equipment, high-efficiency electromagnetic interference (EMI) shielding materials are highly entailed to eliminate radiated electromagnetic pollution. Herein, by assembling hexagonal SrFe12 O19 flakes into a Ti3 C2 Tx MXene/MWCNT substrate, a magnetized Ti3 C2 Tx -based film is successfully fabricated by a facile filtration approach. Carbon nanotubes are used as isolation agents to realize the submicroscopic dispersion of MXene and SrFe12 O19 . The obtained MXene/MWCNTs/SrFe12 O19 film shows a high electrical conductivity of 438 S cm-1 and an excellent EMI shielding effectiveness of 62.9dB in X-band at a thickness of only 40µm. Benefiting from a strong magnetic response ability and an expanded magnetic coupling space, hexagonal SrFe12 O19 sheets can efficiently consume incident magnetic field energy by domain wall migration and the ferromagnetic resonance effect. Boosted EMI shielding performance can be achieved by improving the magnetic loss in the Ti3 C2 Tx MXene/MWCNTs/SrFe12 O19 film, preventing the secondary reflection of electromagnetic waves. Meanwhile, magnetized MXene-based films display the freestanding and flexible features and are suitable for installation in electric devices. It is anticipated that this strategy offers new ideas for designing EMI shielding films and in broadening potential utility of MXene-based materials.

Grain boundary oxide layers in NdFeB-based permanent magnets

The microstructure of grain boundaries (GBs) in the commercial NdFeB-based alloy for permanent magnets has been studied. It is generally accepted that the unique hard magnetic properties of such alloys are controlled by the thin layers of a Nd-rich phase in Nd2Fe14B/Nd2Fe14B GBs. These GB layers ensure the magnetic isolation of Nd2Fe14B grains from each other. It is usually supposed that such GB layers contain metallic Nd or Nd-rich intermetallic compounds. However, the commercial NdFeB-based permanent magnets frequently contain a tangible amount of neodymium oxide Nd2O3 at the triple junctions between Nd2Fe14B grains. The goal of this work was to check whether the Nd2Fe14B/Nd2Fe14B GBs could also contain the thin layers of Nd2O3 oxide phase. Indeed, the screening with EELS-based elemental analysis permitted to observe that some of these Nd-rich layers in Nd2Fe14B/Nd2Fe14B GBs contain not only neodymium, but also oxygen. More detailed analysis of such GBs with high-resolution transmission electron microscopy (HR TEM) showed these GB layers are crystalline and have the lattice of neodymium oxide Nd2O3. In turn, the Lorentz micro-magnetic contrast in TEM permitted to observe that the Nd-oxide GB layers prevent the migration of domain walls from one Nd2Fe14B grain to another during remagnetization. This finding proves that the GB oxide layers, similar to those of metallic Nd or Nd-rich intermetallic compounds, can ensure the magnetic isolation between Nd2Fe14B grains needed for high coercivity. Therefore, the GB oxide layers can be used for further development of NdFeB-based permanent magnets.

Bioelectrical domain walls in homogeneous tissues.

Electrical signaling in biology is typically associated with action potentials, transient spikes in membrane voltage that return to baseline. Hodgkin-Huxley and related conductance-based models of electrophysiology belong to a more general class of reaction-diffusion equations which could, in principle, support spontaneous emergence of patterns of membrane voltage which are stable in time but structured in space. Here we show theoretically and experimentally that homogeneous or nearly homogeneous tissues can undergo spontaneous spatial symmetry breaking through a purely electrophysiological mechanism, leading to formation of domains with different resting potentials separated by stable bioelectrical domain walls. Transitions from one resting potential to another can occur through long-range migration of these domain walls. We map bioelectrical domain wall motion using all-optical electrophysiology in an engineered cell line and in human induced pluripotent stem cell (iPSC)-derived myoblasts. Bioelectrical domain wall migration may occur during embryonic development and during physiological signaling processes in polarized tissues. These results demonstrate that nominally homogeneous tissues can undergo spontaneous bioelectrical symmetry breaking.

Sr-induced dipole scatter in BaxSr1−xTiO3 : Insights from a transferable-bond valence-based interatomic potential

In order to design next-generation ferroelectrics, a microscopic understanding of their macroscopic properties is critical. One means to achieving an atomistic description of ferroelectric and dielectric phenomena is classical molecular dynamics simulations. Previously, we have shown that interatomic potentials based on the bond valence molecular dynamics (BVMD) method can be used to study structural phase transitions, ferroelectric domain nucleation, and domain wall migration in several perovskite oxides and fixed-composition binary and ternary alloys. Most modern devices, however, use variable-composition perovskite oxide alloys such as Ba$_x$Sr$_{1-x}$TiO$_3$ (BST). In this paper, we extend our bond valence approach to BST solid solutions and, in so doing, show that the potential parameters for each element are transferable between materials with different $x$. Using this potential, we perform BVMD simulations investigating the temperature and composition dependence of the lattice constants, Ti displacements, and ferroelectric polarization of BST and find that our predictions match experiments and first-principles theory. Additionally, based on a detailed analysis of local dipole distributions, we demonstrate that substitution of Sr for Ba scrambles dipoles, reduces global polarization, and enhances the order-disorder character of the ferroelectric-paraelectric phase transition.

Domain contribution to the aging characteristics in BaTiO3 ceramics

The domain contribution to the aging characteristics of BaTiO3 (BTO) ceramics in a DC electric field was quantitatively investigated using microwave dielectric spectroscopy. The BTO aging effect was related to its domain size (DS): the reduction in the apparent permittivity by the DC electric field, Δε, increased with decreasing BTO DS. Inspection of the microwave dielectric spectra revealed that dipole polarization governed the overall aging effect regardless of the BTO DS, and that the ionic and electronic polarizations contributions were small. Substitutional loading of acceptor Mg2+ into the Ti4+ site decreased the BTO’s DS because defect pairs consisting of and its counterpart acted as nucleation sites for new domain walls. Magnesium-loaded BTO (Mg-BTO) had smaller Δε values (attributed to dipole polarization) than BTO for similar DS. The – defect complex effectively clamped the domain walls, thereby suppressing domain wall migration in the presence of an electric field. The measured permittivity decrease under a DC field was 423 for BTO but 246 for Mg-BTO with a similar DS, corresponding to a 41.8% improvement.

Abnormal grain growth mediated by fractal boundary migration at the nanoscale

Modern engineered materials are composed of space-filling grains or domains separated by a network of interfaces or boundaries. Such polycrystalline microstructures have the capacity to coarsen through boundary migration. Grain growth theories account for the topology of grains and the connectivity of the boundary network in terms of the familiar Euclidian dimension and Euler’s polyhedral formula, both of which are based on integer numbers. However, we recently discovered an unusual growth mode in a nanocrystalline Pd-Au alloy, in which grains develop complex, highly convoluted surface morphologies that are best described by a fractional dimension of ∼1.2 (extracted from the perimeters of grain cross sections). This fractal value is characteristic of a variety of domain growth scenarios—including explosive percolation, watersheds of random landscapes, and the migration of domain walls in a random field of pinning centers—which suggests that fractal grain boundary migration could be a manifestation of the same universal behavior.

Domain Wall Motion in Perovskite Ferroelectrics Studied by the Nudged Elastic Band Method

Because of its unique physical properties, domain wall (DW) in ferroelectrics not only plays a key role in the electrical properties of ferroelectric film but also has shown tremendous prospect in the DW-based memory and logic devices. However, the motion mechanism of ferroelectric DW, which is of great significance for the application of ferroelectric films, has not been clearly understood. In this paper, the 180° DW motions in BaTiO3 (BTO) and PbTiO3 (PTO) were studied by using the nudged elastic band method based on the density functional theory. The evolutions of atomic structure, local polarization, and energy of the system for the DW motion process between the two adjacent equilibrium positions were systematically revealed. The DW migration across the oxygen vacancy was also simulated, and the corresponding potential well for the DW motion was obtained. It was also found that the DW motion barrier could be significantly influenced by the in-plane strain. The “activation field” deduced from the energ...

Real time and space observation of domain wall migration in a MX-chain complex with a quasi-2D CDW state by using STM

In STM images of [Pd(chxn-d(4))(2)Br]Br(2), which is in a quasi-2D-CDW state, we observed domain wall migration due to fast soliton motion at neighboring chains to the domain wall.

Surface morphology effects on polarization switching in nanoscale ferroelectrics

The effects of surface morphology on polarization switching in thin ferroelectric films areinvestigated using a real-space, time-dependent Ginzburg–Landau model that incorporateselectrostatic interactions. We consider a two-dimensional uni-axial ferroelectric film with athickness that varies sinusoidally. Polarization switching, starting from a singledomain remnant state, is simulated for several surface modulation amplitudes andwavelengths. We demonstrate that surface heterogeneities produce inhomogeneitiesin the electric field within the film. These inhomogeneities become preferentialsites for easy nucleation of reverse domains. This has a profound effect on theexternal field necessary to switch the polarization. Increasing the surface undulationamplitude significantly reduces the coercive field compared to the ideal flat film,even for very small amplitude modulations in the thickness. Although surfaceroughness decreases the field required to form reverse domains, it also hinderssubsequent domain wall migration. In fact, for very high amplitude and smallwavelength surface morphologies, complete switching to a single domain state becomesimpossible. This is because the domain walls become trapped near the peaks inthe modulated surface. The technological implications of the present results forutilization of surface roughness and for surface morphology design are discussed.

Structure-related effects on the domain wall migration in atomic point contacts

We investigate the interplay between magnetic and structural dynamics in magnetic point contacts under current-carrying conditions. In particular we address the dependence of the results upon the specific parameterization of the elastic properties of the material. Our analysis shows a strong influence of the structural relaxation on the energy barrier for domain wall migration, but a negligible dependence of the mechanical properties on the magnetic state. These results are stable against the various material specific choices of parameters describing different transition metals.

Coalescence of Pore Columns by Domain Switching

The present paper studies the coalescence of pore columns in ferroelectric ceramics driven by back and forth domain switching under cyclic electric field. A finite element method that incorporates mass transfer capacity is formulated to simulate the evolution of point defects subjected to the kinetics of pore surface diffusion and domain wall migration. The merge of point defects provides a mechanism for the vacancy agglomeration that leads to the formation of large pores or microcracks.

Magnetomechanical interplay in spin-polarized point contacts

We investigate the interplay between magnetic and structural dynamics in ferromagnetic atomic point contacts. In particular, we look at the effect of the atomic relaxation on the energy barrier for magnetic domain wall migration and, reversely, at the effect of the magnetic state on the mechanical forces and structural relaxation. We observe changes of the barrier height due to the atomic relaxation up to 200%, suggesting a very strong coupling between the structural and the magnetic degrees of freedom. The reverse interplay is weak, i.e. the magnetic state has little effect on the structural relaxation at equilibrium or under non-equilibrium, current-carrying conditions.

Agglomeration of point defects in ferroelectric ceramics under cyclic electric field

This paper studies the agglomeration of point defects in ferroelectric ceramics driven by repeated domain switching under cyclic electric field. A finite element method that incorporates mass transfer capacity is formulated to simulate the movement of point defects subjected to the kinetics of pore surface diffusion and domain wall migration. The simulation of pore agglomeration qualitatively agrees with the experimental data, namely the point defects migrate from the edges to the centre of the sample under cyclic electric field.

Development of magnetic domains in hard ferromagnetic thin films of polytwinned microstructure

Three-dimensional computer simulations are used to examine the development of magnetic domains and migration of domain walls in hard ferromagnetic thin films of polytwinned microstructure. The polytwinned microstructure, consisting of three different orientation variants of the L10 ordered phase with orthogonal c-axes, is characterized by a multi-component long-range order (LRO) parameter. The coupling between the magnetic domains and the polytwinned microstructure is described by the coupling between magnetization and the LRO parameter in the anisotropy free energy of the system. The magnetodynamics is described by Landau–Lifshitz equations that are solved numerically in the reciprocal space using a parallel Fast Fourier Transform algorithm. It is found that the coupling results in a multilevel hierarchy in the magnetic domain structure, with intracolony boundaries (across multiple variants within a single polytwinned colony) and intercolony boundaries (across multiple colonies). In thin films of high magnetic anisotropy we find the intracolony boundary to be of {100} type and the intercolony one to be on average of {110} type. The migration of the magnetic domain boundaries in external magnetic fields is studied in a number of simple cases in which the fields are applied along different directions. The simulation results are analyzed and compared with existing experimental observations.

Domain Wall Migration in 3-d in the Presence of Diffusing Impurities

A new three-dimensional simulation procedure was developed for domain wall (grain boundary, APB, magnetic, etc.) migration in the presence of diffusing impurities. The simulation is based upon a kinetic Monte Carlo algorithm and an extended Ising model, incorporating both conserved and non-conserved dynamics. The simulations show a dependence of the domain wall velocity on driving force which is very similar to that seen in 2-d and in qualitative agreement with experiment. That is, the presence of a low mobility regime at small driving force and an abrupt transition to a high mobility regime at larger forces, under some conditions, and a continuous, non-linear dependence of the velocity on the force in others. The main qualitative difference between the 2-d and 3-d simulation results is in how the domain wall roughness depends on driving force. The velocity-driving force relation is not consistent with classic continuum models, but may be described, in the high velocity regime, by a theory based upon a discrete version of these models.

Modification of the domain wall structure and generation of submicron magnetic formations by local optical irradiation

Experimental and theoretical investigations are made of the generation of vertical Bloch lines in a magnetic iron garnet film exposed to pulsed optical radiation. High-speed photography and anisotropic dark-field microscopy are used to study characteristic features of the generation of Bloch lines and domain structure relaxation processes after the local action of a laser pulse. Optimum optical irradiation parameters to ensure the controlled generation of Bloch lines are established. A theoretical model is developed which links the generation of Bloch lines to the migration of domain walls induced by local changes in the distribution of the degaussing fields caused by a reduction in magnetization with temperature at the optical radiation focusing point. The experimental results indicate that the controlled formation of magnetic structures smaller than or of the order of 0.1 μm by local optical irradiation is quite feasible.

Surface reconstruction in TGS family crystals under humidity and temperature controls

Surface reconstruction of TGS family single crystals has been investigated using AFM at ambient and controlled conditions. Domain boundary migration with simultaneous formation of holes at the boundary has been imaged for the first time. It is found that domain wall migration is accompanied by formation of elongated holes on originally negative end. Using humidity / temperature controlled AFM, holes on the positive end have been obtained with fast dehumidification while there is no such hole-formation with slow dehumidification. These observations provide direct evidences for a dissolution / recrystallization model accounting for hole / terrace formations. On the basis of this work, it is further proposed that the hole-formation upon the fast dehumidification can be ascribed by mass transfer limitation (kinetic control). On the contrary, with a slow dehumidification rate, the same crystal plane would resume a flatten termination to reduce the total surface energy (thermodynamic control).

射出成形 ＭＩＭプロセスによる軟質磁性材料（センダスト）の磁気特性

For miniaturization and complication of the shape of electronic devices in recent years, improvement of magnetic properties are also being required to the soft magnetic components. Fe-Si-Al alloy is well known to be a soft magnetic material suited for a magnetic head because of high permeability and magnetic flux density as well as high wear-resistance. However, this alloy is very brittle and difficultly workable. Therefore, metal injection molding (MIM) process is hoped to be a suitable production route for hard and brittle materials such as Fe-Si-Al alloy because of providing net shape components.In this study, the effect of MIM process conditions on the magnetic properties of Fe-Si-Al alloys has been investigated using a gas-atomized powder. Since this alloy melted at high temperature over 1523K, the optimum sintering temperature was found to be 1523K at which the obtained relative density was more than 98%. However, the magnetic properties of the compacts sintered in both hydrogen and vacuum atmospheres were relatively inferior to that of the wrought materials. This was seemed that the domain wall migration was prevented by the fine grain size and the retained oxides. By the continuous process including the debinding and sintering steps together, the compacts showed good magnetic properties because of the small amomts of oxides and coarse grain size.

In situ domain multiplication and migration in the antiferroelectric ceramic PLSnZT

The domain structures of an antiferroelectric perovskite in the system (Pb, La)(Zr, Sn, Ti)O 3 have been examined over a range of length scales by TEM. The structure consists of multiple orientation variants of a long range incommensurate modulation running along one of the 〈110〉 directions with an approximate wavelength 9 a / √2. The incommensurate modulation gives rise to several characteristic patterns of contrast. Beam-induced charging causes domain wall migration in specimens with an incomplete conductive coating. In one instance, a novel regenerative domain source was observed.