Wall Dynamics Research Articles

Analytical model of the deformation-induced inertial dynamics of a magnetic vortex

We present an analytical model to account for the deformation-induced inertial dynamics of a magnetic vortex. The model is based on a deformation of the vortex core profile based on the Döring kinetic field, whereby the deformation amplitudes are promoted to dynamical variables in a collective-coordinate approach that provides a natural extension to the Thiele model. This extended model describes complex transients due to inertial effects and the variation of the effective mass with velocity. The model also provides a quantitative description of the inertial dynamics leading up to vortex core reversal, which is analogous to the Walker transition in domain wall dynamics. Our work paves the way for a standard prescription for describing the inertial effects of topological magnetic solitons.

Atomically resolved structure of step-like uncharged and charged domain walls in polycrystalline BiFeO3

The dynamics of domain walls (DWs) during polarization switching has a great influence on the electromechanical response of ferroelectrics. To understand the influence of DWs on the functional performance of the material, their local structure and properties must be revealed. There are many investigations of the DWs in ferroelectric thin films, while investigations of the DWs in polycrystalline ferroelectrics are fewer. Here, we report on a comparative study of uncharged and charged (“tail-to-tail”) DWs in BiFeO3 ceramics from the point of view of the atomically resolved strain and structure using scanning-transmission electron microscopy. Both types of DWs have a step-like morphology and have a non-Ising behavior. The strain, expressed as the unit-cell distortion, is higher and more concentrated in the case of uncharged walls, which we associate with a narrower transition region of the Fe-displacement vectors. Conversely, in the case of “tail-to-tail” charged DWs, the unit-cell distortion is smaller and more dispersed because of the wider transition region of the Fe-displacement vectors. All the types of step-like DWs reported here, regardless of their charge state, exhibit a comparable amount of Bi-vacancies segregation. Further details about the structural differences are discussed from the point of view of the Fe-displacement evolution through the wall for uncharged/“tail-to-tail” charged DWs. The results are useful as they provide an insight into the local structure and chemistry of charged and uncharged DWs in polycrystalline BiFeO3.

Domain wall dynamics in antiferromagnetically coupled double-lattice systems

In ferromagnetic materials, the rich dynamics of magnetic domain walls (DWs) under magnetic field or current have been successfully described using the well-known q-{\phi} analytical model. We demonstrate here that this simple unidimensional model holds for multiple-sublattice materials such as ferrimagnetic alloys or synthetic antiferromagnets (SAF) by using effective parameters, and is in excellent agreement with double-lattice micromagnetic simulations. We obtain analytical laws for the DW velocity and internal precession angle as a function of net magnetisation for different driving forces (magnetic field, spin transfer and spin-orbit torques) and different propagation regimes in ferrimagnetic alloys and SAFs. The model predicts that several distinctive dynamical features occur near or at the magnetic and the angular compensation points when the net magnetization or the net angular momentum of the system vanishes, and we discuss the experimental observations that have been reported for some of them. Using a higher degree-of-freedom analytical model that accounts for inter-sublattice distortions, we give analytical expressions for these distortions that agree with the micromagnetic simulations. This model shows that the DW velocity and precession rate are independent of the strength of the inter-sublattice exchange coupling, and justifies the use of the simpler effective parameters model.

Domain-wall dynamics in a nanostrip with perpendicular magnetic anisotropy induced by perpendicular current injection

A numerical and analytical study of the dynamics of domain walls (DWs) in a magnetic tunnel junction with perpendicular magnetic anisotropy in a free layer is presented. Equilibrium states of the domain wall are obtained for various widths of the structure. The corresponding symmetries of the components of spin transfer torques ${\mathbf{T}}_{STT}$ and the polarizer directions favoring stable DW motion under perpendicular current injection are obtained. The DW steady motion with velocities up to 200 m/s at current densities below ${10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{A}/{\mathrm{cm}}^{2}$ is reported. The Walker breakdown is demonstrated, and the dynamics of the postthreshold DW motion is investigated for various configurations of torques and polarizer directions. To have analytical insight into the investigated regimes of DW dynamics a theoretical model is developed and verified by micromagnetic simulations.

Domain structure and domain wall dynamics in topological chiral antiferromagnets from the viewpoint of magnetic octupole

Spintronics has been evolving rapidly; it becomes next-generation electronics exploiting both spin and charge degrees of freedom and a primary research field directly linked to topology and nano-magnetism in solid-state physics. In this article, we put our focus on the domain structure and domain wall dynamics based on a cluster magnetic octupole in topological antiferromagnets. Many issues are still not clear in terms of magnetic octupole domains (MODs) and domain walls. We first discuss the current status of the study on the antiferromagnetic domains and walls and then the MODs and walls from detection and manipulation viewpoints.

Dynamics of domain walls in chiral magnetics

Dynamics of domain walls in chiral magnetics

Quantitative Evaluation of a Telerobotic System for Vascular Ultrasound Measurement on a Short Arm Human Centrifuge

Artificial Gravity generated by Short Arm Human Centrifuges is a promising multi-system countermeasure for physiological deconditioning during long duration space flights. To allow a continuous assessment of cardiovascular hemodynamics during centrifugation, a telerobotic robotic system holding an ultrasound probe has been installed on a Short Arm Human Centrifuge. A feasibility study was conducted to define the use capabilities and limitations of such a novel method. The objective of this study is to estimate the reproducibility and precision of remotely controlled vascular ultrasound assessment under centrifugation by assessing peripheral vascular diameter and wall distension. Four repeated centrifugation runs of 5 min, with 2.4 g at feet level, were performed including a 15 min rest between each run for a group of eight healthy male volunteers. Vascular diameter and distention were assessed for the common carotid artery (CCA) and the femoral artery (FA) by ultrasound imaging using a 10 MHz linear array probe (Mylab1, Esaote). Ultrasound measurements were consecutively performed: a) by an expert user in hand-held mode in standing as well as supine position, b) using the telerobotic arm without centrifugation as baseline and c) using the telerobotic arm during centrifugation. Vascular responses were compared between baseline and under centrifugation. Inter-, intra-registration and group variability have been assessed for hand-held and remotely controlled examination. The results show that intra-registration variability, σh , was always smaller than inter-registration variability, σm, that is in turned smaller than the inter-subject variability σg (σh < σm < σg). Centrifugation caused no significant changes in CCA diameter but a lower carotid distension compared to manual and robotic ultrasound in supine position (p < 0.05). Femoral diameter was significantly decreased in hypergravity compared to robotic sonography without centrifugation. A good reproducibility and precision of the remotely controlled vascular ultrasound assessment under centrifugation could be demonstrated. In conclusion, arterial wall dynamics can be precisely assessed for the CCA and femoral artery during centrifugation using a telerobotic ultrasound measurement system. Potential improvements to further enhance reproducibility and safety of the system are discussed.

Ultrafast nanoimaging of the order parameter in a structural phase transition.

Understanding microscopic processes in materials and devices that can be switched by light requires experimental access to dynamics on nanometer length and femtosecond time scales. Here, we introduce ultrafast dark-field electron microscopy to map the order parameter across a structural phase transition. We use ultrashort laser pulses to locally excite a 1T-TaS2 (1T-polytype of tantalum disulfide) thin film and image the transient state of the specimen by ultrashort electron pulses. A tailored dark-field aperture array allows us to track the evolution of charge-density wave domains in the material with simultaneous femtosecond temporal and 5-nanometer spatial resolution, elucidating relaxation pathways and domain wall dynamics. The distinctive benefits of selective contrast enhancement will inspire future beam-shaping technology in ultrafast transmission electron microscopy.

Multifrequency Phased Tracking Method for Estimating Velocity in Heart Wall

Local high-accuracy velocity estimation is important for the ultrasound-based evaluation of regional myocardial function. The ultrasound phase difference at the center frequency of the transmitted signal has been conventionally used for velocity estimation. In the conventional method, spatial averaging is necessary owing to the frequency-dependent attenuation and interference of backscattered waves. Here, we propose a method for suppressing these effects using multifrequency phase differences. The resulting improvement in velocity estimation in the heart wall was validated by in vivo experiments. In the conventional method, the velocity waveform exhibits spike-like changes. The velocity waveform estimated using the proposed method did not exhibit such changes. Because the proposed method estimates myocardium velocity without spatial averaging, it can be used for measuring heart wall dynamics involving thickness changes.

Dynamics of Composite Domain Walls in Multiferroics in Magnetic Field and Their Instability

We study theoretically the dynamics of composite domain walls (DW) in multiferroic material GdFeO3 driven by magnetic field H. Two antiferromagnetic orders of Fe and Gd spins and Gd-ion displacement interact with each other in this system with coupling c at low temperatures. We have numerically simulated the corresponding time-dependent Ginzburg–Landau equations in which magnetic field H couples to Fe-spin order parameter. We vary H and c systematically and calculate velocity v and the internal structure in a stationary state for magnetic DW and magneto-electric DW. DW mobility v/H increases with H and decreases with c for both DWs, but their characteristics differ between the two. We have also analytically studied the smooth characteristics of magneto-electric DW by a perturbation theory. Another finding is a splitting instability at a large H. A magneto-electric DW splits into a pair of magnetic DW and electric DW when c is large, while a magnetic DW splits when c is small. The internal structure of composite DW deforms with increasing c and H, and modulations in different order parameters separate in space. Their relative distances show a noticeable enhancement with approaching the splitting instability.

Ferroelectric soft mode and microwave dielectric relaxation in BaTiO3−PbMg1/3Nb2/3O3 ceramics

${\mathrm{BaTiO}}_{3}$ (BT) is the best-known ferroelectric and $\mathrm{Pb}{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ (PMN) is the best-known relaxor ferroelectric, but their solid solutions were so far studied only very scarcely. Here the high-density perovskite ceramic solid solutions of $(1\ensuremath{-}x){\mathrm{BaTiO}}_{3}\text{\ensuremath{-}}x\mathrm{Pb}{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ (BT-100$x$%PMN, $x=0.075$, 0.10, 0.15) were studied by the infrared reflectivity, time-domain terahertz transmission and microwave spectroscopies (1 MHz--20 THz) and compared with the undoped BT and PMN ceramics. The spectra show presence of a split soft phonon at all temperatures, similar to that observed in BT and PMN. Microwave dielectric dispersion was observed both below and above temperature of the diffuse permittivity maximum. In the case of BT-7.5%PMN and BT-10%PMN, the main dispersion takes place above 1 GHz, in case of the BT-15%PMN, the dispersion covers more homogeneously the 1 MHz--1 THz range. The dielectric response up to the THz range of the BT-10%PMN and BT-15%PMN was fitted with three excitations: soft mode, central mode (lower-frequency component of the split soft phonon), and microwave relaxation, but the soft mode does not soften appreciably and only the latter two excitations contribute substantially to the dielectric permittivity maximum. Dielectric response of the BT-7.5%PMN is similar except for the presence of a second dielectric relaxation in the MHz range, also observed in BT. The relaxations show no PMN-like freezing and can be tentatively attributed to the dynamics of polar nanoregions in the paraelectric phase and to the domain/nanodomain wall dynamics and/or acoustic wave emission and piezoelectric resonances within the grains in the ferroelectric phase. In this way, the BT-100$x$%PMN ceramics cannot be considered as relaxor ferroelectrics, but rather as ferroelectrics with the diffuse phase transition. The broadband dielectric spectra of undoped BT ceramics revealed a split optical soft mode and two additional dielectric relaxations which are mainly responsible for the dielectric anomaly at the ferroelectric phase transition. It supports the phase transition in BT to be at the crossover from a displacive to an order-disorder type.

Maximum entropy in the dimensional transition of the magnetic domain wall dynamics

Barkhausen effect is a paradigmatic case of crackling noise and it offers insights on the complex microscopic magnetization process associated to the jerky motion of magnetic domain walls in ferromagnetic materials. The study of Barkhausen noise in magnetic films shows that the domain wall dynamics strongly depends on the structural characteristics of the material and film thickness. In fact, this phenomenon presents a dimensional transition that takes place for the thickness in the range from 100 to 50nm in both cases: amorphous and polycrystalline films. Here we explore the entropy in the dynamics of magnetic domain walls using an entropy quantifier S based on the recurrence analysis. From the Barkhausen noise time series recorded in amorphous and polycrystalline films with distinct thicknesses, we address the entropy far from and at the dimensional crossover in the domain wall dynamics. Our findings reveal S presents a maximum at the dimensional transition; the distribution of S follows a Gaussian distribution for three-dimensional films far from the border of the dimensional crossover; the S distribution at the imminence of the dimensional crossover is not Gaussian, but strongly asymmetric, and despite the results are influenced by the film thickness, they seem to be insensitive to the structural characteristics of the materials.

Toward Decoding the Relationship between Domain Structure and Functionality in Ferroelectrics via Hidden Latent Variables

Polarization switching mechanisms in ferroelectric materials are fundamentally linked to local domain structure and the presence of the structural defects, which both can act as nucleation and pinning centers and create local electrostatic and mechanical depolarization fields affecting wall dynamics. However, the general correlative mechanisms between domain structure and polarization dynamics are only weakly explored, precluding insight into the associated physical mechanisms. Here, the correlation between local domain structures and switching behavior in ferroelectric materials is explored using convolutional encoder-decoder networks, enabling image to spectral (im2spec) and spectral to image (spec2im) translations via encoding of latent variables. The latter reflect the assumption that the relationship between domain structure and polarization switching is parsimonious, i.e., is based upon a small number of local mechanisms. The analysis of latent variables distributions and their real-space representations provides insight into the predictability of the local switching behavior and hence associated physical mechanisms. We further pose that the regions where these correlative relationships are violated, i.e., predictability of the polarization dynamics from domain structure is reduced, represent the obvious target for detailed studies, e.g., in the context of automated experiments. This approach provides a workflow to establish the presence of correlation between local spectral responses and local structure and can be universally applied to spectral imaging techniques such as piezoresponse force microscopy (PFM), scanning tunneling microscopy (STM) and spectroscopy, and electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM).

Signal detection based on the chaotic motion of an antiferromagnetic domain wall

The antiferromagnetic domain wall dynamics is currently a hot topic in mesoscopic magnetic systems. In this work, it is found that, based on the Thiele approach, the motion of an antiferromagnetic domain wall is described by the Duffing equation. Numerical simulations demonstrate that the antiferromagnetic domain wall can be used as a Duffing oscillator, and the transition between the periodic and chaotic motion can be used to detect the periodic signal in the presence of the white noise. Furthermore, we calculate the bifurcation diagram and Lyapunov exponents to study the chaotic behavior of an antiferromagnetic domain wall. The numerical simulations are in good agreement with the analytical solutions. Our results may be useful for building spintronic detection devices based on antiferromagnetic domain walls.

Dynamics of cell wall structure and related genomic resources for drought tolerance in rice.

Cell wall plasticity plays a very crucial role in vegetative and reproductive development of rice under drought and is a highly potential trait for improving rice yield under drought. Drought is a major constraint in rice (Oryza sativa L.) cultivation severely affecting all developmental stages, with the reproductive stage being the most sensitive. Rice plants employ multiple strategies to cope with drought, in which modification in cell wall dynamics plays a crucial role. Over the years, significant progress has been made in discovering the cell wall-specific genomic resources related to drought tolerance at vegetative and reproductive stages of rice. However, questions remain about how the drought-induced changes in cell wall made by these genomic resources potentially influence the vegetative and reproductive development of rice. The possibly major candidate genes underlying the function of quantitative trait loci directly or indirectly associated with the cell wall plasticization-mediated drought tolerance of rice might have a huge promise in dissecting the putative genomic regions associated with cell wall plasticity under drought. Furthermore, engineering the drought tolerance of rice using cell wall-related genes from resurrection plants may have huge prospects for rice yield improvement. Here, we review the comprehensive multidisciplinary analyses to unravel different components and mechanisms involved in drought-induced cell wall plasticity at vegetative and reproductive stages that could be targeted for improving rice yield under drought.

Исследование межступенчатого канала комбинированного турбомолекулярного насоса

The calculation is based on the use of Monte Carlo method (method of test particle), which consists in the statistical modeling of processes. The article describes an algorithm to construct a mathematical model step by step. The article defines both a probability for gas molecules to pass through the interstage channel of a hybrid turbomolecular pump in forward and backward direction. The effect of the movable walls limiting the channel is taken into account. Results and, accordingly, recommendations, given in the article, can be used in designing a flow passage of the hybrid turbomolecular pumps. The dependencies of a probability for gas molecules to pass through the interstage canal on the relative parameters determining the geometry and dynamics of the channel walls are given

Investigation on behaviors of acoustoelastic cavities using a novel reduced finite element–dual reciprocity boundary element formulation

This paper presents the acoustic and structure characteristics of chambers with elastic/rigid baffles and elastic walls using a novel formulation. The acoustic fluid is assumed to be compressible and inviscid, and the effect of mean flow is neglected. The acoustic field is formulated based on the dual reciprocity boundary element method (DRBEM), and the finite element method (FEM) is used to model the structural dynamics of elastic walls. The reduced-order model (ROM) of the coupled system is derived based on modal analysis of the acoustic field with rigid boundaries and the structure in a vacuum. Numerical examples are presented, and the acoustic modes and frequencies of the chambers are determined by analysis of eigenvalues of the coupled governing equations. The accuracy of the introduced model, which is based on the reduced-order DRBEM-FEM formulation, is examined by comparing the results with analytical and numerical predictions reported in the literature, and good agreements are achieved. Besides, the effects of baffles are studied for different configurations and the elasticity of a wall in a combustion chamber is investigated.

The effect of morphology and particle-wall interaction on colloidal near-wall dynamics.

We investigated the near-wall Brownian dynamics of different types of colloidal particles with a typical size in the 100 nm range using evanescent wave dynamic light scattering (EWDLS). In detail we studied dilute suspensions of silica spheres and shells with a smooth surface and silica particles with controlled surface roughness. While the near wall dynamics of the particle with a smooth surface differ only slightly from the theoretical prediction for hard sphere colloids, the rough particles diffuse significantly slower. We analysed the experimental data by comparison with model calculations and suggest that the deviating dynamics of the rough particles are not due to increased hydrodynamic interaction with the wall. Rather, the particle roughness significantly changes their DLVO interaction with the wall, which in turn affects their diffusion.

Domain structure and energy losses up to 10 kHz in grain-oriented Fe-Si sheets

We investigate in theory and experiment the frequency dependence of magnetic losses in Grain-Oriented 0.29 mm thick high-permeability steel sheets up to 10 kHz. Such an unusually broad frequency range, while responding to increasing trends towards high-frequency regimes in applications, is conducive to a complex evolution of the magnetization process, as imposed by increasing frequencies to a non-linear high-permeability saturable material. We show that the concept of loss decomposition, supported by observations of the domain wall dynamics through Kerr experiments, is effective in the assessment of the broadband frequency dependence of the energy loss. By calculating, in particular, the instantaneous and time averaged macroscopic induction profiles across the sheet thickness through the Maxwell’s diffusion equation, the classical loss component Wclass, versus frequency f and peak polarization Jp, is obtained. A simplified theoretical approach is pursued in this case by identifying the normal magnetization curve with the magnetic constitutive equation of the material. While the hysteresis loss Whyst is shown to invariably increase with frequency, the excess loss Wexc, the quantity directly associated with the eddy currents circulating around the moving domain walls, tends to vanish upon increasing both frequency and induction values. The Kerr experiments actually show that, while the oscillating 180° domain walls can adjust to the depth of the induction profile by bowing at low Jp values, the magnetization reversal at high inductions and high frequencies occurs by inward motion of symmetric fronts originating at the sheet surface, according to a classical framework.

Wall, Gate and Self-Other Dynamics: A Confucian Ethics of Separation and Interconnection

This paper starts with the social and moral implications of wall in history and in the contemporary world, to usher in the early Confucian discourse on wall and gate. The Confucian discourse implies that walls either actual, virtual or symbolic are there to defend and/or to separate, while gates enable the managed access to and opening-up the self-imposed insularity or moderate the self-centred exclusiveness that walls imply. By way of reinterpretation and reconstruction, we will extract from a variety of Confucian discussions the ethical awareness that however strongly built, walls must be associated with gates, and that the wall and the gate are therefore locked in mutuality to make possible the reality of interconnectedness between the inside and the outside and between the self and the other. It will be argued that by using ethical virtues as tools to moderate separation and exclusiveness, Confucian discourses highlight the dynamics of the self-other relationship, and establishes an ethics that may well be still applicable to contemporary situations and can be drawn upon to help dissolve the tension between the values of populist self-centrism and those of globalist interconnectedness.