Closure Domains Research Articles

Magnitno-silovaya i nelineyno-opticheskaya mikroskopiya pripoverkhnostnoy domennoy struktury epitaksial'noy plenki ferrita-granata

The magnetic properties of the surface layer of an epitaxial bismuth-doped iron garnet film with a weak uniaxial anisotropy are studied by magnetic force, optical, and nonlinear optical microscopy. A significant modulation of optical second harmonic generation intensity along stripe domains is detected near the free film surface. This modulation is caused by periodic distortions of the stripe domain structure near the film surface and the presence of surface closure domains detected by magnetic force microscopy (MFM). A series of MFM images of the film taken in an in-plane magnetic field varying from –400 to 400 Oe demonstrates quasi-static nucleation of closure domains periodically located along stripe domains.

Control of closure domain state circulation in coupled triangular permalloy elements using MFM tip

We demonstrate experimentally and numerically the control of magnetization circulation of the closure domain state in thin permalloy squares divided diagonally into four dipolar-coupled isosceles triangles. The control over the magnetization circulation was achieved by the mutual application of a suitable external in-plane magnetic field and interaction with a magnetic force microscopy (MFM) tip. Numerical simulations confirm that the proposed switching protocol is valid for the broad range of MFM tips. Experiments performed on four diagonally cut squares show that this method is robust against slight imperfections in the fabrication process. Moreover, by varying the polarity of the applied field and the direction of the MFM scanning, proper symmetry response of the switching method is observed.

Induction and Ferroelectric Switching of Flux Closure Domains in Strained PbTiO3 with Neural Network Quantum Molecular Dynamics.

We have developed an extension of the Neural Network Quantum Molecular Dynamics (NNQMD) simulation method to incorporate electric-field dynamics based on Born effective charge (BEC), called NNQMD-BEC. We first validate NNQMD-BEC for the switching mechanisms of archetypal ferroelectric PbTiO3 bulk crystal and 180° domain walls (DWs). NNQMD-BEC simulations correctly describe the nucleation-and-growth mechanism during DW switching. In triaxially strained PbTiO3 with strain conditions commonly seen in many superlattice configurations, we find that flux-closure texture can be induced with application of an electric field perpendicular to the original polarization direction. Upon field reversal, the flux-closure texture switches via a pair of transient vortices as the intermediate state, indicating an energy-efficient switching pathway. Our NNQMD-BEC method provides a theoretical guidance to study electro-mechano effects with existing machine learning force fields using a simple BEC extension, which will be relevant for engineering applications such as field-controlled switching in mechanically strained ferroelectric devices.

Identifying the knowledge, skills, and barriers for self-employment

BACKGROUND: While self-employment is a viable and beneficial employment strategy for people with disabilities, there seems to be considerable variation in how state rehabilitation agencies support self-employment. Because of this variation, research is needed to identify the critical elements for facilitating self-employment. OBJECTIVE: The purpose of this study is to gather information from vocational rehabilitation counselors about what they believe are the knowledge and skills essential to facilitating self-employment and the barriers to business ownership throughout key phases of the vocational rehabilitation process, including (a) application and intake, (b) plan development and service delivery, and (c) employment and closure. METHODS: A Delphi method with three iterations was used to identify information from VRCs and managers about what they believe are the knowledge, skills, and barriers when facilitating self-employment and business ownership during the vocational rehabilitation process. RESULTS: Respondents rated 15 knowledge and skill items for the application and intake domain, 19 knowledge and skill items in the plan development and service delivery domain, and 12 knowledge and skill items in the employment and closure domain. Respondents also rated three barriers for application and intake, six plan development and service delivery barriers, and three barriers were rated in employment and closure. CONCLUSION: The VRCs identified a wide range of knowledge and skills and barriers across each area of the rehabilitation process. More research should be conducted to further operationalize the skills and proficiencies for self-employment.

Observation of Hidden Polar Phases and Flux Closure Domain Topology in Bi2WO6 Thin Films.

Topological textures of ferroelectric polarizations have promise as alternative devices for future information technology. A polarization rotation inevitably deviates from the stable orientation in axial ferroelectrics, but local energy losses compromise the global symmetry, resulting in a distorted shape of the topological vortex or inhibiting the vortex. Easy planar isotropy helps to promote rotating structures and, accordingly, to facilitate access to nontrivial textures. Here, we investigate the domain structure of an epitaxial thin film of bismuth tungsten oxide (Bi2WO6) grown on a (001) SrTiO3 substrate. By using angle-resolved piezoresponse force microscopy and scanning transmission electron microscopy, we find the existence of a hidden phase with ⟨100⟩-oriented ferroelectric polarizations in the middle of the four variant ⟨110⟩-oriented polarization domains, which assists in the formation of flux closure domains. The results suggest that this material is one step closer to becoming an isotropic two-dimensional polar material.

Measurements of Stray Magnetic Fields of Fe-Rich Amorphous Microwires Using a Scanning GMI Magnetometer

A scanning magnetometer based on a magnetoimpedance sensor with a 1 mm spatial resolution and 10 nT sensitivity was used to study stray magnetic fields of Fe74B13Si11C2 amorphous ferromagnetic microwires. Spatial magnetic images and vertical component profiles of stray magnetic fields of the studied microwires were obtained in a longitudinal homogeneous magnetic field of Helmholtz coils with a strength in the range of ±600 A/m. A magnetic calculation method is suggested that allows for using the measured magnetic fields to determine the magnitude and pattern of magnetization for the microwire. Characteristic values of the microwires’ average magnetization and width of closure domains for various values of bias fields were found.

Imaging of Love Waves and Their Interaction with Magnetic Domain Walls in Magnetoelectric Magnetic Field Sensors

AbstractThe complex behavior of horizontally polarized surface shear waves in magnetoelectric surface acoustic wave based magnetic field sensor devices is revealed by time‐resolved magnetooptical microscopy with picosecond temporal and submicron spatial resolution. The imaging of the propagating waves in the magnetoelectric composites is realized through the functional soft‐magnetic layer by coupled magnetoelastic interactions. Partial surface wave reflections, wave front dephasing, and secondary wave generation occur, which originate from structures and magnetic domain walls. Closure domain structures bend and reflect the magnetic surface waves. Strain stimulated magnetic domain walls display dynamic periodic expansions, which propagate along the domain walls and change the magnetomechanical response also in the surrounding regions. The revealed spatial and temporally varying nondeterministic response restricts the noise performance of the surface acoustic wave based magnetic field sensors and thus confines the sensor's limit of detection. Magnetic time‐resolved optical imaging is shown to be a powerful method for the operando characterization of magnetoelectric devices and in‐plane displacement surface acoustic wave fields that are not accessible by other methods.

Sheet thickness dependence of magnetization properties based on domains and walls within the non-linear diffusion-like equation for grain-oriented electrical steels

The definition and development of a dynamic or transient magnetic formulation compatible with the Finite Element Method, able to take the dynamic hysteresis and classical and extra losses into account (not a posteriori but a priori), and using the most adapted state variable that makes the problem well defined and easily convergent requires the knowledge of dynamic behavioral properties of soft magnetic materials whatever the thickness and the working conditions. The dynamic hysteresis of soft magnetic materials corresponds to excess iron losses, due to dynamic magnetization reversal processes within magnetic domains and especially to microscopic eddy currents around the magnetic walls in motion and inside rotating domains. The model properties used are the internal quasi-static permeability μ and the dynamic magnetization property Λ that lumps the magnetization mechanisms (domain walls displacement, bowing, fusion, nucleation and multiplication). The latter are involved in the magnetic field damping due to microscopic eddy currents within the field diffusion that renders the magnetic behavior geometry dependent. This model does not separate the field diffusion process from the magnetization reversal mechanisms. In this paper, the sheet thickness ζ dependence of the dynamic magnetization property Λ(ζ) for a Grain Oriented Electrical Steel (GOES: steel made of 3% Silicon and Iron: SiFe) is analyzed in the Rolling Direction (RD). To this extend, it is proposed to carry out and interpret magnetic measurements on GOES samples with the Epstein frame for four different thicknesses (ζ = 0.23, 0.27, 0.30 and 0.35 mm), but with similar metallurgical and crystallographic properties for the whole specimens. Magnetic properties are first identified at low induction with linear assumptions and at higher induction with non-linearities. It makes it possible to re-compute the dynamic hysteresis loops of the material and to predict the losses whatever the working conditions with frequencies from 50 to 800 Hz. It is found that μ does not depend significantly on the sheet thickness whereas Λ depends on it significantly. Microscopic observation of the magnetic domains width are then performed thanks to the MOIF (Magneto-Optical Indicator Film) technique. It helps us discriminate between three simultaneous origins for the dependence of iron losses to the sheet thickness: a skin-like effect, a domains’ refinement and changes on the walls’ mobility. Results are discussed taking the dipolar magnetic effects, closure domains, the grains and texture and the manufacturing and coating residual stress on the metal surface into account.

Molecular dynamics simulations of void-mediated polarization vortex domain switching in compressed BaTiO3 nanofilm

Molecular dynamics simulations based on the anisotropic shell model with the first-principles parameters are performed to investigate the void-mediated polarization vortex domain switching in compressed BaTiO3 nanofilms. When a unit-cell-size void is located at the nanofilm center, the polarization configuration is in a single-vortex state instead of a multi-vortex state. For different void length fractions along the [100] lattice orientation, a cylindrical vortex is formed around the void while a few small vortexes nucleate closely above and/or below the void. With increasing void area fraction in the [100] plane, the vortex around the void gradually switches from clockwise to counterclockwise. Both the void shape and orientation have important effects on the vortex domain switching. A void near the model center tends to induce a clockwise vortex around it, a void in the lower left region of model to motivate a counterclockwise vortex, but a void in other locations to induce a closure domain with different switching degrees. In addition, it is found that the degree of closure domain switching could be kept if a void changes its location only in a small area. These observations should be instructive for the design and application of ferroelectric devices.

Control of Domain States in Rhombohedral Lead Zirconate Titanate Films via Misfit Strains and Surface Charges

AbstractUsing the Landau–Ginzburg–Devonshire approach and numerical modeling, an influence of the misfit strain and surface screening charges as well as the role of the flexoelectric effect have been studied in the case of a rhombohedral lead zirconate titanate ferroelectric/ferroelastic thin film with an anisotropic misfit caused by a substrate. The magnitude and sign of the misfit strain influence the domain structure, providing misfit‐dependent phases with different favorable polarization components. Whilst strong enough compressive misfit strains favor a phase with orthorhombic‐like polarization directions, strong tensile misfits only yield in‐plane polarization components. The strength of surface screening conditions the existence of closure domain structures and, by increasing, supports the single‐domain state. The flexoelectric effect exhibits a weak influence on the phase diagram of multi‐domain states when compared with the phase diagram of single‐domain states. Its effect, however, becomes significant in the case of skyrmion topological states spontaneously formed near the film surface when compressive misfit strains are applied. Cooperative influence of the misfit strain, surface screening charges, and temperature can set a thin rhombohedral ferroelectric film into a number of different polar and structural states, whereby the role of the flexoelectric effect is pronounced for topologically nontrivial structures.

A new era in ferroelectrics

Topological structures in ferroic materials have drawn great interest in recent years due to the richness of the underlying physics and the potential for applications in next generation electronics. Recent advances in atomically precise thin-film materials synthesis and characterization of structural/physical phenomena at unprecedented length/energy/time scales have enabled us to study exotic phases and their associated physics [Rößler et al., Nature 442, 797 (2006); S. Das, Nature 568, 368 (2019); Yadav et al., Nature 530, 198 (2016); and Stoica et al., Nat. Mater. 18, 377 (2019)]. It is appropriate that, in the second century of ferroelectrics, some dramatic discoveries are propelling the field into directions heretofore unimaginable. In this review, we explore the recent progress in ferroelectric-oxide superlattices in which researchers can control structure and physical properties through the application of epitaxial strain, layer thickness, temperature, electric field, etc. We provide a discussion of exotic topological structures (e.g., closure domains, vortices, polar skyrmions, and other exotic phases) and associated functionalities in ferroelectric/paraelectric superlattices. We conclude with a brief overview of and prospects for how the field may evolve in the coming years.

Anisotropic fractal magnetic domain pattern in bulk Mn1.4PtSn

The tetragonal compound Mn$_{1.4}$PtSn with the $D_{2d}$ symmetry recently attracted attention as the first known material that hosts magnetic antiskyrmions, which differ from the so far known skyrmions by their internal structure. The latter have been found in a number of magnets with the chiral crystal structure. In previous works, the existence of antiskyrmions in Mn$_{1.4}$PtSn was unambiguously demonstrated in real space by means of Lorentz transmission electron microscopy on thin-plate samples ($\sim$100~nm thick). In the present study, we used small-angle neutron scattering and magnetic force microscopy to perform reciprocal- and real-space imaging of the magnetic texture of bulk Mn$_{1.4}$PtSn single-crystals at different temperatures and in applied magnetic field. We found that the magnetic texture in the bulk differs significantly from that of thin-plate samples. Instead of spin helices or an antiskyrmion lattice, we observe an anisotropic fractal magnetic pattern of closure domains in zero field above the spin-reorientation transition temperature, which transforms into a set of bubble domains in high field. Below the spin-reorientation transition temperature the strong in-plane anisotropy as well as the fractal self-affinity in zero field is gradually lost, while the formation of bubble domains in high field remains robust. The results of our study highlight the importance of dipole-dipole interactions in thin-plate samples for the stabilization of antiskyrmions and identify criteria which should guide the search for potential (anti)skyrmion host materials. Moreover, they provide consistent interpretations of the previously reported magnetotransport anomalies of the bulk crystals.

Controlled Nucleation and Stabilization of Ferroelectric Domain Wall Patterns in Epitaxial (110) Bismuth Ferrite Heterostructures

AbstractFerroelectric domain walls, topological entities separating domains of uniform polarization, are promising candidates as active elements for nanoscale memories. In such applications, controlled nucleation and stabilization of domain walls are critical. Here, using in situ transmission electron microscopy and phase‐field simulations, a controlled nucleation of vertically oriented 109° domain walls in (110)‐oriented BiFeO3 (BFO) thin films is reported. In the switching experiment, reversed domains that are nucleated preferentially at the nanoscale edges of the “crest and sag” pattern‐like electrode under external bias subsequently grow into a stable stripe configuration. In addition, when triangular pockets (with an in‐plane polarization component) are present, these domain walls are pinned to form stable flux‐closure domains. Phase field simulations show that i) field enhancement at the edges of the electrode causes site‐specific domain nucleation, and ii) the local electrostatics at the domain walls drives the formation of flux closure domains, thus stabilizing the striped pattern, irrespective of the initial configuration. The results demonstrate how flux closure pinning can be exploited in conjunction with electrode patterning and substrate orientation to achieve a desired topological defect configuration. These insights constitute critical advancements in exploiting domain walls in next generation ferroelectronic devices.

Weak Stripe Angle Determination by Quantitative x-ray Magnetic Microscopy

High-resolution scanning transmission x-ray microscopy (STXM) can provide quantitative access to local magnetization canting angles relative to the surface. We use this cutting-edge microscopy technique to probe the weak stripe magnetic texture hosted by a 180-nm-thick ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ layer. We report a full set of measurements of the weak stripe canting angle, as well as a method that uses only a single-impinging-direction x-ray beam to extract the angle. This method also allows the spatial profile to be extracted with 30 nm resolution, and the canting angle versus applied field can be measured. Combining these information, the macroscopic magnetization versus applied field loop on a complex spin texture can be reconstructed in an unprecedented way and shows undoubtedly that flux closure domains exist. These results show that the presence of flux closure domains, at the surface of such a thick film, is difficult to demonstrate by means of the STXM technique. Beyond the characterization of the weak stripe angle, this quantitative x-ray magnetic microscopy can be used to study the local textures hosted in all materials exhibiting some circular x-ray magnetic dichroism.

Magnetoresistance in Fe0.8Ga0.2 thin films with magnetic stripes: The role of the three-dimensional magnetic structure

In this work we show the existence of closure domains in Fe$_{1-x}$Ga$_x$ thin films featuring a striped magnetic pattern and study the effect of the magnetic domain arrangement on the magnetotransport properties. By means of X-ray resonant magnetic scattering, we experimentally demonstrate the presence of such closure domains and estimate their sizes and relative contribution to surface magnetization. Magnetotransport experiments show that the behavior of the magnetoresistance depends on the measurement geometry as well as on the temperature. When the electric current ows perpendicular to the stripe direction, the resistivity decreases when a magnetic field is applied along the stripe direction (negative magnetoresistance) in all the studied temperature range, and the calculations indicate that the main source is the anisotropic magnetoresistance. In the case of current flowing parallel to the stripe domains, the magnetoresistance changes sign, being positive at room temperature and negative at 100 K. To explain this behavior, the contribution to magnetoresistance from the domain walls must be considered besides the anisotropic one.

Evaluation of internal microcrack evolution in red sandstone based on time–frequency domain characteristics of acoustic emission signals

Rocks are commonly used as building stone and construction materials in many engineering applications. To study the internal microcrack evolution characteristics of red sandstone specimens during deformation and failure, a uniaxial compression experiment was conducted based on RMT-150C rock mechanics, PCI-II acoustic emission (AE), and the strain acquisition testing systems. For eight red sandstone specimens, the relationships among axial stress, axial strain, acoustic emission events rate, and average frequency centroid were established. The variability of the AE events rate and average frequency centroid of the specimens was analyzed for five cracking levels. Additionally, the relationship between the loading stress corresponding to the minimum average frequency centroid of the AE signals and the peak stress during deformation and failure of the red sandstone specimens was discussed, and critical failure precursor characteristics revealed. Moreover, statistical analysis of the AE signal distribution characteristics of different frequency bands and amplitudes during rock deformation and failure was performed. Based on the difference between the AE signal duration ratio and the specimen rise time, a method for extracting AE characteristic signals during deformation and failure was proposed. Time-frequency analysis was performed on the extracted AE characteristic signals and results showed that the signals were closely related to the axial stress–strain change in the specimens, and were affected by the degree and development of internal microcracks. The wave peaks of the crack closure and linear elastic deformation stage AE characteristic signal frequency domain waveforms are relatively scattered. The wave peaks of the frequency-domain waveforms of the AE characteristic signals during stable and unstable crack growth stages are relatively concentrated.

Recent Progress on Topological Structures in Ferroic Thin Films and Heterostructures.

Topological spin/polarization structures in ferroic materials continue to draw great attention as a result of their fascinating physical behaviors and promising applications in the field of high-density nonvolatile memories as well as future energy-efficient nanoelectronic and spintronic devices. Such developments have been made, in part, based on recent advances in theoretical calculations, the synthesis of high-quality thin films, and the characterization of their emergent phenomena and exotic phases. Herein, progress over the last decade in the study of topological structures in ferroic thin films and heterostructures is explored, including the observation of topological structures and control of their structures and emergent physical phenomena through epitaxial strain, layer thickness, electric, magnetic fields, etc. First, the evolution of topological spin structures (e.g., magnetic skyrmions) and associated functionalities (e.g., topological Hall effect) in magnetic thin films and heterostructures is discussed. Then, the exotic polar topologies (e.g., domain walls, closure domains, polar vortices, bubble domains, and polar skyrmions) and their emergent physical properties in ferroelectric oxide films and heterostructures are explored. Finally, a brief overview and prospectus of how the field may evolve in the coming years is provided.

Thickness-dependence of magnetic anisotropy and domain structure in Ni thin films grown on a PMN-PT substrate

High-quality Ni ferromagnetic thin films with the thicknesses of 180, 240, 330, and 510 nm ± 5 nm were grown on (001)Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) single-crystal substrates by magnetron sputtering followed by annealing. The thickness dependence of magnetic anisotropy was investigated using ferromagnetic resonance, indicating that the 180 nm Ni thin film exhibits a large magnetic anisotropy field. Increasing the film’s thickness resulted in a reduction of the magnetic anisotropy. Well-defined stripe domain structures in Ni thin films were observed. Calculations based on the tilted partial flux closure domain structure with Bloch wall and Neel wall indicate an increase in the primary domain size and closure domain size of Ni thin films with increasing film thickness.

Exotic closure domains induced by oxygen vacancies in compressed BaTiO3 nanofilm

The molecular dynamics method based on the shell model is used to investigate the polarization configuration evolution in compressed BaTiO3 nanofilms with oxygen vacancy lattices of different volume fractions and positions. A clockwise closure domain surrounding a head-to-head domain is observed for a single oxygen vacancy nanofilm. With an increasing oxygen vacancy fraction, the closure domain around the vacancy becomes larger and gradually transforms its rotation direction from clockwise to counterclockwise. Inside the vacancy, except for the head-to-head domain remaining unchanged, the domain changes from a random configuration to a clockwise vortex, and finally to a segmented strip polydomain with 90°- and 180°-domain walls. For a single oxygen vacancy, its location region where the closure domain can occur is obtained, and the model center is a favored site for clockwise vortex nucleation. The closure domain partly changes its orientation if the oxygen vacancy shifts off the model center. In addition, homogeneous closure domains can occur when the vacancy changes its location within a small area. This study demonstrates the feasibility of tuning this kind of closure domain in ferroelectric nanofilms through external loadings and oxygen vacancies. This could be instructive to develop novel nanoscale memories and logic devices.

Dynamics of domain wall depinning from closure domain structure at the end of bistable glass coated microwire

The study of closure domain structure dynamics at the end of magnetic bistable glass-coated microwire is presented. The method in which critical parameters of the so-called three-level rectangular field pulse are measured is used. It is shown that dependences measured using three-level field pulses cannot be satisfactorily interpreted by the model in which application of a rectangular field pulse causes the release of a single domain wall from a potential well. A new model based on the simplest possible closure domain structures at the microwire end is proposed. In the framework of this model the application of a non-zero first-level field results in changes in domain wall parameters (inertial mass and mobility) which have opposite effects on the prolonging/shortening of critical field pulse width and do not provide satisfactory explanation of experimental observations. Shortening of the path which the wall has to move along to become free can result from the wall lengthening caused by application of the non-zero first-level field. This effect combined with position-independent force acting on the wall can explain the observed behaviour. Modelling of this process is also presented and supports this interpretation.