Magnetization Reversal Research Articles

Energy loss and hysteresis of reversible magnetization processes in iron-based soft magnetic composites

The paper focuses on investigation of the hysteresis originating from the reversible magnetization processes and the quantification of the corresponding energy dissipation, on soft magnetic composite materials differing in various parameters so exhibiting different proportions of magnetization processes types during quasi-static magnetizing reversal under different maximum inductions. It was confirmed that reversible magnetization processes show hysteresis too and generate energy losses. These losses come from frictional effects accompanying local rotations of spins at the microscopical view of all magnetization processes including the reversible ones. It was found that the area of a hysteresis loop obtained by the integration of reversible permeability reveals approximately the proportion of reversible magnetization vector rotations, which produce energy loss much more than reversible domain wall displacements.

Investigating a Solar Wind Stream Interaction Region using Interplanetary Spacecraft Radio Signals: A Magnetohydrodynamic Simulation Study

Stream interaction regions (SIRs) are spiral heliospheric structures that arise at the interface between fast and preceding slow solar wind regions. SIR enhancements of density and magnetic field intensity, often with magnetic polarity inversion, are potentially geoeffective and therefore important in the analysis of space weather. We studied an MHD heliospheric simulation containing a well-defined SIR using a new instrument concept based on trans-heliospheric radio sensing: Faraday Effect Tracker of Coronal and Heliospheric structures (FETCH). FETCH uses line-of-sight radio propagation techniques to measure Faraday rotation and electron column density. Analysis of the simulated FETCH observations clearly demonstrated density and magnetic field enhancements, and magnetic polarity reversal, all of which were confirmed in Wind spacecraft measurements at 1 au. FETCH provided 4.5–5.7 days lead times for predicting the arrival of SIR features at Earth. The SIR radial speed was estimated to be 350–390 km s−1. These initial results hold promise that FETCH will be valuable in detecting and characterizing the inner heliosphere SIR properties well ahead of their presentation in the local geospace environment.

Exchange coupling effect and magnetic properties in sintered RE-Fe-B magnets

Preparing the magnetic materials bearing both a high coercive field and a high energy product is a great challenge. In this paper the content of Nd was reduced in Nd-Fe-B for enhancing the interaction effect as well as for increasing the energy product, and Tb-Fe-B powders were mixed to enhance the coercivity. In the sintered RE-Fe-B magnets some of Tb atoms diffuse into the outer regions of Nd-Fe-B grains, and Tb-rich RE2Fe14B grains also exist with the crystal size at micrometer scale. The recoil loops bear the spring behaviors, and the reversal susceptibility implies the availability of exchange coupling effect among the micro grains. The activation volume also indicates that the exchange coupling is effective in magnetization reversal. So the magnetization reversal in Nd-Fe-B grains could be restrained by the high anisotropy phase of Tb-rich core. Therefore, besides the effects of Tb-diffusion into grain outer region and the intergranular phase, the exchange coupling effect could not be neglected in improving the coercivity in the Nd-content-reduced magnets, and both a high coercive field of 20.0 kOe and a high energy product of 50.1 MGOe are achieved in the micro-crystal composite magnets.

Magnetization reversals in core–shell sphere clusters: finite-element micromagnetic simulation and machine learning analysis

Recently developed permanent magnets, featuring specially engineered microstructures of inhomogeneous magnetic phases, are being considered as cost-effective alternatives to homogeneous single-main-phase hard magnets composed of Nd2Fe14B, without compromising performance. In this study, we conducted a comprehensive examination of a core–shell sphere cluster model of Ce-substituted inhomogeneous Nd2-δCeδFe14B phases versus homogeneous magnetic phases, utilizing finite-element micromagnetic simulation and machine learning methods. This involved a meticulous, sphere-by-sphere analysis of individual demagnetization curves calculated from the cluster model. The grain-by-grain analyses unveiled that these individual demagnetization curves can elucidate the overall magnetization reversal in terms of the nucleation and coercive fields for each sphere. Furthermore, it was observed that Nd-rich spheres exhibited much broader ranges of nucleation and coercive field distributions, while Nd-lean spheres showed relatively narrower ranges. To identify the key parameter responsible for the notable differences in the nucleation fields, we constructed a machine learning regression model. The model utilized numerous hyperparameter sets, optimized through the very fast simulated annealing algorithm, to ensure reliable training. Using the kernel SHapley Additive eXplanation (SHAP) technique, we inferred that stray fields among the 11 parameters were closely related to coercivity. We further substantiated the machine learning models’ inference by establishing an analytical model based on the eigenvalue problem in classical micromagnetic theory. Our grain-by-grain interpretation can guide the optimal design of granular hard magnets from Nd2Fe14B and other abundant rare earth transition elements, focusing on extraordinary performance through the careful adjustment of microstructures and elemental compositions.

In unison magnetization reversal in a magnetoelastic bilayer structure

Due to the magnetostriction effect, a magnetic material changes its dimensions when it is magnetized. In this work, we show how in a magnetostrictive bilayer structure comprising two materials with magnetostriction constants of opposite signs, the magnetic switching is affected by magnetoelastic coupling. While the layer with positive magnetostriction tries to elongate in the direction of the applied magnetic field, the layer with negative magnetostriction tries to contract. In the studied bilayers, the mechanical influence of each magnetostrictive layer on the other is of the opposite sign because of their opposite magnetostrictive constants. Since magnetoelasticity is not an interfacial interaction but an intrinsic property of magnetic materials, the mechanical strain promoted by the applied magnetic field affects the layers as a whole. The net effect is a simultaneous reversal of magnetization of the two layers regardless of their thicknesses. This behavior has been studied in bilayers with different thickness ratios comprising Ni90Fe10, negative magnetostriction and Fe70Ga30, positive magnetostriction. These results demonstrate the possibility of using this physical mechanism to beat the critical limit thickness imposed by interfacial interactions in magnetically coupled multilayers in such a way that the magnetization reversal is made in unison regardless of the layer thickness.

First-principles study on the magnetic properties in non-metallic co-doped cubic LaNiO3

Using the first principles spin polarized calculations, the electronic structures and magnetic properties of two C atoms and two S atoms doped LaNiO3 (LNO) are investigated. For the double C-doped LNO (C-LNO), C–O–C chain exhibits the ferromagnetic (FM) order arrangement. The introduction of C atoms increases the average magnetic moment of each Ni atom by 0.23 μB/cell and enhances the hybridization between Ni-3d and O-2p states. The C atom generates a weak magnetic moment of 0.07 μB/atom due to the magnetization in the FM ground state. For the case of double S-doped LNO (S-LNO), S–O–S chain exhibits the antiferromagnetic (AFM) order. In the ferromagnetic configuration of S-LNO, each S atom generates a magnetic moment of 0.18 μB/atom due to the Ni–O hybridization. In addition, the interesting phenomenon of magnetic moment reversal of S atoms is observed in both the FM and AFM structures of S-LNO.

Direct observation and stochastic analysis on thermally activated nucleation and growth of individual magnetic domain

The nucleation and subsequent growth of the reversed magnetic domain at a finite temperature are stochastic processes, which are often analyzed using an ensemble of magnetic elements. In this study, we investigate the stochastics of magnetization reversal in multiple trials for the sole magnetic domain. Specifically, we utilize the robust magnetic domain structure formed in a dual-exchange-biased Pt/Co/Au/Cr2O3/Pt thin film. Our investigation encompasses the latency of reversed domain nucleation and the subsequent domain wall motion based on time-lapse magnetic domain observations. The time evolution of the magnetic domain is observed after applying a pulsed magnetic field superimposed on a DC field. Magnetization reversal is triggered by the nucleation of a small embryo with finite latency, followed by domain wall propagation. The nucleation probability of the embryo increases exponentially with the DC field, thus indicating that the nucleation process obeys the Poisson process. An analysis of the relaxation time for nucleation provides a suitable expression for the energy barrier. The nucleated domain wall propagates with temporal stops, thus indicating creep motion. The temperature dependence of the relaxation time and domain wall creep motion reveal that the magnetic anisotropy in the antiferromagnetic layer significantly affect both the energy barrier for nucleation and the depinning potential of domain wall propagation. This study provides comprehensive understanding into the coercivity mechanism and contributes to the thermal stability of magnetic/spintronic devices.

Quantized resistance revealed at the criticality of the quantum anomalous Hall phase transitions

In multilayered magnetic topological insulator structures, magnetization reversal processes can drive topological phase transitions between quantum anomalous Hall, axion insulator, and normal insulator states. Here we report an examination of the critical behavior of two such transitions: the quantum anomalous Hall to normal insulator (QAH-NI), and quantum anomalous Hall to axion insulator (QAH-AXI) transitions. By introducing a new analysis protocol wherein temperature dependent variations in the magnetic coercivity are accounted for, the critical behavior of the QAH-NI and QAH-AXI transitions are evaluated over a wide range of temperature and magnetic field. Despite the uniqueness of these different transitions, quantized longitudinal resistance and Hall conductance are observed at criticality in both cases. Furthermore, critical exponents were extracted for QAH-AXI transitions occurring at magnetization reversals of two different magnetic layers. The observation of consistent critical exponents and resistances in each case, independent of the magnetic layer details, demonstrates critical behaviors in quantum anomalous Hall transitions to be of electronic rather than magnetic origin. Our finding offers a new avenue for studies of phase transition and criticality in QAH insulators.

Deep learning in bifurcations of particle trajectories

We show that deep learning algorithms can be deployed to study bifurcations of particle trajectories. We demonstrate this for two physical systems, the unperturbed Duffing equation and charged particles in magnetic reversal by using the AI Poincaré algorithm. We solve the equations of motion by using a fourth-order Runge–Kutta method to generate a dataset for each system. We use a deep neural network to train the data. A noise characterized by a noise scale L is added to data during the training. By using a principal component analysis, we compute the explained variance ratios for these systems which depend on the noise scale. By plotting explained ratios against the noise scale, we show that they change at bifurcations. For different values of the Duffing equation parameters, these changes are of the form of different patterns of growth-decline of the explained ratios. For the magnetic reversal, the changes are of the form of a change in the number of principal components. We comment on the use of this technique for other dynamical systems with bifurcations.

Energetic particle dynamics in a simplified model of a solar wind magnetic switchback

Context.Recent spacecraft observations in the inner heliosphere have revealed the presence of local Alfvénic reversals of the magnetic field, while the field magnitude remains almost constant. These are called magnetic switchbacks (SBs) and are very common in the plasma environment close to the Sun explored by the Parker Solar Probe satellite.Aims.A simple numerical model of a magnetic field reversal with constant magnitude is used in order to explore the influence of SBs on the propagation of energetic particles within a range of energy typical of solar energetic particles.Methods.We model the reversal as a region of space of adjustable size bounded by two rotational discontinuities. By means of test particle simulations, beams of mono-energetic particles can be injected upstream of the SB with various initial pitch- and gyro-phase angles. In each simulation, the particle energy may also be changed.Results.Particle dynamics is highly affected by the ratio between the particle gyroradius and the size of the SB, with multiple pitch-angle scatterings occurring when the particle gyroradius is of the order of the SB size. Further, particle motion is extremely sensitive to the initial conditions, implying a transition to chaos; for some parameters of the system, a large share of particles is reflected backwards upstream as they interact with the SB. These results could have a profound impact on our understanding of solar energetic particle transport in the inner heliosphere, and therefore possible comparisons with in situ spacecraft data are discussed.

Cosmic ray transport in large-amplitude turbulence with small-scale field reversals

ABSTRACT The nature of cosmic ray (CR) transport in the Milky Way remains elusive. The predictions of current microphysical CR transport models in magnetohydrodynamic (MHD) turbulence are drastically different from what is observed. These models usually focus on MHD turbulence with a strong guide field and ignore the impact of turbulent intermittency on particle propagation. This motivates our studying the alternative regime of large-amplitude turbulence with δB/B0 ≫ 1, in which intermittent small-scale magnetic field reversals are ubiquitous. We study particle transport in such turbulence by integrating trajectories in stationary snapshots. To quantify spatial diffusion, we use a set-up with continuous particle injection and escape, which we term the turbulent leaky box. We find that particle transport is very different from the strong guide-field case. Low-energy particles are better confined than high-energy particles, despite less efficient pitch-angle isotropization at small energies. In the limit of weak guide field, energy-dependent confinement is driven by the energy-dependent (in)ability to follow reversing magnetic field lines exactly and by the scattering in regions of ‘resonant curvature’, where the field line bends on a scale that is of the order of the local particle gyro-radius. We derive a heuristic model of particle transport in magnetic folds that approximately reproduces the energy dependence of transport found numerically. We speculate that CR propagation in the Galaxy is regulated by the intermittent field reversals highlighted here and discuss the implications of our findings for CR transport in the Milky Way.

WITHDRAWN: Parametric mechanism of the magnetization reversal as a low-power recording mechanism for MRAM. Measurement of spin-accumulation-induced in-plane magnetic field in a FeB nanomagnet

WITHDRAWN: Parametric mechanism of the magnetization reversal as a low-power recording mechanism for MRAM. Measurement of spin-accumulation-induced in-plane magnetic field in a FeB nanomagnet

Plastic-strain-induced magnetocaloric effect of Pt3Fe ordered alloy

We report a magnetocaloric effect of a plastically strained Pt3Fe antiferromagnet, in which ferromagnetism is induced due to the changes in the atomic arrangement around the {110} glide plane. The magnetic entropy change after the application of magnetic field increases with increasing applied plastic strain and shows a peak value of ∼0.1 J/K kg for an applied field of 50 kOe around the Néel temperature of 170 K. The magnetic entropy change can be due to the magnetization reversal of Fe magnetic moments in ferromagnetic domains formed around the {110} glide planes, and the peak temperature is influenced by the magnetic interaction between ferromagnetic domains and antiferromagnetic matrix. These observations suggest that a Pt3Fe chemically ordered alloy is a unique type of antiferromagnets in which the magnetocaloric effect can be induced and controlled by applied plastic strain.

Surface Crystallization and Magnetization Reversal Processes in Amorphous Microwires

Surface Crystallization and Magnetization Reversal Processes in Amorphous Microwires

Изучение влияния слабых импульсов магнитного поля на локальные свойства ленточных аморфных сплавов Fe(Ni, Cu)(SiB)

Atomic force microscopy and magnetic force microscopy have been used to study the influence of weak magnetic field pulses on the local properties of ribbon amorphous Fe(Ni, Cu)(SiB) alloys about 100 µm thick, 10 mm wide, and 50 mm long, which were obtained by ultrafast cooling of the melt on a rotating copper drum. On the surface of the tape adjacent to the copper drum, there were practically no areas with low rough-ness, which did not allow subsequent studies of this side of the tape by magnetic force microscopy. This method was used to investigate another, free surface of the foil, which was not adjacent to the copper drum and did not have significant roughness. Prior to the impact of magnetic field pulses on the foil, no magnetic contrast was observed on the free side of the ribbon. After magnetic pulse processing, a magnetic contrast was registered on this side of the foil: stripe domains 0.6–0.8 μm wide became visible, and closing domains, became visible on structural defects, wedge-shaped Neel domains, from 1 µm to 1.6 µm wide. The results of the study allow us to say that the magnetization reversal losses are to a large extent associated with losses due to eddy currents and are associated with the domain width, which depends slightly on the modes of magnetic pulse processing. The obtained results of the research can be used to refine the method for relieving stresses arising in the process of manufacturing amorphous ribbons.

The Temperature, Electron, and Pressure Characteristics of Switchbacks: Parker Solar Probe Observations

Parker Solar Probe observes unexpectedly prevalent switchbacks, which are rapid magnetic field reversals that last from seconds to hours, in the inner heliosphere, posing new challenges to understanding their nature, origin, and evolution. In this work, we investigate the thermal states, electron pitch-angle distributions, and pressure signatures of both inside and outside the switchbacks, separating a switchback into spike, transition region (TR), and quiet period (QP). Based on our analysis, we find that the proton temperature anisotropies in TRs seem to show an intermediate state between spike and QP plasmas. The proton temperatures are more enhanced in the spike than in the TR and QP, but the alpha temperatures and alpha-to-proton temperature ratios show the opposite trend to the proton temperatures, implying that the preferential heating mechanisms of protons and alphas are competing in different regions of switchbacks. Moreover, our results suggest that the electron-integrated intensities are almost the same across the switchbacks, but the electron pitch-angle distributions are more isotropic inside than outside switchbacks, implying switchbacks are intact structures, but strong scattering of electrons happens inside switchbacks. In addition, the examination of pressures reveals that the total pressures are comparable through an individual switchback, confirming switchbacks are pressure-balanced structures. These characteristics could further our understanding of ion heating, electron scattering, and the structure of switchbacks.

Magnetization reversal of [Co/Pd] perpendicular magnetic thin film dot on (Bi,La)(Fe,Co)O3 multiferroic thin film by applying electric field

A multilayer structure with a high-quality (Bi,La)(Fe,Co)O3 multiferroic thin film/[Co/Pd] perpendicular magnetic thin film dots was fabricated for demonstrating magnetization reversal of [Co/Pd] dots under an applied electric field. Although the magnetization direction of the multiferroic thin film was reversed under the electric field, the magnetic properties of the multiferroic thin films were generally low. If the multiferroic thin film in this structure can control the magnetization direction of the highly functional magnetic thin film under an electric field, high-performance magnetic devices with low power consumption are easily obtained. The magnetic domain structure of the [Co/Pd] dots fabricated on the (Bi,La)(Fe,Co)O3 thin film was analyzed by magnetic force microscopy (MFM). The structure was de-magnetized before the local electric-field application and magnetized after applying the field, showing reduced magnetic contrast of the dot. The line profile of the MFM image revealed a downward magnetic moment of 75%, which reversed to upward under the local electric field. Magnetic interaction between the (Bi,La)(Fe,Co)O3 and [Co/Pd] layers was also observed in magnetization hysteresis measurements. These results indicate that the magnetization direction of the [Co/Pd] dots was transferred through the magnetization reversal of the (Bi,La)(Fe,Co)O3 layer under a local electric field. That is, the magnetization of [Co/Pd] dots were reversed by applying a local electric field to the multilayer structure. This demonstration can potentially realize high-performance magnetic devices such as large capacity memory with low power consumption.

Revealing intrinsic domains and fluctuations of moiré magnetism by a wide-field quantum microscope

Moiré magnetism featured by stacking engineered atomic registry and lattice interactions has recently emerged as an appealing quantum state of matter at the forefront of condensed matter physics research. Nanoscale imaging of moiré magnets is highly desirable and serves as a prerequisite to investigate a broad range of intriguing physics underlying the interplay between topology, electronic correlations, and unconventional nanomagnetism. Here we report spin defect-based wide-field imaging of magnetic domains and spin fluctuations in twisted double trilayer (tDT) chromium triiodide CrI3. We explicitly show that intrinsic moiré domains of opposite magnetizations appear over arrays of moiré supercells in low-twist-angle tDT CrI3. In contrast, spin fluctuations measured in tDT CrI3 manifest little spatial variations on the same mesoscopic length scale due to the dominant driving force of intralayer exchange interaction. Our results enrich the current understanding of exotic magnetic phases sustained by moiré magnetism and highlight the opportunities provided by quantum spin sensors in probing microscopic spin related phenomena on two-dimensional flatland.

Interlayer antiferromagnetic coupling in Tb3Fe5O12/Y3Fe5O12 bilayers

The interlayer antiferromagnetic (AFM) coupling between thin films plays a significant role in the application of spintronics and magnetic memory devices. Previously, we observed AFM coupling phenomenon at low temperatures in rare-earth iron garnet bilayers epitaxially grown on Y3Al5O12 substrates. Here, we report a detailed study on the impacts of various factors, including temperature, crystallographic orientation, and layer thickness, on the AMF coupling and magnetization reversal behavior of such a bilayer system. A simple energy model qualitatively described the coupling behavior of the two layers during the magnetization reversal process. The interlayer coupling strength was calculated by measuring the minor magnetic hysteresis loops. The current results can serve as a reminder for future research on interlayer AFM coupling phenomena and highlight the potential of manipulating the magnetic properties in rare-earth garnet bilayers for spintronics studies and other applications.

Optimization of Magnetoimpedance Effect and Magnetic Properties of Fe-Rich Glass-Coated Microwires by Annealing.

As-prepared Fe-rich microwires with perfectly rectangular hysteresis loops present magnetization reversal through fast domain wall propagation, while the giant magnetoimpedance (GMI) effect in Fe-rich microwires is rather low. However, the lower cost of Fe-rich microwires makes them attractive for magnetic sensors applications. We studied the effect of conventional (furnace) annealing and Joule heating on magnetic-propertied domain wall (DW) dynamics and the GMI effect in two Fe microwires with different geometries. We observed that magnetic softness, GMI effect and domain wall (DW) dynamics can be substantially improved by appropriate annealing. Observed experimental results are discussed considering the counterbalance between the internal stresses relaxation and induced magnetic anisotropy associated with the presence of an Oersted magnetic field during Joule heating.