Magnetization Reversal Process Research Articles

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.

Surface Crystallization and Magnetization Reversal Processes in Amorphous Microwires

Surface Crystallization and Magnetization Reversal Processes in Amorphous Microwires

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.

Convoluted Magnetoresistance and Magnetic Reversal Processes in Ni–Fe Segmented Cylindrical Nanodots with Tunable Size and Composition for Technological Applications

AbstractDue to their unique properties, bi‐segmented systems are currently used in several technological applications such as sensing devices, high density magnetic data storage systems, spintronics and microelectromechanical components, among others. In this study, the magnetoresistance and magnetic properties of Ni–Fe bi‐segmented cylindrical nanodots in a broad range of diameters and heights are discussed. The power of the First Principles approach, as considered in the density functional theory formulation, is used to study the structural and magnetic relaxation effects, and atomistic simulations, through the Fast Monte Carlo methodology, are employed to explore the magnetoresistance and magnetic behaviors of these systems. By means of the magnetic hysteresis and magnetoresistance signals, convoluted magnetization reversal schemes are discussed. These effects take place depending on the size of the Ni and Fe components as a result of the interplay among exchange interactions and size and shape effects induced by dipolar interactions. Since size has become an experimental controllable parameter, due to the enriched phenomena, the effects discussed in these bi‐component systems are useful for the design and production of devices for technological applications with relevance beyond the observed in the more restricted single component systems.

Enhancing the Squareness and Bi-Phase Magnetic Switching of Co2FeSi Microwires for Sensing Application.

In the current study we have obtained Co2FeSi glass-coated microwires with different geometrical aspect ratios, ρ = d/Dtot (diameter of metallic nucleus, d and total diameter, Dtot). The structure and magnetic properties are investigated at a wide range of temperatures. XRD analysis illustrates a notable change in the microstructure by increasing the aspect ratio of Co2FeSi-glass-coated microwires. The amorphous structure is detected for the sample with the lowest aspect ratio (ρ = 0.23), whereas a growth of crystalline structure is observed in the other samples (aspect ratio ρ = 0.30 and 0.43). This change in the microstructure properties correlates with dramatic changing in magnetic properties. For the sample with the lowest ρ-ratio, non-perfect square loops are obtained with low normalized remanent magnetization. A notable enhancement in the squareness and coercivity are obtained by increasing ρ-ratio. Changing the internal stresses strongly affects the microstructure, resulting in a complex magnetic reversal process. The thermomagnetic curves show large irreversibility for the Co2FeSi with low ρ-ratio. Meanwhile, if we increase the ρ-ratio, the sample shows perfect ferromagnetic behavior without irreversibility. The current result illustrates the ability to control the microstructure and magnetic properties of Co2FeSi glass-coated microwires by changing only their geometric properties without performing any additional heat treatment. The modification of geometric parameters of Co2FeSi glass-coated microwires allows to obtain microwires that exhibit an unusual magnetization behavior that offers opportunities to understand the phenomena of various types of magnetic domain structures, which is essentially helpful for designing sensing devices based on thermal magnetization switching.

Investigation of magnetization reversal and domain structures in perpendicular synthetic antiferromagnets by first-order reversal curves and magneto-optical Kerr effect

Perpendicular synthetic-antiferromagnet (p-SAF) has broad applications in spin-transfer-torque magnetic random access memory and magnetic sensors. In this study, the p-SAF films consisting of (Co/Ni)3]/Ir(t Ir)/[(Ni/Co)3 are fabricated by magnetron sputtering technology. We study the domain structure and switching field distribution in p-SAF by changing the thickness of the infrared space layer. The strongest exchange coupling field (H ex) is observed when the thickness of Ir layer (t Ir) is 0.7 nm and becoming weak according to the Ruderman–Kittel–Kasuya–Yosida-type coupling at 1.05 nm, 2.1 nm, 4.55 nm, and 4.9 nm in sequence. Furthermore, the domain switching process between the upper Co/Ni stack and the bottom Co/Ni stack is different because of the antiferromagnet coupling. Compared with ferromagnet coupling films, the antiferromagnet samples possess three irreversible reversal regions in the first-order reversal curve distribution. With t Ir increasing, these irreversible reversal regions become denser and smaller. The results from this study will help us understand the details of the magnetization reversal process in the p-SAF.

Recent Unrevealing on Magnetic Hysteresis of Permanent Magnets

Magnetic hysteresis and coercivity mechanism of a permanent magnet have been long discussed, and their understandings have been significantly deepened during the last decade. Magnetization reversal in a permanent magnet proceeds with thermally activated nucleation and domain wall depinning processes followed by magnetic domain expansions. To treat theoretically the thermally activated magnetization reversal process, atomistic spin model and continuum calculations have been performed. Through these multi-scale calculations, the physical meanings of fluctuation field and activation volume which have been used phenomenologically become very clear. Experimental approaches for both microscopic and macroscopic thermally activated magnetization reversal processes and magnetic domain growths have been also performed. It is expected that these theoretical and experimental multi-scale studies will be integrated to fully understand the magnetic hysteresis and coercivity mechanism with the aid of data science technologies in the future.

Barkhausen noise from formation of360∘domain walls in disordered permalloy thin films

Micromagnetic simulations reveal formation of 360${}^{\ensuremath{\circ}}$ domain walls in disordered, thin permalloy films during the field-driven magnetic reversal process. With stronger disorder, the reversal process is increasingly governed by discontinuous jumps, resulting in Barkhausen noise without explicit domain wall movement.

Analysis of the magnetization reversal mechanism of heterogenous grain boundary diffused Nd-Fe-B sintered magnets

The grain boundary diffusion (GBD) method could effectively improve the coercivity of Nd-Fe-B sintered magnets, but the uneven distribution of heave rare earth (HRE) elements would lead to the decrease of squareness factor of the demagnetization curve of the magnets, which limited the application of this method. In this paper, three typical-structure regions were summarized based on microstructure characterization, and the magnetic properties and magnetization reversal mechanism were analyzed. The differences in nucleation positions and magnetization reversal processes of the three regions were discussed, and it found that the remarkable differences among them resulted in the low squareness factor of GBD Nd-Fe-B sintered magnets. In addition, the efficiency of Tb elements could be further improved to enhance the coercivity of Nd-Fe-B sintered magnets by forming homogeneous and complete core-shell structure grains, which could strengthen the nucleation field and even transfer the nucleation position from the grain surface defect layer to the grain interior region. Our results could provide guidance and reference for further improving the permanent magnetic properties of Nd-Fe-B sintered magnets by optimized GBD method.

First-Order Reversal Curves of Sets of Bistable Magnetostrictive Microwires.

Amorphous microwires have attracted substantial attention in the past decade because of their useful technological applications. Their bistable magnetic response is determined by positive or negative magnetostriction, respectively. First-order reversal curves (FORC) are a powerful tool for analyzing the magnetization reversal processes of many-body ferromagnetic systems that are essential for a deeper understanding of those applications. After theoretical considerations about magnetostatic interactions among microwires, this work introduces a systematic experimental study and analysis of the FORC diagrams for magnetostrictive microwires exhibiting an individually bistable hysteresis loop, from a single microwire to sets of an increasing number of coupled microwires, the latter considered as an intermediate case to the standard many-body problem. We performed the study for sets of quasi-identical and different hysteretic microwires where we obtained the coercivity Hc and interaction Hu fields. In the cases with relevant magnetostatic interactions, FORC analysis supplies deeper information than standard hysteresis loops since the intrinsic fluctuations of the switching field generate a complex response. For sets of microwires with very different coercivity, the coercivity distributions of the individual microwires characterize the FORC diagram.

Pinning of vortices during the passage of turbulent magnetization reversal waves in antidot films with through and non-through holes

Micromagnetic modeling of the process of magnetization reversal in films with antidot arrays, initiated by a moving domain wall, is performed. Under certain conditions, a narrow magnetization reversal zone appears, filled with vortex structures (turbulent wave). Changes in speed, temporary or final stops of the wave are observed. The existence of a connection between the features of wave motion and the structure of the vortex zone is shown.

Disentangling the magneto-optic Kerr effect of manganite epitaxial heterostructures

The magneto-optic Kerr effect can probe the process of magnetization reversal in ferromagnetic thin films and can, thus, be used as an alternative to magnetometry. The Kerr effect is wavelength-dependent and the Kerr rotation can reverse sign, vanishing at particular wavelengths. We investigate the epitaxial heterostructures of ferromagnetic manganite, La0.7Sr0.3Mn0.9Ru0.1O3, by using the polar Kerr effect and magnetometry. The manganite layers are separated by or interfaced with a layer of nickelate, NdNiO3. The Kerr rotation hysteresis loops of trilayers, with two manganite layers of different thicknesses separated by a nickelate layer, have intriguing humplike features when measured with light of 400 nm wavelength. By investigating additional reference samples, we disentangle the contributions of the individual layers to the loops: we show that the humps originate from the opposite sense of the Kerr rotation of the two different ferromagnetic layers, combined with the additive behavior of the Kerr signal. The change of sign of the Kerr rotation for the thinner manganite layer is most likely caused by optical interference.

Magnetic skyrmions in FePt square-based nanoparticles around room-temperature

Magnetic skyrmions formed at temperatures around room temperature in square-based parallelepiped magnetic FePt nanoparticles with perpendicular magnetocrystalline anisotropy (MCA) were studied during the magnetization reversal process using micromagnetic simulations. Finite differences method were used for the solution of the Landau–Lifshitz–Gilbert equation. Magnetic configurations exhibiting Néel skyrmionic formations were detected. The magnetic skyrmions can be created in different systems by the variation of external field, side length and width of the squared-based parallelepiped magnetic nanoparticles. Micromagnetic configurations revealed a variety of states which include skyrmionic textures with one distinct skyrmion formed and stabilized for a range of external fields around room-temperature. The size of the nucleated Néel skyrmion is calculated as a function of the external field, temperature, MCA and nanoparticle’s geometrical characteristic lengths which can be adjusted to produce skyrmions on demand having diameters down to 12 nm. The micromagnetic simulations revealed that stable skyrmions in the temperature range of 270–330 K can be created for FePt magnetic nanoparticle systems lacking of chiral interactions such as Dzyaloshinskii–Moriya.

Magnetization reversal mechanisms in highly corrugated thin films

Nanopatterned ferromagnetic (FM) thin films have specific characteristics that make them a workhorse for sensors based on magnonic, magnetoplasmonic, or anisotropic magneto-resistive effects. Undulated FM thin films have been studied because of their tunable uniaxial anisotropy. They have been traditionally understood by means of Schlömann's model taking account of shape-induced magnetic anisotropies in softly corrugated systems. Here, we show how it cannot describe accurately the magnetic behavior of highly corrugated FM systems within a thickness region of less than the ripple amplitude. We report on the magnetization reversal processes detected in Permalloy films deposited onto highly corrugated patterns (250 nm in periodicity, 180 nm in amplitude) in a wide thickness range (15–150 nm), finding both that the anisotropy of the system does not correspond to a uniaxial type for FM thicknesses larger than 40 nm and that the anisotropy of the system increases with the FM thickness. Based on the results, we hypothesize that whereas Schlömann's model is valid for softly corrugated thin films, it fails to explain magnetization reversal processes of highly corrugated thin films, especially when the ripple amplitude is much greater than the deposited FM layer thickness. By means of micromagnetic simulations, we find an increment of anisotropy with thickness, just as in the experimental, as well as determine the arise of magnetic domains at the ridges of high thickness corrugated FM thin films. This approach will help to get a better understanding of operating mechanisms in magnetic field sensors based on undulated ferromagnetic materials.

Coercivity Mechanism and Magnetization Reversal Process in Nanocomposite Pr2Fe14B/α‐Fe Alloy with Phosphorous Addition

An investigation is conducted for the coercivity mechanism in as‐spun Pr2Fe14B/α‐Fe ribbons containing different phosphorus (P) prepared at a wheel speed of 30 ms−1. With the increase of P addition, the volume of the amorphous phase in the ribbons is increased, and the grain size of Pr2Fe14B is decreased. The hysteresis loops of the ribbons with 0.1 at% P exhibit a homogeneous magnetization reversal process similar to single‐phase magnet, demonstrating that soft magnetic grains are fully coupled with the hard ones. The highest coercivity of 12.0 kOe (300 K)–38.0 kOe (100 K) is achieved in the ribbon with 0.3 at% P‐addition, which shows weak exchange coupling interaction between grains, and the high coercivity comes mainly from the decoupled hard phase. The values of the microstructure parameter in all compositions are less than 0.3 and imply that the main coercivity mechanism is the mixture of the nucleation of reverse domain and domain wall pinning. The minor hysteresis loops and initial magnetization curves show the dominant domain wall pinning in the ribbon with 0–0.1 at% P, nucleation and pinning in the ribbon with 0.3 at% P, and dominant domain nucleation in the ribbon with 0.5 at% P.

Magnetic structure and interaction of nickel nanowires studied by electron holography and micromagnetic simulation

In this study, high resolution transmission electron microscopy (HR-TEM) was used to analyze the crystal structure of nickel nanowires with a diameter of 40 nm. The micromagnetic structure of the nickel nanowire was analyzed using Lorentz microscopy and off-axis electron holography. This method is confirmed to be an effective means to obtain intuitive information of low dimensional magnetic nanomaterials. To investigate the influence factors in the magnetization reversal process, dynamic magnetization simulations were performed on an individual nanowire and a two nanowires system, respectively. By comparing the experimental data with simulation results, we find that the magnetostatic interactions between adjacent nanowires have significant influence on the magnetization behavior of nanowire arrays.

Micromagnetic Simulation of Increased Coercivity of (Sm, Zr)(Co, Fe, Cu)z Permanent Magnets

The finite element micromagnetic simulation is used to study the role of complex composition of 2:17R-cell boundaries in the realization of magnetization reversal processes of (Sm, Zr)(Co, Cu, Fe)z alloys intended for high-energy permanent magnets. A modified sandwich model is considered for the combinations of 2:7R/1:5H phase and 5:19R/1:5H phase layers as the 2:17R-cell boundaries in the alloy structure. The results of the simulation represented in the form of coercive force vs. total width of cell boundary showed the possibility of reaching the increased coercivity at the expense of 180°-domain wall pinning at the additional barriers within cell boundaries. The phase and structural states of the as-cast Sm1-xZrx(Co0.702Cu0.088Fe0.210)z alloy sample with x = 0.13 and z = 6.4 are studied, and the presence of the above phases in the vicinity of the 1:5H phase was demonstrated.

Temperature dependence of magnetization reversal mechanism in misch-metal substituted Nd-Fe-B magnets sintered by dual alloy method

The substitution of Nd by misch-metal (MM) in Nd-Fe-B permanent magnets has attracted extensive attention due to the effective reduction of production cost and rational utilization of rare earth resources. In particular, multi-main phase (MMP) magnets prepared by dual alloy method possess a higher permanent magnetic performance than that of single main phase (SMP) magnets. However, the coercivity mechanism of MMP magnets is different from the nucleation and propagation of SMP magnets, especially the interactions of different 2:14:1 main phases have not been clarified. In this study, we investigated the temperature dependence of magnetization reversal mechanism in (MM,Nd)-Fe-B magnets by using recoil loops, minor hysteresis loops, first-order and second-order reversal curves. It found that with the increase of temperature, the magnetic moments motion is mainly affected by long-range magnetostatic interaction rather than exchange coupling. The weakened coupling interaction makes the demagnetization process of softer and harder magnetic phases asynchronous. The thermal activation energy barrier index related to coercivity mechanism is 1.45, 1.18 and 1.0 at 200 K, 300 K and 380 K, respectively. It indicates that nucleation and expansion of reversed domains are the main mechanisms controlling the magnetization reversal process at low temperature. While the coercivity at high temperature is mainly determined by the domain wall pinning mechanism. In addition, we noted that both two coercivity mechanisms coexist in MMP magnets at 300 K. Micromagnetism has also been used to verify the coercivity mechanism of (MM,Nd)-Fe-B magnets sintered by dual alloy method, which is of great significance for improving the magnetic properties of high abundance rare earth magnets.

Key Parameters for Detectivity Improvement of Low Noise Anisotropic Magnetoresistive Sensors Made of La2/3Sr1/3MnO3 Single Layers on Vicinal Substrates

The current trend in magnetoresistive sensors development is to increase the sensitivity of single sensing elements by using multilayer structures and to design them into arrays. Such arrays are designed to compensate the excess low frequency noise of individual elements, which limits their magnetic resolution. Here, we report the modeling, design, and fabrication of single layer anisotropic magnetoresistive (AMR) sensors using low noise epitaxial La2/3Sr1/3MnO3 (LSMO) oxide thin films deposited on vicinal SrTiO3 substrates. The fabrication process is simple, and the operation of the sensor is based on a step-induced uniaxial magnetic anisotropy, described using the Stoner–Wohlfarth model. A coherent magnetization reversal process is observed by magneto-optical Kerr effect imaging. A good agreement between experimental data and the expected sensor response confirms the correct operation of the device. Three main fabrication parameters, namely the vicinal angle of the substrate, the deposition temperature, the thin film thickness, and their effects on film anisotropy field and device detectivity have been studied. Detectivity levels as low as 1.4 nT Hz–1/2 at 1 Hz and 240 pT Hz–1/2 in the white noise region are achieved with a single Wheatstone bridge element operating at 310 K. Compared to GMR and AMR sensors, these results are promising for further development and for their use as single layer LSMO low field AMR sensors, including applications as implantable biomedical devices.

Influence of Physical Symmetries on the Magnetization Dynamics in Magnetic Fibers

Magnetic nanofibers belong to the geometries which are intensively investigated in simulations and experiments due to their unique magnetic properties, varying in their lengths, cross-sections, and bending radii. Besides basic research of different magnetization reversal processes and magnetization dynamics in bent nanofibers, these structures are of potential interest for data storage applications, data transport, or other tasks in spintronics devices. While previous simulations concentrated on the domain wall transport through coupled bent nanofibers, creating networks with many in- and outputs to establish nanofiber-based domain wall logics, here we show the influence of the constricted area, in which a rotating magnetic field is applied in the middle of bent or straight magnetic nanofibers, on the magnetization dynamics. Our micromagnetic simulations, performed by Magpar, reveal a strong impact not only of this area, but also of the curvature of the nanofiber as well as of an additional Dzyaloshinskii–Moriya interaction (DMI).