Magnetization Reversal Process Research Articles

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).

Role of heterophase interfaces on local coercivity mechanisms in the magnetic Al0.3CoFeNi complex concentrated alloy

Microstructural features across different length scales have a profound influence on the coercivity of magnetic alloys. Whereas the role of homophase boundaries on the pinning of magnetic domain walls is well established, the influence of heterophase interfaces on domain wall motion is complex and poorly understood. Here, we use state-of-the-art electron microscopy techniques to show that the magnetization reversal process in an Al0.3CoFeNi magnetic complex concentrated alloy (CCA), which is responsible for its coercivity, changes dramatically from a nucleation-type mechanism in the FCC+L12 state of the CCA, with a domain wall width of 171 nm, to a pinning type mechanism in the microstructure with colonies of FCC/L12 nanorods embedded in a BCC/B2 matrix, with a domain wall width of 35 nm. Our work reveals that heterophase FCC/BCC interfaces have a much stronger effect on coercivity than isostructural chemically ordered/disordered interfaces and provides a powerful guide to the rational design of microstructure to tune magnetic properties in both complex concentrated alloys and conventional magnetic alloys.

Electric field control of magnetic states in ferromagnetic-multiferroic nanostructures.

Multiferroic oxides are considered as key elements of energy-consuming devices required for the development of scalable logic and information storage technologies. In this regard, understanding the mechanisms of magnetoelectric switching and finding the optimal way to switch magnetization by an electric field is of crucial importance. In this study, we develop a model for studying magnetic states in a nanoscale exchange-coupled ferromagnetic-multiferroic heterostructure subjected to the action of an electric field. Based on bias effects emerging due to the coupling between a ferromagnetic subsystem and an antiferromagnetically ordered multiferroic material, we explore the magnetic textures and the magnetization reversal processes in a ferromagnet. As the multiferroic material, we consider BiFeO3, where magnetic ordering and ferroelectric ordering are determined by the mutually perpendicular antiferromagnetic (L), weak ferromagnetic (M) and polarization (P) vectors. Application of an electric voltage removes degeneration from eight energetically equivalent positions of P|| 〈111〉, allocates the definite directions of vectors P, M, and L and as a consequence the unidirectional magnetic anisotropy axis in the reference ferromagnetic layer. Our study reveals the features of the magnetic configurations in systems of different geometries, with varying exchange and magnetic anisotropy, necessary to determine the optimal conditions for switching magnetic states in a multiferroic bi-layer by an electric field.

Crystal structure and peculiarities of microwave parameters of Co1-xNixFe2O4 nano spinel ferrites.

Nanosized spinel ferrites Co1-xNixFe2O4 (where x = 0.0-1.0) or CNFO have been produced using a chemical method. The crystal structure's characteristics have been determined through the utilization of X-ray diffraction (XRD). It has been demonstrated that all samples have a single phase with cubic syngony (space group Fd3̄m). The lattice parameter and unit cell volume behavior correlate well with the average ionic radii of Co2+ and Ni2+ ions and their coordination numbers. Thus, an increase in the Ni2+ content from x = 0.0 to x = 1.0 leads to a decrease in the lattice parameter (from 8.3805 to 8.3316 Å) and unit cell volume (from 58.86 to 57.83 Å3). Elastic properties have been investigated using Fourier transform infrared (FTIR) analysis. The peculiarities of the microwave properties have been analyzed by the measured S-parameters in the range of 8-18 GHz. It was assumed that the energy losses due to reflection are a combination of electrical and magnetic losses due to polarization processes (dipole polarization) and magnetization reversal processes in the region of inter-resonant processes. A significant attenuation of the reflected wave energy (-10 … -21.8 dB) opens broad prospects for practical applications.

Spin-phonon coupling and magnetic relaxation in single-molecule magnets.

Electron-phonon coupling is important in many physical phenomena, e.g. photosynthesis, catalysis and quantum information processing, but its impacts are difficult to grasp on the microscopic level. One area attracting wide interest is that of single-molecule magnets, which is motivated by searching for the ultimate limit in the miniaturisation of binary data storage media. The utility of a molecule to store magnetic information is quantified by the timescale of its magnetic reversal processes, also known as magnetic relaxation, which is limited by spin-phonon coupling. Several recent accomplishments of synthetic organometallic chemistry have led to the observation of molecular magnetic memory effects at temperatures above that of liquid nitrogen. These discoveries have highlighted how far chemical design strategies for maximising magnetic anisotropy have come, but have also highlighted the need to characterise the complex interplay between phonons and molecular spin states. The crucial step is to make a link between magnetic relaxation and chemical motifs, and so be able to produce design criteria to extend molecular magnetic memory. The basic physics associated with spin-phonon coupling and magnetic relaxation was outlined in the early 20th century using perturbation theory, and has more recently been recast in the form of a general open quantum systems formalism and tackled with different levels of approximations. It is the purpose of this Tutorial Review to introduce the topics of phonons, molecular spin-phonon coupling, and magnetic relaxation, and to outline the relevant theories in connection with both the traditional perturbative texts and the more modern open quantum systems methods.

Study of the Effect of Plastic Deformation and Annealing on the Complex of Magnetic Characteristics of Nickel

It is shown that the coercive force decreases monotonically by a factor of 25 with an increase in the annealing temperature. It has been found that the magnetostrictive sensitivity dλ/dH of nickel increases almost linearly by more than a factor of 3 with an increase in the annealing temperature. The complex of magnetic characteristics shows an increase in irreversible magnetization reversal processes with an increase in the annealing temperature of cold-deformed nickel.

Role of interfacial nickel silicides in shaping magnetic anisotropy in nickel films grown on Silicon substrates

Using different experimental techniques such as X-ray reflectivity, X-ray diffraction, X-ray photoelectron spectroscopy and magneto-optical Kerr effect (MOKE) microscopy, the structural and magnetic properties of Ni films grown on Si substrates are investigated. Various Ni-silicide phases are found to be formed at the Ni–Si interface as a function of depth following the sequence Ni-Ni2Si-NiSi-NiSi2 from the nickel to silicon side. This has been explained in terms of the effective heat of formation and inter-diffusion of the elements at the interface. In-plane magnetic anisotropy of the system is investigated by measuring in-plane magnetization using MOKE microscopy. The film exhibits an uniaxial in-plane magnetic anisotropy. An anomalous behavior, i.e., the collapse of the hard axis, has also been observed. This anomaly has been explained in terms of the creation of V-state domains during the magnetization reversal process at the hard axis. Interestingly, in-plane magnetic anisotropy could not be found in a Ni film grown with a Pt buffer layer on the Si substrate. Ni-silicide phases could not be detected in this system. This may suggests that the formation of silicides and diffusion at the interface causes the atypical magnetic anisotropy in Ni–Si sample.

Evidence for vortex state in Fe2CoGe thin films using FORC and magnetic imaging

We report on the evidence for the vortex state in the thin films of Fe2CoGe through first order reversal curves, magnetic force microscope, longitudinal magneto-optical Kerr effect and micro-magnetic simulations. Phase purity of the films confirmed through X-ray diffraction, which confirms the A2 type disorder Heusler alloy structure. Contour graph of first order reversal curves infers the formation of vortex state that is useful to understand magnetization reversal and switching process. We do observe the vortex state ∼1 μm with in – plane curling of the magnetization using magnetic force microscope phase analysis. We believe that realization of vortex state formation in Fe2CoGe thin films may cater applications in future magnetic data storage and microwave oscillators.

Visualization of magnetization reversal processes by dynamic Kerr microscopy

Abstract Dynamic Kerr microscopy enables the analysis of structural defects in magnetic materials and their impact on the demagnetization behavior. Using new visualization techniques, it is possible to image the demagnetization process in the microstructure. It has been found that residual stresses and inclusions in soft magnets, for example, will impede magnetization. In permanent magnets, abnormally large grains, for example, will demagnetize first and then cause an accelerated demagnetization of adjacent grains.

Tunable Perpendicular Magnetoresistive Sensor Driven by Shape and Substrate Induced Magnetic Anisotropy

AbstractControl of magnetization reversal processes is a key issue for the implementation of magnetic materials in technological applications. The modulation of shape magnetic anisotropy in nanowire structures with a high aspect ratio is an efficient way to tune sharp in‐plane magnetic switching. However, control of fast magnetization reversal processes induced by perpendicular magnetic fields is much more challenging. Here, tunable sharp magnetoresistance changes, triggered by out‐of‐plane magnetic fields, are demonstrated in thin permalloy strips grown on LaAlO3 single crystal substrates. Micromagnetic simulations are used to evaluate the resistance changes of the strips at different applied field values and directions and correlate them with the magnetic domain distribution. The experimentally observed sharp magnetic switching, tailored by the shape anisotropy of the strips, is properly accounted for by numerical simulations when considering a substrate‐induced uniaxial magnetic anisotropy. These results are promising for the design of magnetic sensors and other advanced magnetoresistive devices working with perpendicular magnetic fields by using simple structures.

Super-hierarchical and explanatory analysis of magnetization reversal process using topological data analysis

Super-hierarchical and explanatory analysis of magnetization reversal process using topological data analysis

Construction of high performance nanocomposites based on a shape anisotropic soft phase: A micromagnetic simulation study

Nanocomposite permanent magnets are considered as a strong candidate for the next generation of high-performance permanent magnet materials due to their ultra-high theoretical magnetic energy product. In this paper, a nanocomposite theoretical model based on the shape anisotropy of the soft phase (Fe65Co35) is constructed to guide the improvement of coercivity, the lack of which has become a critical problem in improving the performance of Nd-Fe-B nanocomposites further. The results of micromagnetic simulation show that adding a shape anisotropic soft phase to nanocomposites can effectively improve coercivity, delay nucleation during the magnetization reversal process, and help obtain a demagnetization curve with high squareness. When the length size of the soft phase ds ≤ 21 nm and the aspect ratio of the soft phase I = 5 for the Nd2Fe14B/Fe65Co35 nanocomposites, almost square demagnetization curves can be obtained, particularly when ds = 21 nm, the size of the soft phase is 21 × 21 × 105 nm3, the content of the soft phase is 42.1 vol. %, and the Nd2Fe14B/Fe65Co35 nanocomposite achieves a maximum magnetic energy product of 94.4 MGOe. In addition, the results also show that, compared with the cubic nanocomposite model (I = 1), the larger size of the soft phase can be accommodated into the nanocomposites by the addition of shape anisotropy, on the premise of ensuring the soft–hard coupling effect. Our design provides a new strategy and approach for preparing high-performance nanocomposite permanent magnets.

Temperature-Dependent Mechanism of Magnetization Reversal in the Spintronic Electrode Material La0.67Sr0.33MnO3

We investigate magnetization-reversal processes in half-metallic epitaxial films of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_{3}$ on ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ (110) substrates. At 150 K, major-loop magnetization reversal (MLMR) occurs beyond minor-loop switching fields, implying that MLMR is limited by domain nucleation. At 280 K, MLMR overlaps with minor-loop switching fields if there are deliberately introduced precipitates, implying that the precipitates limit MLMR by pinning domain walls. To confirm that the precipitates pin domain walls at 280 K, we use a two-dimensional analogue of the Gaunt pinning model to show that pinning-limited switching occurs at fields consistent with the observed precipitate density of 0.3--2.0 \ensuremath{\mu}m${}^{\ensuremath{-}2}$. Our work demonstrates that comparing major and minor loops at any temperature of interest represents a simple way to check whether spintronic electrode materials show nucleation-limited MLMR, as circumstantially required for single-domain switching in micron-scale elements of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_{3}$ that represent spintronic electrodes [L. C. Phillips et al., Phys. Rev. Appl. 4, 064004 (2015)].

Micromagnetic study of the effect of Dzyaloshinskii-Moriya interaction on magnetization reversal in magnetic nanodots of different shape

Thorough understanding of magnetization reversal process in perpendicularly magnetized nanostructures of different shapes in the presence of anti-symmetric exchange interaction is crucial in designing energy efficient magnetic storage devices. Here, we investigate the effect of interfacial Dzyaloshinskii-Moriya interaction (IDMI) and in-plane bias field (µ0H) on the magnetization reversal mechanism in perpendicularly magnetized square, circular and triangular nanodot of side length (diameter) 128 nm and thickness 1 nm using micromagnetic simulation Mumax3. Our results indicate a monotonic reduction in the coercive field with increase in the strength of IDMI and the nucleation of the reversal is observed at the edge of the nanodot irrespective of the shape. Additionally, the application of bias field pins the nucleation centre at the far edge of the dots and further lowers the energy barrier for the magnetization reversal with the extent of lowering related to the symmetry of the shape. Detailed analysis of energy profile reveals that due to the lateral asymmetry of the triangular nanodot (large spin canting), the total energy value is smaller for it at small magnitude of D (≤2 mJ/m2) and µ0H (≤100 mT) thereby making it preferred for energy efficient magnetization reversal in comparison to other two shapes. However, at sufficiently large magnitude of D (>2 mJ/m2) and µ0H (>100 mT), due to the laterally symmetric sides of the square (small spin canting), remarkable reduction of the energy barrier for the magnetization reversal is noticed. These findings indicate that the effect of D and µ0H in lowering the energy barrier for the magnetization reversal is intricately related to the symmetry associated with the shape of the nanodots. We thus believe that these results provide useful insight into the role of the shape of the nanoelement in designing spintronics devices where energy efficient magnetization reversal is required.

Feature extended energy landscape model for interpreting coercivity mechanism

Coercivity is expressed as a complex correlation between magnetisation and microstructures. However, owing to multiple intrinsic origins, coercivity has not been fully understood in the framework of the conventional Ginzburg–Landau theory. Here, we use machine learning to draw a realistic energy landscape of magnetisation reversal to consider missing parameters in the Ginzburg–Landau theory. The energy landscape in the magnetisation reversal process is visualised as a function of features extracted via machine learning; the correlation between the reduced feature space and hysteresis loop is assigned. Features in the lower dimension dataset strongly correlate with magnetisation and are embedded with morphological information. We analyse the energy landscape for simulated and experimental magnetic domain structures; a similar trend is observed. The landscape map enables visualisation of the energy of the system and coercivity as a function of feature space components.