Rotation Of Magnetization Vector Research Articles

Magnetization Vector Rotation Reservoir Computing Operated by Redox Mechanism.

Physical reservoir computing is a promising way to develop efficient artificial intelligence using physical devices exhibiting nonlinear dynamics. Although magnetic materials have advantages in miniaturization, the need for a magnetic field and large electric current results in high electric power consumption and a complex device structure. To resolve these issues, we propose a redox-based physical reservoir utilizing the planar Hall effect and anisotropic magnetoresistance, which are phenomena described by different nonlinear functions of the magnetization vector that do not need a magnetic field to be applied. The expressive power of this reservoir based on a compact all-solid-state redox transistor is higher than the previous physical reservoir. The normalized mean square error of the reservoir on a second-order nonlinear equation task was 1.69 × 10-3, which is lower than that of a memristor array (3.13 × 10-3) even though the number of reservoir nodes was fewer than half that of the memristor array.

Microstructural Model of Magnetic and Deformation Behavior of Single Crystals and Polycrystals of Ferromagnetic Shape Memory Alloy

In this article, a microstructural model of the Heusler alloy with the shape memory effect caused by the application of an external magnetic field is constructed. The dynamics of the magnetization process are described using the Landau–Lifshitz–Gilbert equation. For the numerical implementation of the model using the finite element method, the variational equations corresponding to the differential formulation of the magnetic problem are used. Such an approach makes it possible to reduce (weaken) the requirements for the smoothness of the sought solution. The problem of magnetization of single crystals of the Ni2MnGa alloy, which has a “herringbone”-type martensitic structure (a twinned variant of martensite), is considered. In each element of the twin, the magnetic domains with walls of a certain thickness are formed. The motion and interaction of these walls and the rotation of magnetization vector in the walls and domains under the action of the external differently directed magnetic fields are studied. These processes in the Heusler alloy are also accompanied by the detwinning process. A condition for the detwinning of a ferromagnetic shape memory alloy in a magnetic field is proposed, and the effect of the reorientation (detwinning) of martensitic variants forming a twin on the magnetization of the material and the occurrence of structural (detwinning) deformation in it are taken into account. First, the processes of magnetization and structural deformation in a single grain are considered at different angles between the anisotropy axes of twinned variants and the external magnetic field. For these cases, the magnetization curves are constructed, and the deformed states are identified. The model described such experimental facts as the detwinning process and the jump in magnetization on these curves as a result of this process. It was shown that the jump occurred at a certain magnitude of the strength of the applied external magnetic field and a certain direction of its action relative to the twinning system. Then, based on the obtained results, deformed states arising due to the detwinning process were determined for various (isotropic and texture-oriented) polycrystalline samples, and magnetization curves taking into account this process were constructed for these materials.

Magnetic properties of a Fe-based amorphous alloy with stress gradient

The magnetic properties and domain structure of a Fe78Si9B13 amorphous alloy with a stress gradient from upper to the lower surface were investigated. We found that the magnetic properties of the samples vary significantly upon stress variation. Specifically, the magnetic anisotropy increases linearly with the maximum stress, whereas the coercivity, although higher than the one without stress, remains reasonably small. The phase contrast in magnetic force microscopy image and the fingerprint-like domains in magneto-optical Kerr microscopy images for ribbon surface with compressive stress demonstrate the presence of perpendicular anisotropy due to the magneto-elastic coupling. As a result, the compressive stress hinders the reversible magnetization vector rotation and makes the magnetization hard to be saturated. However, the stress causes no irreversible effects, neither on the domain wall movement nor on the magnetization rotation. Thus it is irrelevant to the coercivity. These results are different from the situation of the inhomogeneously distributed quenched-in stress enlarging the coercivity in Fe-based amorphous alloys.

Shear-strain-induced over 90° rotation of local magnetization in FeCoSiB/PMN-PT (011) multiferroic heterostructures

Strain-mediated magnetoelectric effect can be utilized as an energy-efficient approach for spin manipulation. However, over 90° magnetization rotation is still challenging in un-patterned magnetic films, as the piezo-strain driven by ferroelectric domain switching is generally uniaxial rather than unidirectional, which limits the developments of non-volatile magnetic memory and logic devices. Here we demonstrate the rotation of local magnetization with a large angle of 136° by applying strains with a shear component at a fixed magnetic field of 45 Oe in FeCoSiB/PMN-PT (011) multiferroic heterostructures, revealed by a vector-resolved quantitative magneto-optical Kerr effect (MOKE) microscopy. Phase-field simulations confirm that the approximate 140° rotation of magnetization vectors is a consequence of the shear strain associated with ferroelectric/ferroelastic switching of PMN-PT (011) substrates. The visualization of over 90° magnetization rotation induced by the strain with a shear component paves the way for deterministic magnetization switching that has important implications in the energy-efficient spintronic devices.

Visualization of Magnetic Domains and Magnetization Vectors in Magnetic Shape Memory Alloys Under Magneto-Mechanical Loading

$$\hbox {Ni}_{2}\hbox {MnGa}$$ magnetic shape memory alloys have a microstructure consisting of martensite variants, and magnetic domains exist in each martensite variant. In the absence of a magnetic field, the magnetic domains are equally distributed so that the net magnetization of the material is zero. Application of a magnetic field or mechanical stress can rearrange the martensite variants and magnetic domains. This study focuses on understanding the effects that magnetic field, compressive stress and magneto-mechanical loading have on the magnetic domains, by measuring and quantifying magnetic domain volume fraction and magnetization vector rotation using direct imaging. In particular, a magneto-optical indicator film in conjunction with polarization microscopy was used to visualize the evolution of the magnetic domains and magnetization vector rotation of a $$\hbox {Ni}_{2}\hbox {MnGa}$$ sample for different load cases, namely varying magnetic field at constant strain, and varying compressive stress at a constant magnetic field. Our experiments revealed that the applied magnetic field causes change in domain volume fraction at different rates in each variant, and that domain wall motion is not always fully reversible. Magnetization vector rotation, however, was found to be reversible for all loading cases tested.

Three-dimensional constitutive model for magneto-mechanical deformation of NiMnGa ferromagnetic shape memory alloy single crystals

Existing experimental results have shown that four types of physical mechanisms, namely, martensite transformation, martensite reorientation, magnetic domain wall motion and magnetization vector rotation, can be activated during the magneto-mechanical deformation of NiMnGa ferromagnetic shape memory alloy (FSMA) single crystals. In this work, based on irreversible thermodynamics, a three-dimensional (3D) single crystal constitutive model is constructed by considering the aforementioned four mechanisms simultaneously. Three types of internal variables, i.e., the volume fraction of each martensite variant, the volume fraction of magnetic domain in each variant and the deviation angle between the magnetization vector, and easy axis are introduced to characterize the magneto-mechanical state of the single crystals. The thermodynamic driving force of each mechanism and the thermodynamic constraints on the constitutive model are obtained from Clausius’s dissipative inequality and constructed Gibbs free energy. Then, thermodynamically consistent kinetic equations for the four mechanisms are proposed, respectively. Finally, the ability of the proposed model to describe the magneto-mechanical deformation of NiMnGa FSMA single crystals is verified by comparing the predictions with corresponding experimental results. It is shown that the proposed model can quantitatively capture the main experimental phenomena. Further, the proposed model is used to predict the deformations of the single crystals under the non-proportional mechanical loading conditions.

Pure spin current manipulation in antiferromagnetically exchange coupled heterostructures

We present a model to describe the spin currents generated by ferromagnet/spacer/ferromagnet exchange coupled trilayer systems and heavy metal layers with strong spin-orbit coupling. By exploiting the magnitude of the exchange coupling (oscillatory RKKY-like coupling) and the spin-flop transition in the magnetization process, it has been possible to produce spin currents polarized in arbitrary directions. The spin-flop transition of the trilayer system originates pure spin currents whose polarization vector depends on the exchange field and the magnetization equilibrium angles. We also discuss a protocol to control the polarization sign of the pure spin current injected into the metallic layer by changing the initial conditions of magnetization of the ferromagnetic layers previously to the spin pumping and inverse spin Hall effect experiments. The small differences in the ferromagnetic layers lead to a change in the magnetization vector rotation that permits the control of the sign of the induced voltage components due to the inverse spin Hall effect. Our results can lead to important advances in hybrid spintronic devices with new functionalities, particularly, the ability to control microscopic parameters such as the polarization direction and the sign of the pure spin current through the variation of macroscopic parameters, such as the external magnetic field or the thickness of the spacer in antiferromagnetic exchange coupled systems.

Energy Losses in Composite Materials Based on Two Ferromagnets

We focused on the low and high induction loss separation in soft magnetic composites (SMC), which determine the regions of different predominant magnetization processes. These are the low induction losses which are related to domain wall displacements and the high induction losses related to magnetization vector rotation, domain wall annihilation, and nucleation. The composite samples prepared by replacing of Somaloy 700 (polycrystalline iron powder covered by insulating layer) with Vitrovac 6155 (amorphous co-based alloy) have been investigated. We found out that the porosity remained constant up to 5 wt.% of Vitrovac fraction, which led to slight improvement of magnetic properties of the resulting SMC-the maximum permeability increased and the magnetization process was confirmed to be realized to a greater extent by the processes of domain wall displacement, up to a higher magnetic induction. The higher inner demagnetizing fields caused by higher porosity in sample with 20 wt.% of Vitrovac resulted in lower mobility of domain walls, exhibiting by higher energy losses and lower maximum permeability.

Magnetoelastic phenomena in residually magnetized cobalt steel

Results of research into magnetoelastic change of residual magnetization of cobalt steel annealed at 200—800°C are provided. The value of irreversible magnetoelastic demagnetization is shown to decrease with annealing temperature and become nearly zero after annealing at 600–800°C. In this case, a quasi-reversible (with nearly no hysteresis) change of magnetization is observed. It is explained by the processes of rotation of magnetization vectors when a load is applied.

Vibrating sample magnetometer 2D and 3D magnetization effects associated with different initial magnetization states

Differences in VSM magnetization vector rotation associated with various initial magnetization states were demonstrated. Procedures and criteria were developed to select sample orientation and initial magnetization states to allow for the combination of two different 2D measurements runs (with the same field profiles) to generate a dataset that can be representative of actual 3D magnetization rotation. Nickel, cast iron, and low moment magnetic tape media were used to demonstrate these effects using hysteresis and remanent magnetization test sequences. These results can be used to generate 2D and 3D magnetic properties to better characterize magnetic phenomena which are inherently three dimensional. Example applications are magnetic tape-head orientation sensitivity, reinterpretation of 3D coercivity and other standard magnetic properties, and multi-dimensional shielding effectiveness.

Anisotropic magnetic entropy change in RFeO3 single crystals (R = Tb, Tm, or Y).

Compared with traditional gas-compression/expansion refrigeration, magnetic refrigeration based on magnetocaloric effect (MCE) exhibits the advantages of high energy efficiency and environment friendliness. Here, we created large MCE in RFeO3 (R = Tb or Tm) single crystals by the magnetization vector rotation of single crystal with strong magnetocrystalline anisotropy (MCA), rather than merely via the order-disorder magnetic phase transition or magnetic structural transition. Owing to the difference in charge distribution of 4f-electrons between Tb3+ and Tm3+ ions, the rotating field entropy with different signs, −ΔSMR = 17.42 J/kg K, and –ΔSMR = −9.01 J/kg K are achieved at 9 K and 17 K for TbFeO3 and TmFeO3 single crystals from b axis to c axis, at 50 kOe, respectively. The finding of the large anisotropic MCE not only advances our understanding of the anisotropy of MCE, but also extends the application for single crystals to magnetic refrigeration.

Stroboscopic imaging of an alternating magnetic field from a perpendicular magnetic recording head by frequency-modulated magnetic force microscopy

The alternating current (ac) magnetic field from perpendicular magnetic recording head was imaged by using our developed frequency-modulated magnetic force microscopy (FM-MFM), which uses the frequency modulation of cantilever oscillation caused by applying ac magnetic field to a mechanically oscillated cantilever. The amplitude and phase signals (or orthogonal X and Y signals) of the ac magnetic field can be obtained separately by using the FM-MFM technique incorporated with a lock-in amplifier. A signal transformation technique to characterize the AC magnetic field source for frequency modulated magnetic force microscopy was proposed. Two-dimensional vector analysis together with the magnetization vector rotation of the perpendicular recording head was demonstrated using the FM-MFM method combined with the signal transformation algorithm. By taking advantage of this technique, the present method opens a possibility to stroboscopic analyze the AC magnetic field source and gives a very useful feedback for the development of advanced magnetic recording heads.

Micromagnetic Simulation of Recording Write Heads a Comparison of Various Micromagnetic Software

The performance of three micromagnetic software were compared: (a) originally developed by Hitachi and refined at Kogakuin Univeristy and Niigata Institute of Technology, (b) developed by Fujitsu, and (c) Magpar, a free, public domain software. A perpendicular write head was modeled to compare the performance of these software. It was found that the finite-difference method (FDM) based micromagnetic software (a) had difficulties with complex structures but required less RAM, while finite-element, boundary integral method (FEM-BIM) based micromagnetic software (b) and (c) could handle complex structures but required more RAM. We derived the quasi-static recording field distributions and the dynamic recording field responses to high-frequency currents of up to 2.0 GHz. We found that there were no major differences in the head field distributions and responses, while FEM-BIM was superior to the FDM for observing magnetization vector rotation as FEM-BIM can handle complex structures with fine meshes in regions of interest.

Study of the magnetic interaction in nanocrystalline Pr–Fe–Co–Nb–B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr–Fe–Co–Nb–B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd2Fe14B grains.

Magnetic Properties of Crystalline NiFe Alloy Prepared by High-Energy Ball Milling and Compacting

The structure and magnetic properties of compacted microcrystalline NiFe (81 wt% of Ni) powder is investigated. The powder of NiFe alloy prepared by ball milling of ribbon (prepared by melt spinning) remains single phase material suitable for compaction in order to prepare soft magnetic material. The bulk samples were consolidated by uniaxial compaction of the powder in vacuum. By measuring of AC and DC magnetic properties it was found out that in bulk samples the displacement of domain walls is the dominant magnetization process, while rotation of magnetization vectors prevails in powder material.

Magnetization dependence on dynamic strain in ferromagnetic shape memory Ni–Mn–Ga

The characterization of commercial Ni–Mn–Ga for use as a dynamic deformation sensor is addressed. The flux density is experimentally determined as a function of cyclic strain loading at frequencies from 0.2to160Hz. With increasing frequency, the stress versus strain response remains almost unchanged whereas the flux density versus strain response shows increasing hysteresis. This behavior indicates that twin-variant reorientation occurs in concert with the mechanical loading, whereas the rotation of magnetization vectors occurs with a delay as the loading frequency increases. The increasing magnetization hysteresis must be considered when utilizing the material in dynamic sensing applications.

Investigation of the magnetization vector hodographs corresponding to the 90° pulse magnetization of ferrite-garnet films with easy-plane anisotropy

The magnetization vector hodographs corresponding to the regime of 90° pulse magnetization of ferrite-garnet films with easy-plane anisotropy have been analyzed. It is found that when the magnetizing pulse amplitude Hm exceeds some threshold value H*m (≈15 Oe), magnetization occurs through the mechanism of uniform magnetization vector rotation. The weak dependence of the intensity of nonlinear oscillations (accompanying the magnetization process) on the rise time of the magnetizing pulse, observed in the fields Hm > H*m, is explained by the presence of biaxial anisotropy in the plane of real films.

The magnetic properties of powdered and compacted microcrystalline permalloy

The aim of this work is to investigate the magnetic properties of powdered and compacted microcrystalline Ni–Fe (81 wt% of Ni) permalloy. It was found by investigating the influence of mechanical milling on the magnetic properties of powder samples prepared by milling of the ribbon that the alloy remains a solid solution with stable structure during the whole milling process. With decreasing particle size the rotation of magnetization vector gradually becomes dominant magnetization process and thus coercivity increases. After compaction of the powder by uniaxial hot pressing the magnetic contact between powder particles is recreated and for resulting bulk the displacement of the domain walls becomes dominant magnetization process with coercivity of 11 A/m (comparable with the coercivity of conventional permalloy).

Measuring the effective field of biaxial anisotropy in magnetic films magnetized in their plane on pulsed induction setups

The process of 90° pulsed magnetization is proposed to be used to measure the effective field of biaxial anisotropy HK2. The method has been developed as applied to ferrite-garnet films with a magnetization lying in their plane and is based on the determination of threshold field Hm0, at which magnetization is fully effected by rotation of magnetization vector M. The analysis performed in this study has shown that HK2 = 4(Hm0cosϕ* − H0sinϕ*)/sin4ϕ*, where H0 is the setting field determining the initial direction of vector M along one of the easy magnetization axes, and ϕ* is the critical angle of the initial rotation of vector M reached in field Hm0, beginning with which the magnetization rotation is not decelerated by anisotropy forces. It is shown that, for typical values of HK2, ϕ* ≅ 26°. The threshold field is determined from a kink in the dependence of the initial-peak amplitude in the longitudinal signal of 90° magnetization on the amplitude of magnetizing field Hm. For homogeneous films, the measurement accuracy of the effective field of biaxial anisotropy ensured in this method is no worse than 5–6%.

Magnetostrictive hysteresis of TbCo∕CoFe multilayers and magnetic domains

Magnetic and magnetostrictive hysteresis loops of TbCo∕CoFe multilayers under field applied along the hard magnetization axis are studied using vectorial magnetization measurements, optical deflectometry, and magneto-optical Kerr microscopy. Even a very small angle misalignment between hard axis and magnetic field direction is shown to drastically change the shape of magnetization and magnetostrictive torsion hysteresis loops. Two kinds of magnetic domains are revealed during the magnetization: big regions with opposite rotation of spontaneous magnetization vector and spontaneous magnetic domains which appear in a narrow field interval and provide an inversion of this rotation. We show that the details of the hysteresis loops of our exchange-coupled films can be described using the classical model of homogeneous magnetization rotation of single uniaxial films and the configuration of observed domains. The understanding of these features is crucial for applications (for microelectromechanical systems or microactuators) which benefit from the greatly enhanced sensitivity near the point of magnetic saturation at the transverse applied field.