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(tIr)/[(Ni/Co)3are 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(Hex) is observed when the thickness of Ir layer(tIr) 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 tIrincreasing, 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.

Magnetoresistive behavior and magnetization reversal of NiFe/Cu/CoFe/IrMn spin valve GMRs in nanoscale

The magnetoresistance behavior and the magnetization reversal mode of NiFe/Cu/CoFe/IrMn spin valve giant magnetoresistance （SV-GMR） in nanoscale were investigated experimentally and theoretically by nanosized magnetic simulation methods. Based on the Landau-Lifshitz-Gilbert equation, a model with a special gridding was proposed to calculate the giant magnetoresistance ratio （MR） and investigate the magnetization reversal mode. The relationship between MR and the external magnetic field was obtained and analyzed. Studies into the variation of the magnetization distribution reveal that the magnetization reversal mode, that is, the jump variation mode for NiFe/Cu/CoFe/IrMn, depends greatly on the antiferromagnetic coupling behavior between the pinned layer and the antiferromagnetic layer. It is also found that the switching field is almost linear with the exchange coefficient.

Gate-Voltage-Induced Magnetization Reversal and Tunneling Anisotropic Magnetoresistance in a Single Molecular Magnet with Temperature Gradient

We study the coutrol of gate voltage over the magnetization of a single-molecule magnet （SMM） weakly coupled to a ferromagnetic and a normal metal electrode in the presence of the temperature gradient between two electrodes. It is demonstrated that the SMM＇s magnetization can change periodically with periodic gate voltage due to the driving oI the temperature gradient. Under an appropriate matching of the electrode polarization, the temperature difference and the pulse width of gate voltage, the SMM＇s magnetization can be completely reversed in a period of gate voltage. The corresponding flipping time can be controlled by the system parameters. In addition, we also investigate the tunneling anisotropic magnetoresistance （TAMFt） of the device in the steady state when the ferromagnetic electrode is noncollinear with the easy axis of the SMM, and show the jump characteristic of the TAMR.

Magnetization reversal of ultrathin Fe film grown on Si（111） using iron silicide template

Ultrathin Fe films were epitaxially grown on Si（lll） by using an ultrathin iron silicide film with p（2 × 2） surface reconstruction as a template. The surface structure and magnetic properties were investigated in situ by low energy electron diffraction （LEED）, scanning tunnelling microscopy （STM）, and surface magneto-optical effect （SMOKE）. Polar SMOKE hysteresis loops demonstrate that the Fe ultrathin films with thickness t 〈 6 ML （monolayers） exhibit perpen-dicular magnetic anisotropy. The characters of M-H loops with the external magnetic field at difference angles and the angular dependence of coercivity suggest that the domain-wall pinning plays a dominant role in the magnetization reversal process.

Thermally activated magnetization reversal in magnetic tunnel junctions

In this paper, the magnetization reversal of the ferromagnetic layers in the IrMn/CoFe/AlOx/CoFe magnetic tunnel junction has been investigated using bulk magnetometry. The films exhibit very complex magnetization processes and reversal mechanism. Thermal activation phenomena such as the training effect, the asymmetry of reversal, the loop broadening and the decrease of exchange field while holding the film at negative saturation have been observed on the hysteresis loops of the pinned ferromagnetic layer while not on those of the free ferromagnetic layer. The thermal activation phenomena observed can be explained by the model of two energy barrier distributions with different time constants.

SIMULATION OF THE MAGNETIZATION REVERSAL PROCESS OF RECTANGLE-SHAPED NiFe FILM ELEMENTS UNDER AN ORTHOGONAL MAGNETIC FIELD

The magnetization reversal process of nano-size rectangle-shaped NiFe film elements with different aspect ratios have been investigated under the orthogonally applied magnetic fields by micromagnetic simulation. Different magnetization reversal modes can appear depending on whether the bias field is applied or not. When there is no bias field, double “C” state is the initial reversal state. However, when there is a bias field, “S” state is the starting mode. The larger the aspect ratio is, the larger the switching field is. But, when the aspect ratio is larger than 3, the increase of the switching field ceases. These results can provide useful information to the application of the patterned NiFe film with rectangular elements.

In-plane current-induced magnetization reversal of Pd/CoZr/MgO magnetic multilayers

High critical current density(>10^(6)A/cm^(2))is one of major obstacles to realize practical applications of the currentdriven magnetization reversal devices.In this work,we successfully prepared Pd/CoZr(3.5 nm)/MgO thin films with large perpendicular magnetic anisotropy and demonstrated a way of reducing the critical current density with a low out-of-plane magnetic field in the Pd/CoZr/MgO stack.Under the assistance of an out-of-plane magnetic field,the magnetization can be fully reversed with a current density of about 10^(4)A/cm^(2).The magnetization reversal is attributed to the combined effect of the out-of-plane magnetic field and the current-induced spin-orbital torque.It is found that the current-driven magnetization reversal is highly relevant to the temperature owing to the varied spin-orbital torque,and the current-driven magnetization reversal will be more efficient in low-temperature range,while the magnetic field is helpful for the magnetization reversal in high-temperature range.

Magnetization Reversal for Ni Nanowires Studied by Micromagnetic Simulations

The magnetization reversal mechanisms for Ni nanowires with different diameters were investigated by micromagnetic simulations. The results show that the reversal mechanisms are significantly dependeht on the diameter of wire. For very thin wires, the reversal occurs by pseudo-coherent rotation. With increasing diameter, magnetization reversal takes place via different nucleation （the transverse domain wall and the vortex domain wall） and subsequent propagation. The reason of transition from the transverse domain wall to the vortex domain wall is given by analytical studies. With further increase of the diameter, the reversal nuclear domain wall becomes tundishoshaped form. As the diameter increases, the width of wall becomes larger.

Investigation of magnetization reversal process in pinned CoFeB thin film by in-situ Lorentz TEM

Exchange bias effect has been widely employed for various magnetic devices.The experimentally reported magnitude of exchange bias field is often smaller than that predicted theoretically,which is considered to be due to the partly pinned spins of ferromagnetic layer by antiferromagnetic layer.However,mapping the distribution of pinned spins is challenging.In this work,we directly image the reverse domain nucleation and domain wall movement process in the exchange biased Co Fe B/Ir Mn bilayers by Lorentz transmission electron microscopy.From the in-situ experiments,we obtain the distribution mapping of the pinning strength,showing that only 1/6 of the ferromagnetic layer at the interface is strongly pinned by the antiferromagnetic layer.Our results prove the existence of an inhomogeneous pinning effect in exchange bias systems.

Magnetic anisotropy and magnetization reversal of ultrathin iron films with in-plane magnetization on Si（111） substrates

The magnetic anisotropy and magnetization reversal of single crystal Fe films with thickness of 45 monolayer （ML） grown on Si（111） have been investigated by ferromagnetic resonance （FMR） and vibrating sample magnetometer （VSM）. Owing to the significant modification of the energy surface in remanent state by slight misorientation from （111） plane and a uniaxial magnetic anisotropy, the azimuthal angular dependence of in-plane resonance field shows a six-fold symmetry with a weak uniaxial contribution, while the remanence of hysteresis loops displays a two-fold one. The competition between the first and second magnetoerystalline anisotropies may result in the switching of in-plane easy axis of the system. Combining the FMR and VSM measurements, the magnetization reversal mechanism has also been determined.

Topology-like dynamical behavior of magnetization reversal in exchange-bias systems

In an exchange-bias system, the barriers with intrinsic potential energy may be asymmetric due to unidirectional anisotropy. Based on the Stoner–Wohlfarth model, we show that the asymmetric barriers may lead to four kinds of dynamical process underlying the hysteresis-loop measurement. These kinds of dynamical processes are different in a topology-like property, which can be controlled by the orientation of the external field. In our study, a new analysis approach has been proposed to reveal the dynamical behaviors of magnetization reversal. With this approach, coercivity, exchange-bias field, and asymmetry of hysteresis loops can be quantitatively obtained.

Current-induced synchronized magnetization reversal of two-body Stoner particles with dipolar interaction

We investigate magnetization reversal of two-body uniaxial Stoner particles, by injecting spin-polarized current through a spin-valve structure. The two-body Stoner particles perform synchronized dynamics and can act as an information bit in computer technology. In the presence of magnetic dipole–dipole interaction（DDI） between the two particles,the critical switching current Ic for reversing the two dipoles is analytically obtained and numerically verified in two typical geometric configurations. The Ic bifurcates at a critical DDI strength, where Ic can decrease to about 70% of the usual value without DDI. Moreover, we also numerically investigate the magnetic hysteresis loop, magnetization self-precession,reversal time and synchronization stability phase diagram for the two-body system in the synchronized dynamics regime.

Magnetization Reversal Process of Single Crystal a-Fe Containing a Nonmagnetic Particle

The magnetization reversal process and hysteresis loops in a single crystal α-iron with nonmagnetic particles are simulated in this work based on the Landau-Lifshitz-Gilbert equation. The evolutions of the magnetic domain morphology are studied, and our analyses show that the magnetization reversal process is affected by the interaction between the moving domain wall and the existing nonmagnetic particles. This interaction strongly depends on the size of the particles, and it is found that particles with a particular size contribute the most to magnetic hardening.

Study of magnetization reversal and anisotropy of single crystalline ultrathin Fe/MgO (001) film by magneto-optic Kerr effect

The magnetization reversal process of Fe/MgO （001） thin film is investigated by combining transverse and longi- tudinal hysteresis loops. Owing to the competition between domain wall pinning energy and weak uniaxial magnetic anisotropy, the typical magnetization reversal process of Fe ultrathin film can take place via either an ＂l-jump＂ process near the easy axis, or a ＂2-jump＂ process near the hard axis, depending on the applied field orientation. Besides, the hysteresis loop presents strong asymmetry resulting from the variation of the detected light intensity due to the quadratic magneto-optic effect. Furthermore, we modify the detectable light intensity formula and simulate the hysteresis loops of the Kerr signal. The results show that they are in good agreement with the experimental data.

Electrically manipulating magnetization reversal via energy band engineering

Realization of a magnetization reversal by an external electric field is vital for developing ultra-low-power spintronic devices.In this report,starting from energy band engineering,a general design principle is proposed for achieving electrical manipulation of a nonvolatile 180°magnetization reversal.A half semiconductor(HSC)and a bipolar magnetic semiconductor(BMS)are selected as the model of magnetic layers,whose conduction-band minimum and valence-band maximum are in the same and opposite spin states,respectively.Based on the analysis of virtual hopping and tight-binding models,the interlayer coupling of HSC/insulator/BMS devices is successfully tuned between ferromagnetic and antiferromagnetic interactions by varying electric field directions.Moreover,the interlayer coupling nearly disappears after removing the electric field,proving the nonvolatile magnetization reversal.Using first-principles calculations,the feasibility of present design strategy is further confirmed by a representative device with the structure of CrBr3/h-BN/2H-VSe_(2).This design guideline and physical phenomena may open an avenue to explore magnetoelectric coupling mechanisms and develop next-generation spintronic devices.

Effect of zirconium content on exchange coupling and magnetization reversal of nanocrystalline Nd_(12.3)Fe_(81.7-x)Zr_xB_6 alloy

The effect of Zr content on exchange coupling and magnetization reversal of the Ndl2.3Fe81.7_xZrxB6 （x=0-3.0） ribbons was systematically investigated. Interaction domains were imaged by magnetic force microscopy （MFM）. The strength of interactions determined by Wohlfarth＇s analysis increased first with Zr content x increasing, reached the maximum value at x=l.5, and then decreased with x further increasing. Initial magnetization curves and dependence of coercivity and remanence on applied magnetic field showed that the mechanism of coercivity in all samples was mainly of exchange coupling pinning type, which was enhanced with x increasing. It was found by three-dimensional atom probe （3DAP） that Zr atoms did not partition into the Nd2Fe14B hard magnetic phase, but significantly enriched at the interfacial region.

Effect of niobium addition on magnetization reversal behavior for SmCo-based magnets with TbCu7-type structure

The effect of Nb addition on the microstructure and magnetic properties of nanocrystaUine Sm（CobaiNbxZr0.02）7 permanent magnet were investigated, The magnetization reversal behavior for ball milled Sm（CobaiNbxZr0.02）7 samples with high coercivity was investigated by analyzing hysteresis curves and recoil loops of demagnetization curves. Nb addition proved to result in relevant improvement in the magnetic properties, especially in the coercivity He. It was shown that the magnetic properties of Sm（CobalNbx- Zr0.02）7 nanocrystalline magnets were improved by an additional 0.06 at.% Nb. In particular, Hc was improved from 602 to 786 kA/m at room temperature. The maximum value of the integrated recoil loops area for 0.06 at.% Nb-doped samples of 1.81 kJ/m3 was much lower than that of the Nb-free sample, which could be explained by a smaller recoverable portion of the magnetization remaining in the Nb-doped sample when the applied field was below the coercivity Hc. The nucleation field Hn for irreversible magnetization reversal of the magnetically hard phase were calculated by analyzed in terms of the△Mirrev-H curve and the Kondorsky model.

Magnetization reversal behavior and first-order reversal curve diagrams in high-coercivity Zr-doped α-Fe/Nd_(2)Fe_(14)B nanocomposite alloys

In the present work,the magnetization reversal behavior for the melt spinning(Nd_(0.8)Ce_(0.2))_(2)Fe_(12)Co_(2-x)Zr_(x)B(x=0,0.5)permanent alloys with high coercivity was investigated by analyzing the hysteresis curves and the recoil loops.Compared to the Zr-free alloy,the Zr-doped sample obtains higher magnetic properties:coercivity of H_(cj)=650.5 kA·m^(-1),squareness of H_(k)/H_(cj)=0.76 and maximum energy product of(BH)_(max)=131.0 kJ·m^(-3).The first-order reversal curves(FORCs)analysis was taken to identify optimal conditions of exchange coupling for the Zr-free and Zr-doped alloys.The coercivity mechanism of theα-Fe/Nd_(2)Fe_(14)B nanocomposite alloys was analyzed by the angular dependence of the coercive field as measured for the Zr-doped sample.The results show that the magnetic reverse process of the Zr-doped sample can be explained by the pinning model.

Electric field induced magnetization reversal in magnet/insulator nanoheterostructure

Electric-field control of magnetization reversal is promising for lowpower spintronics.Here in a magnet/insulator nanoheterostructure which is the fundamental unit of magnetic tunneling junction in spintronics,we demonstrate the electric field induced 180°magne-tization switching through a multiscale study combining firstprinciples calculations and finite-temperature magnetization dynamics.In the model nanoheterostructure MgO/Fe/Cu with insu-lator MgO,soft nanomagnet Fe and capping layer Cu,through firstprinciples calculations we find its magnetocrystalline anisotropy linearly varying with the electric field.Using finite-temperature magnetization dynamics which is informed by the first-principles results,we disclose that a room-temperature 180°magnetization switching with switching probability higher than 90%is achievable by controlling the electric-field pulse and the nanoheterostructure size.The 180°switching could be fast realized within 5 ns.This study is useful for the design of low-power,fast,and miniaturized nanoscale electric-field-controlled spintronics.

An Improved Preisach Distribution Function Identification Method Considering the Reversible Magnetization

This paper presents an identification method of the scalar Preisach model to consider the effect of reversible magnetization in the process of distribution function identification.By reconsidering the identification process by stripping the influence of reversible components from the measurement data,the Preisach distribution function is identified by the pure irreversible components.In this way,the simulation accuracy of both limiting hysteresis loops and the inner internal symmetrical small hysteresis loop is ensured.Furthermore,through a discrete Preisach plane with a hybrid discretization method,the irreversible magnetic flux density components are computed more efficiently through the improved Preisach model.Finally,the proposed method results are compared with the traditional method and the traditional method considering reversible magnetization and validated by the laboratory test for the B30P105 electrical steel by Epstein frame.