Micromagnetic Simulations Of Magnetization Reversal Research Articles

Magnetization reversal and domain wall pinning in nanowires with nonlinear modulation of diameter

Micromagnetic simulation of magnetization reversal is studied on cylindrical tapered nanowire with radii 50nm and 10nm at wider and narrower end respectively. The radius decreases from wider end to the narrower end non-linearly as R-βzδ where δ is the degree of non-linearity. Value of δ is either an integer (n) or reciprocal of integer (1/n). At remanence, a vortex and flower state at the wider and narrower extremes respectively are exhibited combined with ferromagnetic ordering in the majority of volume. When δ is below 1/2, pinning and depinning of domain wall drives the hysteresis following a Modified Kondorsky model. Otherwise, the magnetization reversal proceeds with nucleation and propagation of vortex domain wall according to Kondorsky model. The propagation of domain wall is dependent upon the value of δ which suggests tunability of magnetic characteristics with modulation of diameter. For instance, large values of coercivity is observed for wires that have strong domain wall pinning which is considerably reduced when depinning and propagation of domain wall takes over. A large range (20kA/m to 130kA/m) of coercivity can be obtained by changing the modulation.

Micromagnetic simulations of magnetization reversal in kagome artificial spin ice

This work reports micromagnetic simulations used to study magnetization reversal in kagome artificial spin ice, taking into account the actual edge imperfections presented by the magnetic nanoislands as a source of disorder. An important advantage of the micromagnetic approach is to discretize a magnetic element in nanometric domains, allowing access to the sample magnetic moments distribution in detail. The limit case of zero disorder is accessed considering a system composed of perfect magnetic islands. The main result is the prediction of a critical angle between the applied magnetic field and the direction of one of the sublattices, above which Dirac strings emerge. For lower values, the reversal occurs through a bidimensional fashion, being smooth or abrupt depending on whether or not roughness is present, respectively. Finally, our simulations open the question of how the contour imperfections really influence the magnetic response of artificial spin ice.

Magnetization Reversal Modes in Short Nanotubes with Chiral Vortex Domain Walls.

Micromagnetic simulations of magnetization reversal were performed for magnetic nanotubes of a finite length, L, equal to 1 and 2 μm, 50 and 100 nm radii, R, and uniaxial anisotropy with “easy axis” parallel to the tube length. I.e., we considered relatively short nanotubes with the aspect ratio L/R in the range 10–40. The non-uniform curling magnetization states on both ends of the nanotubes can be treated as vortex domain walls (DW). The domain wall length, Lc, depends on the tube geometric parameters and the anisotropy constant Ku, and determines the magnetization reversal mode, as well as the switching field value. For nanotubes with relative small values of Lc (Lc/L < 0.2) the magnetization reversal process is characterized by flipping of the magnetization in the middle uniform state. Whereas, for relative large values of Lc, in the reverse magnetic field, coupling of two vortex domain walls with opposite magnetization rotation directions results in the formation of a specific narrow Néel type DW in the middle of the nanotube. The nanotube magnetization suddenly aligns to the applied field at the switching field, collapsing the central DW.

Micromagnetic simulation of magnetization reversal on the annular free layer with nick in magnetic random access memory

Based on magnetic tunnel junctions (MTJs), the magnetic random access memory with the pseudo-spin value film model, the annular structure with slanted cuts is used as free layer and the way to vary coercivity by changing thickness is discarded. With this improvement, the area resistance of the MTJs is reduced. The analysis of the cuts on the annular layer generated from the secondary effects of deposition in the IC process, is made by the micromagnetic simulations. The magnetization reversal characteristics from the analysis reveal the properties of low crosstalk, low RA, high magnetic reluctance, and strong anti-interference.

Magnetic-field-orientation dependent magnetization reversal and spin waves in elongated permalloy nanorings

The field-orientation dependence of magnetization reversal and spin wave modes in nanorings has been investigated by focused magneto-optical Kerr effect spectroscopy, Brillouin spectroscopy, and micromagnetic simulations. The samples studied are periodic arrays of 150 nm wide Ni80Fe20 rings with long axes of 850, 1200, and 1500 nm, and respective short axes of 550, 700, and 900 nm. For the smallest ring, the onion-to-vortex transition is observed for both the applied magnetic field directed along, and 5° from its magnetization easy axis. In contrast for the two larger rings, this transition is not observed for the field in the latter orientation. Splitting of spin wave modes is observed for the transition from the onion to vortex state. Micromagnetic simulations of magnetization reversal and spin waves are in good agreement with experiments. The simulated temporal recording of magnetization distributions during transition reveals that different switching mechanisms result from different applied magnetic field orientations.

Magnetic vortices in nanowires with transverse easy axis

We present magnetotransport measurements as well as micromagnetic simulations of magnetization reversal in magnetic nanowires having strong transverse magnetocrystalline anisotropy. The interplay between exchange, demagnetizing, and magnetocrystalline energies gives rise to a micromagnetic configuration involving vortices with alternative chirality along the wire. Despite the complexity of the field angle-dependent magnetization reversal process, a very good agreement is obtained between experiments and simulations. This provides evidence that the reported magnetization process appears generally in nanowires with strong transverse magnetocrystalline anisotropy. Moreover an analytical expression is established for the angular dependency of the core switching field. The simulations indicate the occurrence of Bloch points during the core reversal.

Micromagnetic simulation of magnetization reversal in submicron dots with laterally gradient magnetization

Micromagnetic simulation is performed on a ferromagnetic submicron dot with lateral gradient magnetization. The simulations show that the sub-dot with lateral gradient magnetization produces a two-stage magnetization reversal during nucleation. It is different from the magnetization reversal process in a dot with lateral uniform magnetization, which has only one sharp jump in magnetization, corresponding to an irreversible transition from a collinear to a vortex spin structure. The reversal process of the ferromagnetic dot with lateral gradient magnetization is also dependent on the direction of applied magnetic field.

Micromagnetic simulation of magnetization reversal in ultrathin Co magnetic films on Si(1 1 1) vicinal substrates

Micromagnetic simulations were performed to describe the processes for reversal of the magnetization of ultrathin Co magnetic films on Si(1 1 1) vicinal substrates. The micromagnetic model is discussed, taking into account the following contributions to the total magnetic anisotropy energy: perpendicular uniaxial anisotropy, step-induced uniaxial in-plane anisotropy and magnetocrystalline anisotropy. There is good agreement between the results of the simulation and the experiment.

Micromagnetic Study of Switching in Ring-Shaped Spin Valve Structures

In this paper, we present a micromagnetic simulation of magnetization reversal in a ring-shaped pseudo-spin valve (PSV), consisting of a storage layer made of permalloy (3 nm), a pinned layer of Co (3 nm), and a Cu spacer layer (3 nm). The transition between the two vortex stable states of the storage layer is modeled by applying a write current pulse. This pulse generates an Oersted field and an effective exchange field due to spin torque. The switching mechanism is investigated for different amplitude and duration of the current pulse, and the conditions for the desired switching behavior is discussed. Ideal switching behavior is obtained for a write pulse of at least 3 ns.

Micromagnetic simulation of magnetization reversal in small particles with surface anisotropy

In order to study the effect of surface anisotropy, we have developed a three-dimensional finite element micromagnetic model. The finite element mesh of the particles is split into a core region with bulk properties and a surface region (shell) of variable thickness with modified material parameters. We have studied the magnetization reversal in FePt nanoparticles. The effect of surface anisotropy strongly reduces their coercivity. An Fe–oxide shell with low anisotropy in the plane of the surface has been considered as well as surface anisotropy with high anisotropy constants and axis orientation perpendicular to the surface.

Micromagnetic simulation of magnetization reversal in controlled mesoscopic ferromagnetic junction

Magnetisation switching process and corresponding domain structures of mesoscopic ferromagnetic junction with different thickness are studied with micromagnetic simulation using Landau–Lifshiz–Gilbert equations. It is demonstrated by simulation results that the reversal process and switching field are dominated by the wide pad parts and only weakly depend on the wire parts.