Micromagnetic Configurations Research Articles

Tunnel magnetoresistance versus micromagnetism in magnetic tunnel junctions

The impact of the micromagnetic configuration within the ferromagnetic layers on transport properties of hard/soft magnetic tunnel junctions is presented. An artificial ferrimagnetic (AFi) trilayer structure is used as a magnetically hard subsystem. Fluctuations in magnetization in the AFi affect the resistance of the tunnel junctions and are fully reflected in the shape and amplitude of the tunnel magnetoresistance signal.

Magnetic ground state of a thin-film element

By means of three-dimensional numerical calculations we studied possible micromagnetic configurations in a rectangular Permalloy-like thin-film element. The parameters were chosen to be compatible with the so-called micromagnetic standard problem 1. We demonstrate that for these parameters a diamond domain pattern is the lowest energy state that replaces cross-tie patterns favorable in larger elements. Only at smaller sizes does the originally envisaged Landau pattern form the ground state. The transition to high-remanence structures (or what would be comparable to a single-domain state) is found for lateral sizes that are an order of magnitude smaller than the benchmark parameters. The transitions among the different domain patterns become plausible in view of the energy of symmetric Neel walls in extended thin films. The features of the high-remanence structures can be derived from the principle of uniform charge distribution.

Correlation between micromagnetism and tunnel magnetoresistance in magnetic tunnel junctions using artificial ferrimagnets

The impact of the micromagnetic configuration on transport properties of magnetic tunnel junctions is presented. An artificial ferrimagnet system (AFi), made of two ferromagnetic layers with different magnetic moments antiferromagnetically coupled by exchange interactions through a Ru spacer layer, is used as a hard magnetic subsystem of the magnetic junction. Magnetic-force microscopy studies have shown that the magnetization reversal in artificial ferrimagnet systems occurs through the formation of N\'eel-type $360\ifmmode^\circ\else\textdegree\fi{}$ domain walls. The stability of these walls is demonstrated to be different in the two magnetic layers of the AFi. These aspects have major effects in magnetic tunnel junction devices, due to the extreme sensitivity of spin tunneling to spatial variations in local magnetic order. The large tunnel magnetoresistance (TMR) of our samples, in the range of 20--30 % at room temperature, makes the tunnel junctions highly sensitive to magnetic fluctuations. A sharp switching of the soft magnetic layer upon field reversal prevents a domain structure from occurring in the soft magnetic layer, in the field window used for our micromagnetic studies. The tunnel device can thus be used as a sensitive probe for measuring small magnetic fluctuations associated with micromagnetic defects, domains, and walls in the AFi system. These fluctuations modulate the resistance of the tunnel junction and are fully reflected in the shape and the amplitude of the TMR signal.

Effect of material parameters on magnetization process in microstructured thin film

Effects of material parameters on the magnetization process in a sub-micron strip pattern have been studied by micromagnetic simulations. A grain structure in the thin film is modeled by a discretized square element array. A structural parameter is introduced considering discontinuous film structure. The exchange stiffness is also varied as parameters. Simulated transverse susceptibility χ and the longitudinal magnetization reversal field H rev in continuous film structure are somewhat smaller than the analytical predictions based on the coherent rotation model. The values of χ and H rev significantly decrease with the decrease of inter-grain coupling due to discontinuous structure. Numerical analysis for transients of the internal energy during the magnetization process reveals that the demagnetizing term dominates the transverse magnetization process. While, the demagnetizing and exchange energy terms comparatively contribute to the potential barrier for the longitudinal magnetization reversal. Various types of micromagnetic configurations of edge-curl, buckling and vortex were observed in the magnetization reversal depending on the material parameters.

Effects of stacking faults on magnetic viscosity in thin film magnetic recording media

There is much interest in crystallographic defects in thin film magnetic recording media and their role in influencing recording performance such as media noise and thermal loss. In this article we report a correlation between the magnetic viscosity in CoPtCr thin film media, which is the origin of thermal loss effects, and the concentration of local fcc-like regions. The concentration of these defects (the type and density of stacking faults) was varied by growth on different underlayers (Cr and CrTa/Cr) and was measured with grazing incidence x-ray scattering using synchrotron radiation. We show that a substantial percentage of local fcc regions in an otherwise hcp cobalt alloy film leads to significant magnetic viscosity effects at quite modest magnetic fields. We find that the activation volume is reduced for a sample with a higher percentage of fcc-like regions and suggest that this can be understood in terms of the effect of weak links acting to stabilize local micromagnetic configurations.

Systematic Analysis of Micromagnetic Switching Processes

Guided by the properties of the Stoner-Wohlfarth model, the switching behavior of general inhomogeneous micromagnetic configurations is investigated. In most cases the extrapolation of a suitably defined local susceptibility to infinity is demonstrated to lead to the switching point. Only conventional tools of numerical micromagnetics are needed in this calculation. For certain, symmetric classes of switching events we have to resort to indirect methods to determine the switching fields. Examples for the successful application of the proposed methods to various switching phenomena in cube-shaped particles with uniaxial anisotropy are given to validate the procedures.

Arrott's ideal soft magnetic cylinder, revisited

The stray-field-free, curling type magnetization configuration proposed by Arrott et al. (IEEE Trans. Magn. 15 (1979) 1228) for an ideally soft-magnetic finite cylinder is re-evaluated. Because this magnetization pattern represents an important example for micromagnetic configurations in compact bodies, the examination of the foundations of this model is considered important. Two insufficiencies of the original proposal are pointed out while confirming the general correctness of the idea.

Micromagnetic configurations of martensite in Fe-Ni alloys

The magnetic domain structure and the surface relief of the martensite in Fe1−xNix alloys were investigated by using both noncontact atomic force microscopy (NC-AFM) and magnetic force microscopy (MFM). In the Ni compositional range 0.25<x<0.29, the lathlike martensites and the complex magnetic domains were formed, while the lenslike martensite and the stripe domain were observed in 0.29<x<0.34. It is found that the magnetic domain structures on the martensites are connected with magnetoelasticity due to residual stress near the mid-rib.

Micromagnetics of cubic particles with easy axes along the body diagonals

Numerical calculations of equilibrium micromagnetic configurations in small ferromagnetic cubes are reported. Negative cubic anisotropy (K/sub 1/, K/sub 2/ as for nickel) leads to opposition between magnetocrystalline and configurational anisotropies. For particles /spl les/540 /spl Aring/ the remanent state has a flower configuration about [111]. From 540 /spl Aring/ to at least 900 /spl Aring/, it normally consists of a about [100] though stable [111] vortex states also exist from 600 /spl Aring/ to 72O /spl Aring/. M-H loops along [100] and [111] show switching between these configurations. >

Magnetic interactions in giant magnetoresistive Co-Ag heterogeneous films

The effect of the micromagnetic configuration on the giant magnetoresistance of a metastable cobalt-silver thin film prepared by sputtering is investigated. The difference between the magnetoresistance immediately after cooling in zero field and after field cycling is interpreted by invoking interactions between the active magnetic regions in the sample. >

The micromagnetics of cubic particles with cubic anisotropy favouring body diagonal magnetization

Results of a numerical study of the micromagnetic configurations of mesoscopic cubic particles (side length L) for which the magnetocrystalline and configurational anisotropies conflict are reported. Using magnetic constants appropriate to nickel, it is found that in zero field stable flower states, symmetric about [111], exist for L ≤ 576 Å. for L ≥ 540 Å, the ground state is a vortex, V 100, about [100]. V 111 and V 110 states are also stable over limited ranges of particle size. The [100], [110] and [111] hysteresis loops are presented for particles having L = 300 and 780 Å. For the smaller particle the loops resemble those of Stoner and Wohlfarth (SW), but with a [110] switching field ∼ 50% lower. For the 780 Å particle it is found that V 100 persists in [100] fields with a switching field some ten times larger than that for SW. Both the [111] and [110] loops involve four state changes, with a very robust V 100 configuration in low-fields.

Magnetization curling in a fine cylindrical particle

Micromagnetic curling configurations in finite magnetically soft cylinders are studied by means of computer simulation. A simple model based on the variational principle is suggested. The model quantitatively describes the obtained resu;ts over a wide range of particle sizes.

A generalized harmonic map equation for micromagnetics (abstract)

Static micromagnetic configurations are determined by minimizing the sum of anisotropy, demagnetization, exchange, and external interaction energies subject to the constraint that the magnitude of the magnetization vector is constant throughout a sample. If this constraint is not explicitly imposed in a numerical or analytical investigation, a Lagrange multiplier type of term must be appended to the energy functional. In a region R, two candidates are λ∫R(1 − ‖m(r)‖2)2 dV and ∫Rλ(r)(1 − ‖m(r)‖2)dV. The first expression is a Higgs or penalty term in the terminology of physicists and numerical analysts. The penalty parameter λ is given a physical meaning through the Ginsburg–Landau theory.1 The second expression is avoided by physicists and numerical analysts since the scalar Lagrange multiplier λ(r) represents an unwelcomed proliferation of variables. Following the theory of harmonic maps, the scalar Lagrange multiplier field is eliminated in this paper and a generalized harmonic map equation for micromagnetics is obtained. This equation has no extraneous phenomenological constants and enables one to ignore the original constraint in numerical solutions. It also provides a means for tying together results concerning singularities of harmonic maps2 with Block point phenomena believed to exist in VBL memories. This, in turn, enables one to make general inferences about the energetics of Bloch points which are not possible through purely topological investigations or simulations involving specific geometries.<ks>

A topological invariant for the accessibility problem of micromagnetics

A singularity-free three-dimensional micromagnetic configuration is accessible from a given initial state if there is a continuous family of interpolating states. This paper proposes an integral formula for the accessibility invariant which classifies (up to homotopy) the possible final states which are inaccessible from a given initial state. For infinite samples, we give explicit formulas describing initial states from which the final state of uniform saturation cannot be accessed.

Stray‐Field‐Free Magnetization Configurations

AbstractThe concept of stray‐field‐free micromagnetic configurations in applied to Bloch‐ and Néel‐walls, to stripe domains, and to the ripple structure in thin magnetic films. We propose a new structure of Bloch‐walls with a lower energy than the most general classical Bloch‐walls as calculated by LaBonte. In general a stray‐field‐free configuration has a lower symmetry than the simpler structures considered previously. It is concluded that the real structure, if not completely stray‐field‐free, should have at least the same symmetry as a possible stray‐field‐free solution. The significance of Brown's equations, which in general cannot be satisfied without admitting stray‐fields, is analysed with regard to the micromagnetic boundary condition.