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To better understand the response of domain walls to current-induced spin transfer torques, we have directly imaged the internal magnetic structure of domain walls in current-carrying ferromagnetic nanostripes. Domain wall images were acquired both while a constant current was flowing through the wire, and after applying current pulses. Domain walls ranging from vortex walls in wide (1 µm) wires to transverse walls in narrow (100 nm) wires were quantitatively analyzed using scanning electron microscopy with polarization analysis. The domain wall motion is characterized by strong interactions with random pinning sites along the wire. The walls either jump with the electron flow between pinning sites, or the pinned walls are distorted by the current. The domain wall propagation is also associated with transverse motion of the vortex core. [ABSTRACT FROM AUTHOR]

A recent article [Van de Wiele et al., IEEE Trans. Magn. 43, 2917 (2007)] presents a semianalytical method to solve the Landau–Lifshitz (LL) equation. Spin motion is computed analytically as precession about the effective field H, where H is assumed fixed over the time step. However, the exchange field dominates at short range and varies at the time scale of neighbor spin precessions, undermining the fixed field assumption. We present an axis corrected version of this algorithm. We add a scalar multiple of m to H (preserving torque and hence the LL solution) to produce a more stable precession axis parallel to the cross product of the torques m×H at two closely spaced time steps. We build a predictor-corrector solver on this foundation. The second order convergence of the solver enables calculation of adjustable time steps to meet a desired error magnitude. [ABSTRACT FROM AUTHOR]

The inclusion of eddy currents into micromagnetic programs is important for the proper analysis of dynamic effects in conducting magnetic media. This paper introduces a numerical implementation for eddy current calculations, in a limited geometry, for a thick domain wall. In the special case of a zero-thickness wall, our results are directly comparable with an analytical model previously presented. Our calculations provide some computational results for testing more complex programs. [ABSTRACT FROM AUTHOR]

Micromagnetic simulation of domain wall motion in thin, narrow strips leads to a simplified analytical model. The model accurately predicts the same domain wall velocity as full micromagnetic calculations, including dependence on strip width, thickness, and magnitude of applied field pulse. Domain wall momentum and retrograde domain wall motion are both observed and explained by the analytical model. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An array of 42 μm square, 3 μm thick garnet particles has been studied. The strong crystalline uniaxial anisotropy of these particles results in the stable remanent state being single domain with magnetization parallel to the film normal. Magneto-optic measurements of individual particles provide distribution statistics for the easy-axis switching field H[SUBsw], and the in-plane hard-axis effective anisotropy field, H[SUBeff], which induces the formation of a metastable stripe domain structure. Both H[SUBsw] and H[SUBeff] are much smaller than the crystalline anisotropy field. Micromagnetic simulations show that the small H[SUBsw] cannot be attributed to shape anisotropy, but is consistent with smooth, localized reductions in the crystalline anisotropy caused by defects in either the particles or the substrate. [ABSTRACT FROM AUTHOR]

A two-dimensional micromagnetic model is extended to support simulation of films with nonuniform thickness. Zeeman and crystalline anisotropy energies of each cell scale with the cell thickness, while the exchange energy of a pair of neighbor cells scales by a weight dependent on the thicknesses of both cells. The self-magnetostatic energy is computed by scaling the moment of each cell by its thickness, and adding a local correction to the out-of-plane field. The calculation of the magnetostatic field for a 10x10x1 oblate spheroid is shown to be more accurate by the nonuniform thickness model than by a uniform thickness model. With the extended model a 530x130x10 nm film in the shape of a truncated pyramid with tapering over the 15 nm nearest the edges is shown to have smaller switching field and different reversal mechanism compared with uniform thickness films of similar size and shape. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report results for μMAG standard problem No. 4, a 500 nmx125 nmx3 nm rectangle of material with properties to mimic Permalloy. Switching dynamics are calculated for fields applied instantaneously to an initial s state: Field 1 at 170° and Field 2 at 190° (-170°) from the positive long axis. Reversal in Field 1 proceeds by propagation of end domains toward the sample center. Reversal in Field 2 involves rotation of the end domains in one direction while the center of the particle rotates in the opposite direction, resulting in collapsing 360° walls with complex dynamics on fine length scales. Approaching the static coercivity, H[sub c], in small field steps, we find that the ring down frequency, f, and susceptibility, χ, are in approximate agreement with a single spin model that predicts f∝(H[sub c]-H)[sup 1/4] and χ∝(H[sub c]-H)[sup -1/2]. We show a correlation between the modes of oscillation that become unstable at the critical ! field and the switching behavior. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Nano-size ferromagnetic dots, wires, and stripes are of great interest for future high speed magnetic sensors and ultrahigh density magnetic storage. High frequency dynamic excitation is one way to investigate the time scale of the magnetization reversal in submicron particles with lateral nanometer dimension. Macroscopic models like the Landau-Lifshitz (LL) model are often used to describe the switching process. However, these models do not take into account the nonuniformity of the magnetization structure. In this article dynamic micromagnetic calculations are used in determining the high frequency susceptibility of a 1 μmx50 nmx5 nm Permalloy stripe. The studied structure exhibits two resonance modes. The higher, primary peak is around 10 GHz and can be identified with the uniform resonance mode predicted by the macroscopic LL model. The low frequency peak is attributed to the splay of the magnetization distribution near the end of the stripe. © 2001 American! Institute of Physics. [ABSTRACT FROM AUTHOR]

For a uniformly magnetized rectangular particle with dimensions in the ratio 5 : 1 : 0.1, the coercive and switching fields in the (1,1,1) direction are determined to be H[sub c]/M[sub s]=0.057069478 and H[sub s]/M[sub s]=0.057142805. Previous micromagnetic computations of coercive and switching fields that did not approach these values for small particles are analyzed. It is shown that the disagreement was primarily due to a disparity in the method of calculating demagnetization energy. Corrected simulations are shown to agree with analytically determined values. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In magnetic random access memory, power consumption depends on the coercivity of the magnetic elements in the memory cells. In this article a new method is described that uses a “domain wall trap” element shape to reduce both the coercivity and the dependence of coercivity on element size in submicron magnetic elements. Micromagnetic simulations of a shaped permalloy element show coercivity less than one tenth the coercivity calculated for a rectangular permalloy element of the same size. The switching times for the domain wall traps are shown to be comparable to those of rectangular elements. [ABSTRACT FROM AUTHOR]

Presents a study of the magnetization process of granular cobalt-silver films. Use of the magneto-optical indicator film technique and magnetic force microscopy in the study; Quality characterization of high-temperature superconductors; Analysis of magnetization domains.

Presents a study that investigated artifacts in magnetic resonance imaging (MRI) from metals. Importance of magnetic properties of clips or other surgical materials placed in the human body; Information on the artifacts produced by MRI by material magnetic susceptibility; Description of the nuclear MRI facility.