Noncoherent Switching in Permalloy Films

Abstract

High resolution Bitter pattern studies of the domain structure of permalloy films, with uniaxial anisotropy Hk and under the influence of applied fields in the film plane, are reported, and from these studies are inferred some aspects of noncoherent flux reversal processes. The threshold field for irreversible domain propagation across the film is measured for a variety of films differing in thickness (150 to 1100 A), rate of deposition (30 to 1100 A/min), substrate temperature (100 to 350°C), and composition (80 to 83.5% Ni), as a function of the angle α between the reversing field and the easy axis, after saturating along the opposite easy direction. For α<αc, where αc depends on thickness and preparation variables and varies from 0° to about 60°, reversal takes place by parallel wall displacement, with a wall coercive force Hw characteristic of the film. Two principal structural features have been identified as affecting Hw: Nonmagnetic inclusions, and local easy axes in the film plane normal to the main easy axis (negative Hk). In films thin enough to support Néel-type walls, long perturbation walls are generated when a domain wall collides with an inclusion (or hole); these perturbation walls, which appear to be some form of 360° (or 720°, etc.) wall separating head-on parallel domains, require several hundred oersteds for removal. Negative Hk, which is demonstrated by high-angle buckling combined with low angular anisotropy dispersion, is readily found in films with hw=Hw/Hk>1, and is believed to be directly correlated with hw. For α>αc, reversal takes place by labyrinth propagation, in which the walls remain relatively fixed, and long, slender domains develop from the tip leaving behind regions of unswitched material, resulting in a labyrinth-like flux pattern. The labyrinth threshold is nearly parallel to the threshold for coherent rotation, but below it by an amount which increases with decreasing film thickness. A model for labyrinth propagation, which treats the film as being composed of regions of differing Hk, with magnetostatic interactions coupling a region undergoing switching to unswitched and previously switched regions, predicts the direction of propagation and its thickness dependence in reasonable agreement with experiment, and is partially successful in explaining the labyrinth threshold. It is suggested that labyrinth propagation is the primary mechanism of intermediate-speed pulse switching (switching time ≳0.1 μsec); the correlation between intermediate- and high-speed threshold fields, and angular and amplitude anisotropy dispersion, is examined. Finally, the high-speed switching mode (τ<0.1 μsec) is considered; the intrinsic damping for this mode is found to be about four times the damping deduced from ferro-magnetic resonance linewidth or from free oscillations of the magnetization, indicating that even for switching times approaching 1 mμsec reversal does not take place by a completely coherent rotation.