Mechanisms of Reversal with Bias Fields, Deduced from Dynamic Magnetization Configuration Photographs of Thin Films

Abstract

Flux reversal with a transverse field in Ni–Fe thin films has been studied using a Kerr magnetooptic apparatus which permits 10-nsec exposures to be taken of the dynamic magnetization configuration. Whenever the applied fields just exceed the critical asteroid, noncoherent rotation occurs, and a 50–100 μ wavelength striped configuration appears in the film. Before the stripes form there is a fast relaxation of the ripple as the magnetization rotates coherently to angles greater (5°–20°) than either the critical angle for reversal or the uniform torque minimum. Therefore, previous models of noncoherent rotation based on instabilities in the ripple which were expected to occur before or at the critical asteroid or uniform torque minimum do not explain the observed reversal process. After the stripes form, depending on the magnitude of the applied fields, one of two processes occurs: either (1) the rotation of the magnetization stops until the stripes break up into small nucleated regions or (2) the rotation is slowed by an order of magnitude and the stripes decrease in amplitude until they vanish. With lower pulse fields diffuse boundary propagation occurs. Magnetostatic stray fields from the boundary cause the magnetization in small regions to reverse by noncoherent rotation so that the propagation is predominant in the direction of the stripes.