Unidirectional Hysteresis in Thin Permalloy Films

Abstract

A study of torque curves in thin Permalloy films with hc>0.5, where hc=Hc/H̄k is reported. The behavior of a 1400 A, 77% Ni film with hc=0.9 is discussed as a typical example. For applied fields h>0.5, the torque curves are generally complex and the discrepancies with the Stoner-Wohlfarth (S-W) model wide-spread. For h<0.5, if the film is previously saturated, the torque has a period of 2π in agreement with the S-W model. However, when 0.3<h<0.5, it is irreversible and the hysteresis is unidirectional, occurring around the easy direction of the bulk of the film. A model is proposed to explain this effect in which it is assumed that the film contains small regions with negative anisotropy −K2, where K2 is of the order of K1, the anisotropy of the bulk of the film. The torque curves calculated for this model are found to agree qualitatively very well with the experimental results.