Isotropic Permalloy Films

Magnetically isotropic Permalloy films have been prepared by evaporation or vacuum-annealing in a 50-cps rotating field of crossed pairs of Helmholtz coils. The magnetization reversal in magnetically isotropic films is similar to that of films with uniaxial anisotropy in the hard axis or that of inverted films (Hc>Hk) in the easy and the hard axis. This is shown by hysteresis measurements on 25 film-matrix patterns and by electron micrographs. At zero field the magnetic structure is caused by the film inhomogeneities and by the wall and edge stray fields. The locked state characterized by a domain splitting perpendicular to the applied field occurs when the reversed field is increased. Locking disappears at the coercive field strength by migration of Bloch lines on the Néel walls. Locking by partial rotation is not possible with Hk=0. Maze-like domains occurring in an increasing field applied obliquely or perpendicularly to the previous saturation direction are caused by stray field coupling between strips of varying magnetization direction already present at zero field.