Unidirectional Anisotropy in Multilayer Films of Cr and NiFe

Abstract

Layered films of Cr and NiFe alloy have been made which exhibit biased hysteresis loops in the easy direction and asymmetric hysteresis loops in the hard direction. The asymmetry in the hard direction is reversed by rotating the film a few degrees in the loop tester. The magnitude of the bias is 0.5 Oe for films consisting of a 500-Å layer of 76% NiFe on top of a 500-Å layer of Cr on a soft glass substrate. No loop bias was observed for films of 82% NiFe, for films on hard glass substrates, or for films with layer thicknesses departing from 500 Å by a factor of two. The films exhibit high sensitivity to externally applied stress. Loop bias can be eliminated by raising the temperature above the Néel point of Cr. The unidirectional axis is a function of the direction of a magnetic field applied during cooling of a film through the Néel point. The observed phenomena are believed due to exchange anisotropy between antiferromagnetic (strain sensitive) Cr and ferromagnetic NiFe. Crystal anisotropy, shape anisotropy, magnetostriction, etc. are not unidirectional. The only unidirectional effects reported in the literature have resulted from exchange anisotropy.