Critical Thickness Tc Research Articles

Rotatable Anisotropy in Negative Magnetostriction Ni–Fe Films

The phenomenon of rotatable anisotropy (RA) in Ni–Fe films refers to a reorientation of the easy axis along the direction of a previously saturating magnetic field in a time <1 sec at room temperature. The effect was first unambiguously observed by Lommel and Graham in Ni films and attributed by them to ferromagnetic-antiferromagnetic interaction between Ni and NiO. The present study shows that the RA effect occurs under the same conditions of film thickness and composition as a previous studied ``mottling'' effect reported by Huber and Smith. In both cases the effects only occur for compositions with negative magnetostriction λ and for thicknesses greater than a critical thickness tc which is nearly inversely proportional to λ. These thickness and compositional dependencies suggest that magnetostriction and strain are responsible for both effects; a detailed microdomain model is being developed.

Properties of Permalloy Films Having a Magnetoelastic Easy Axis Normal to the Film

Internal isotropic strain in Permalloy films having a negative magnetoelastic constant creates a magnetoelastic easy axis normal to the film. Below a critical thickness tc such films are single domain in the film plane and have a square B-H loop; above this thickness new configurations are established, characterized by a B-H loop of special form and having relaxation properties. Above tc the Bitter pattern is “mottled” and continues so even in the presence of a 10 000 oe field. A domain structure is proposed in qualitative agreement with experiment; mottling is interpreted in terms of centers of microstrain.