Dipolar mechanisms for magnetic anisotropy in amorphous ferrimagnetic alloys

Abstract

Classical magnetic dipolar interactions can make significant contributions to magnetic anisotropy in amorphous ferrimagnetic alloys with slight structural anisotropies on either atomic or microstructural scales. Simple expressions have been derived for this magnetic anisotropy in terms of structural anisotropy parameters, p, V, and Δx. For an alloy with atomic scale structural anisotropy (pair ordering), p is a measure of alignment of unlike nearest neighbor pairs. For an alloy with aligned, elongated composition inhomogeneities, V is the volume fraction of the inhomogeneities and Δx is the composition difference (atom fraction) between the inhomogeneities and their surroundings. Accounting for magnetic anisotropy observed experimentally in bias-sputtered GdCo-based alloy films solely in terms of pair ordering and classical dipolar interactions requires p∼0.015. Accounting for the anisotropy solely in terms of composition inhomogeneities requires V∼0.5 and Δx∼0.1. Although such atomic scale structural anisotropy will be very difficult to detect with currently available experimental techniques, composition inhomogeneities of this magnitude should be easily detectible in x-ray, electron, or neutron small-angle scattering experiments. No evidence for anisotropic composition inhomogeneities is seen in x-ray scattering experiments on micron-thick bias sputtered GdCo-based alloy films, although such inhomogeneities may be present in oxygen-doped, evaporated Gd-Co films and in some annealed Gd-Co films prepared by evaporation and by zero-bias sputtering. Magnetic anisotropy in GdCo-based films probably involves spin orbit interactions as well as classical magnetic dipolar interactions.