Origin of Biaxial Anisotropy in Polycrystalline Films

Abstract

A fourfold symmetry of anisotropy is found in polycrystalline films evaporated on unidirectionally scratched glass substrates. The origin of the biaxial anisotropy is attributed to a shape effect resulting from fine structure of the scratches. In essence, the fine structure of films deposited on such substrates is an array of oriented rectangularly shaped areas as confirmed by electron microscopic observation. Such arrays are simulated by depositing polycrystalline films by several methods in a regular array of small, discrete squares. Torque curves of the films have a large sin4θ component with the easy directions along the diagonals of the squares. The biaxial anisotropy constant estimated from the torque curves has the maximum value at a low measuring field and does not extrapolate to zero at high fields. Orthogonal switching experiments at low fields, where the magnetization rotates by 90° instead of 180°, also confirm the biaxial anisotropy related to the square shape. Based on these results, a domain configuration in the small square film at low fields, that consists of a central closure domain with an outer unclosed region, is proposed.