Origin of magnetization-induced anisotropy of magnetic films

Abstract

It is proposed that the magnetization-induced anisotropy of magnetic films of cubic crystal structure originates from the anisotropy of atomic pair ordering, shape anisotropy, and strain anisotropy resulting from the constraint of the magnetostriction strain imposed on the film by the substrate. Calculated are the three anisotropy constants and their sum K vs temperature for Ni, Fe, and 55%Ni-Fe films; the room temperature (RT) constants vs the substrate temperature T-t during deposition or annealing after deposition for Ni and 50%Ni-Co films; the RT constants vs composition fraction for Fe-Ni films with T-t = RT, 250 degrees C and 450 degrees C, Co-Ni films at T-t = RT, 100 degrees C and 320 degrees C, and Fe-Co films with T-t = RT and 300 degrees C; the spread of RT K vs composition fraction for Fe-Ni films; and RT Delta K/K vs composition fraction for Fe-Ni and Co-Ni films, where Delta K denotes the variation of K of the film that is detached from its substrate. The calculated curves well accord with the measurements. The irrelevancy of K to the substrate material and the fast kinetics of the annealing in a field applied in the direction of the hard axis are explained reasonably. The anisotropies of Fe and Ni films originate mainly from the shape anisotropy and the strain anisotropy, respectively. The major anisotropy component in many cases depends not only on composition fraction but also on T-t. For example, the RT anisotropy of 40-70%Ni-Fe films, when T-t; is RT, mostly comes from the anisotropy of atomic pair ordering while it stems mostly from the shape anisotropy when T-t is 450 degrees C. The most important cause of the spread in values of K is the spread of the intrinsic anisotropic stresses superimposed on the intrinsic isotropic planar stress. It is suggested that the field cooling induced magnetic anisotropy originating from the induced crystal texture observed in the bulk alloys is also a major origin for Co and Co-rich alloy films of hexagonal crystal structure.