Microstructure and magnetism in amorphous rare-earth–transition-metal thin films. II. Magnetic anisotropy

Abstract

Amorphous thin films of GdxCo1−x, HoxCo1−x, GdxFe1−x, and YxFe1−x (0.1<x<0.7) possess an anisotropic microstructure when prepared by vapor or sputter deposition onto room-temperature substrates. The structure consists of mutually parallel columnar regions of high density that are surrounded by a network of low-density material of 10–30 Å thickness. This structure is produced by geometrical shadowing of the incident vapor atoms by the growing film. If separate vapor sources are used to deposit each alloy constituent, compositional inhomogeneities are also formed by shadowing effects. These produce an in-plane magnetic anisotropy that may be eliminated by deposition onto rotating substrates. Annealing at temperatures between 250 and 400 °C, deposition in oxygen-contaminated environments, or bias-sputter deposition produces compositional inhomogeneities at the surface of the columns. Development of this inhomogeneity is accompanied by production of an easy axis of magnetization perpendicular to the film surface in Gd-Co alloys, and development of a diffuse diffraction maximum at 0.38 Å−1. Upon annealing at 500 °C, this diffuse ring is transformed into a set of closely spaced Bragg maxima that correspond to Gd2O3 crystallites. All of the films exhibit large biaxial tensile stresses in the as-deposited state. These are reduced by deposition in oxygen and by bias-sputter deposition. Tensile stress and perpendicular magnetic anisotropy are linearly correlated in Gd-Co alloys, but the magnetostrictive contribution to perpendicular anisotropy is only 0.01 of the observed value.