Structural and Magnetic Anisotropy in Amorphous Terbium-Iron Thin Films

High density, removable media magnetooptic disk drives have recently begun to make significant gains in the information mass storage market. The media in these disks are amorphous rare-earth/transition-metal (RE-TM) alloys. One vital property of these materials is a large perpendicular magnetic anisotropy; that is, an easy axis of magnetization which is perpendicular to the plane of the film. A variety of theories, sometimes contradictory, have been proposed to account for this surprising presence of an anisotropic property in an amorphous material. Recent research indicates that there is an underlying atomic-scale structural anisotropy which is responsible for the observed magnetic anisotropy. Several different types of structural anisotropy have been proposed to account for the observed magnetic anisotropy, including pair-ordering anisotropy (anisotropic chemical short-range order) and bond orientation anisotropy (an anisotropy in coordination number or distances independent of chemical ordering). We have studied the structural origins of perpendicular magnetic anisotropy in amorphous Tb-Fe thin films by employing high-energy and anomalous dispersion x-ray scattering. The as-deposited films show a clear structural anisotropy, with a preference for Tb-Fe near neighbors to align in the out-of-plane direction. These films also have a large perpendicular magnetic anisotropy. Upon annealing, the magnetic anisotropy energy drops significantly, and we see a corresponding reduction in the structural anisotropy. The radial distribution functions indicate that the number of Tb-Fe near-neighbors increases in the in-plane direction, but does not change in the out-of-plane direction. Therefore, the distribution of Tb-Fe near-neighbors becomes more uniform upon annealing. We propose that the observed reduction in perpendicular magnetic anisotropy energy is a result of this change in structure. Our results support the pair -ordering anisotropy model of the structural anisotropy in amorphous Tb-Fe thin films. We see no evidence to support the bond orientation anisotropy model.