ALBERT

Notes: Thin films of amorphous rare earth–transition metal alloys are well known to possess perpendicular magnetic anisotropy. The vapor deposition process possesses factors that break the symmetry of an idealized, bulk, isotropic, amorphous material. These include the substrate, the incident atomic beams, incident energetic Ar atoms, and any applied field. Structural anisotropy such as columnar microstructure, nonperpendicular growth direction, stress and strain, and preferential growth of crystalline grains with low energy surfaces, are known consequences of the growth process. Evidence exists, however, that the anisotropy in these amorphous alloys is of a more subtle local nature. The perpendicular magnetic anisotropy in amorphous Tb-Fe in a broad range of compositions increases dramatically with increasing deposition temperature, including temperatures well above the Curie temperature. It is not strongly dependent on stress and is independent of film thickness. Upon annealing, the anisotropy vanishes. The nonrandomness in the amorphous structure thus does not appear to be related to nanocrystallites, which would presumably be enhanced by annealing, nor to kinetic effects of incident atomic beam directions which would be reduced by an increased deposition temperature. The dependence on deposition temperature can be fitted to a thermally activated form, implying an important role of surface diffusion and surface energy in the formation of the anisotropic local structure. We suggest that this process be generically thought of as a texturing of the amorphous phase, analogous to the preferred texturing of the low surface energy grains in polycrystalline growth. The dependence of the magnetic anisotropy of a-Tb-Fe on deposition temperature and rate will be presented and compared to a particular model involving reorientation of adatom configurations into lower energy orientations. Comparisons to other a-RE-TM alloys will be made, in particular to a-Gd-Fe and to a-Er-Fe (with the opposite sign Steven's coefficient). The results of an extensive computer simulation of a competing model involving chemically driven preferential site occupation will also be presented.