Tetragonal-to-Cubic Transformation of CoPt in Post-Annealed CoPt and CoPt/Au Thin Films

CoPt and CoPt/Au films prepared by magnetron sputtering were studied in this paper. The films were deposited at room temperature followed by a post annealing at temperatures (T _a) from 400degC to 800degC. Ordering of the CoPt and CoPt/Au films occurred at T _a = 600degC and 500degC, respectively, which results in magnetic hardening. With the increase of T _a up to 700degC, coercivity of the both films increased to a maximum value of about 12 kOe. As T _a reached 800degC, a drastic magnetic softening occurs in both samples; meanwhile, the diffraction data indicate a corresponding phase change from tetragonal to cubic. Scanning electron microscopy results revealed an intensive grain growth from 70 nm at 700degC to 160 nm at 800degC for CoPt/glass film and from 110 nm at 700degC to 250 nm at 800degC for CoPt/Au/glass sample. The relation between the abnormal phase change and intensive grain growth indicates that the formation of the cubic phase is driven by large biaxial tensile stress. Further investigations support this explanation and reveal that the cubic phase may be chemically ordered.