Effects of Pt and Ta on the magnetic anisotropy of Co and Co-Cr thin films

We have investigated the magnetic anisotropy energy (MAE) of binary Co-X(X = Cr, Ta, Pt) and ternary Co-Cr with Ta or Pt. It is found that Cr considerably decreases the MAE of Co, and a severer reduction of the MAE has been found in Co-Ta and Co-Cr-Ta within an extremely low solubility limit of Ta less than a few atomic percent. In contrast, Pt gives no noticeable change in the MAE of Co, but causes remarkable enhancement of the saturation magnetization and the MAE of Co-Cr alloy. These enhancement can be explained by the reduction of the Cr content due to Pt in ferromagnetic Co-enriched regions in a segregated phase. A thermodynamical consideration taking into account the magnetic term of Gibbs free energy gives a qualitative explanation for the role of Pt.