Enhancement of perpendicular coercivity for CoPt top layer in CoPt/AlN multilayer structure

The magnetic behavior of sputter deposited AlN20 nm/[CoPt2 nm/AlN2 nm] ₅/CoPt (x) (x as the thickness of the top CoPt layer) multilayer structure has been studied. It has been found that the magnetic anisotropy of the structure strongly depends on thermal annealing. With increasing the annealing temperature, the film changes from the in-plane magnetic anisotropy to the perpendicular anisotropy. Especially, for the top CoPt layer, the perpendicular coercivity increases much rapidly compared with that of the CoPt layers inside the base multilayer, when increasing the annealing temperature to 500 °C or above. The coercivity difference between the top CoPt layer and the base multilayer caused in this way results in an antiparallel alignment state during the magnetization process. The perpendicular magnetization and the coercivity enhancement for the CoPt top layer are correlated with the change in the residual stress inside this layer.