Effects of Co layer thickness and annealing temperature on the magnetic properties of inverted [Pt/Co] multilayers

The effects of Co layer thickness and annealing temperature on the perpendicular magnetic anisotropy (PMA) properties of inverted [Pt (0.2 nm)/Co (t_Co)]₆ multilayers (where t_Co indicates the thickness of the Co layer) have been investigated. The cross-sectional microstructure, as observed from the high-resolution transmission electron microscope images, shows a clear layered structure with atomically flat interfaces both in the as-deposited state as well as after annealing, indicating the interface effects for PMA. The effective PMA energy density (K _eff) increases significantly with an increase in t_Co from 0.2 to 0.28 nm and then becomes almost saturated with further increases in t_Co, followed by a slight reduction at the highest Co thickness, t_Co = 0.6 nm. In order to explain the t_Co dependence on K_eff, the intrinsic PMA energy density (K_i) is calculated by additionally measuring a similar set of results for the saturation magnetization. The K_i value increases nearly linearly with the increase in t_Co from 0.2 to 0.5 nm, followed by saturation at a higher t_Co value of 0.6 nm. Owing to a close relationship between K_i and the quality of the interfaces, these results indicate a similar t_Co dependence on the quality of the interfaces. This is further supported from the magnetic measurements of the samples annealed at the highest temperature of 500 °C, where a second phase is formed, which show a similar t_Co dependence on the amount of the second phase. The K _i value is nearly independent of the annealing temperature at t _Co ≤ 0.4 nm, above which a substantial reduction is observed, when the annealing temperature exceeds 500 °C.