Magnetic properties in ferrimagnetic GdCo alloy films with bulk perpendicular magnetic anisotropy (PMA) are investigated as a function of annealing temperature (Tanneal) and annealing time for several capping layers. Magnetic properties in films capped by TaOx vary markedly with Tanneal; the saturation magnetization and coercivity vary progressively with increasing Tanneal up to 300°C, and above that temperature, PMA is lost abruptly. By comparing the annealing temperature dependence for Co-dominated and Gd-dominated compositions close to the magnetization compensation point, the data are readily explained by preferential oxidation of Gd during annealing. When films are capped by a Ta/Pt bilayer, the film properties are stable up Tanneal = 300 °C, indicating that oxidation at high temperatures is effectively blocked, but the abrupt loss of PMA for Tanneal > 300 °C is still observed. X-ray diffraction measurement reveals that the amorphous structure of the films remains the same after high-temperature annealing that is sufficient to remove PMA, indicating that crystallization from the amorphous phase is not responsible for the lack of PMA. Instead, our results suggest that high annealing temperatures may cause segregation of Co and Gd atoms in the films, which reduces anisotropic pair-pair correlations responsible for the observed bulk PMA in the as-grown state.
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