High coercivity (H-c) and magnetic anisotropy constants (K-u) can be achieved by postannealing in Co-Cr-Pt-SiO2/Ru perpendicular magnetic recording media without grain size growth or interdiffusion. Cr oxide formation is promoted by annealing, especially at the interface of the Ru underlayer and Co-Cr-Pt-SiO2 layer due to oxygen depth profile. Annealing beyond a particular temperature results in grain growth of the Co alloy and enhanced exchange coupling.