A granular perpendicular magnetic recording medium, comprising: (a) a non-magnetic substrate having a surface; and (b) a layer stack on the substrate surface, the layer stack including a granular perpendicular magnetic recording layer formed by: (1) reactively sputtering a target comprised of a magnetic alloy in an atmosphere containing at least one ionized oxygen species derived from a source gas comprised of a compound of oxygen and at least one other non-metallic element; and (2) oxidizing an exposed upper surface of the granular perpendicular magnetic recording layer by generating a plasma containing at least one ionized oxygen species derived from a source gas comprised of a compound of oxygen and at least one other non-metallic element and treating the exposed upper surface of the granular perpendicular magnetic recording layer with the plasma.