A medium in which a magnetic layer is composed mainly of Co and oxygen and has a principal axis of magnetic anisotropy rising unidirectionally at 20.degree. to 80.degree. to a nonmagnetic substrate surface, the coercive force Hs on the surface side of the magnetic layer is 600 to 2,000 Oe and the coercive force Hb on the substrate interfacial side is less than 600 Oe, the medium being produced by treating a nonmagnetic substrate surface with a plasma in an atmosphere of an inert gas or an oxygen gas, and then depositing an alloy composed mainly of Co on the above plasma-treated nonmagnetic substrate surface while an oxygen gas is introduced into the layer-forming atmosphere.