In order to apply a thin-film medium with a high-density magnetic recording capability to organic substrates for a magnetic tape or flexible disk, reduction of preparation temperature is the key technology. We found that an Ru under layer is effective to increase coercivity for Co-Pt-based thin films at low preparation temperature. Increasing the Ru under layer thickness and Pt composition of the magnetic layer increases coercivity. Addition of boron and oxygen to the magnetic layer is effective to reduce medium noise. With addition of boron and oxygen, Co-Pt grains are separated magnetically by a B-O non-magnetic boundary.