Granular CoPt/C films for high-density recording media

Granular CoPt/C films, consisting of nanoparticles of the face-centered-tetragonal (fct) CoPt phase embedded in a carbon matrix, have been made by cosputtering from CoPt and C targets using a tandem deposition mode. The as-made films have a disordered face-centered-cubic structure and low coercivity (below 100 Oe). The granular structure is obtained after an aging heat treatment, which is followed by a drastic increase in coercivity to values up to 9 kOe due to the formation of the highly anisotropic fct phase. Transmission electron microscope studies showed the size of the CoPt particles to increase from 5 nm in the as-made state to 7–12 nm in the optimally annealed state. The measured coercivities are much lower than the value expected for noninteracting, randomly distributed single-domain particles. The large discrepancy is attributed to interparticle interactions, particle-size effects, and mainly to a lower anisotropy of the fct phase because of incomplete ordering.