CoPt thin films sputtered under high and low Ar pressures were found to have different microstructures. Films deposited under high Ar pressures contained an amorphous phase with segregated oxygen. Those prepared under low Ar pressures exhibited Pt modulation within crystal grains, but did not contain an amorphous phase. The magnetic properties of low-Ar pressure films depended on the perpendicular magnetic anisotropy and on the crystal grain size. In the case of films prepared at high Ar pressures, it is thought that other factors affect the coercivity. The dc-erased media noise decreased with decreasing crystal grain size for both types of media. Transition-related media noise also declined as crystal grain sizes shrank for high-Ar pressure films, but not for low-Ar pressure films.