Platinum-cobalt thin films were prepared by rf sputtering with substrate biasing to contol composition. Contrary to what was expected from equilibrium bulk properties, films deposited at room temperature have the fcc structure, in-plane magnetization, and low coercivity. At deposition temperatures above about 500 °C the coercivity increases and x-ray analysis indicates some tetragonal phase. Annealing at 600 °C causes rapid conversion to the tetragonal phase. In the annealed state the films have a large component of magnetization normal to the film plane with typical remanence-to-saturation ratios of 0.8 and coercivities of 8 kOe perpendicular to the film plane. The saturation Faraday rotation of annealed films is about 5×105 deg/cm in the visible and near ir. Curie temperatures are in the range 270-430 °C for compositions with 35-44 at.% cobalt. Dynamic read-write experiments show that this material is an attractive candidate for digital magneto-optic recording. Problems due to the high Curie-point and high room-temperature coercivity are avoided by operating in a minor hysteresis loop mode.