The characteristics of TbCo film samples left in the open air were examined. The bias voltage (Vb) of these samples ranged from 0 to Â¿200 V and as the bias voltage became higher, the film structure became more coarse, allowing oxygen to penetrate it and oxidize the Tb atoms. Saturation magnetization of these films occured with time in the vicinity of 550 emu/cm3. It is thought that interface reaction and the diffusion reactions both contribute to this oxidation process. The Faraday rotation angle, nearly constant with time, is thought to be due to the unchanging concentration of the transition metal Co. Changes in the optical constant due to oxidation are thought to cause the increase in Â¿K of films made at Vb = Â¿150 V.