Effects of Additive Elements on Corrosion Resistance of RE-TM Films

Rare earth-transition metal amorphous films are promising for use in erasable optical memory media. However, they have the drawback of poor reliability due to poor corrosion resistance. Corrosion of films is classified as either surface oxidation, selective oxidation of RE metals, or pitting corrosion. We have studied the corrosion behavior and succeeded in improving the corrosion resistance. To raise the latter, one of the elements Co, Ni, Pt, Al, Cr, or Ti was added to Tb-Fe and Tb-Co films by simultaneous sputtering. The films (no protection) were subjected to an accelerated aging test. The elements were classified into two groups, Co, Ni, and Pt, on the one hand, Al, Cr, and Ti, on the other. Elements in the former group are only effective in preventing surface oxidation and pitting corrosion. The latter group of elements, however, is effective in inhibiting corrosion in all of the above modes. This high corrosion resistance can be explained in terms of the elements´ ability to bind oxygen and to form passive states. The elements are effective in improving corrosion resistance in the order Cr, Ti, Al. These additives cause no appreciable change in the magneto-optical properties.