Bi substitute DyIG thin films having high coercive force and good squareness have been prepared on glass substrates by sputtering. The Faraday rotation amounts to about 7.5°/μm at wavelength of 500 nm. The magnetic domains are stable after thermal magnetic recording. The interference of magnetooptical thin films is investigated when the surface of films is deposited with different metals. Using this charecteristics the optimum disign for garnet recording discs can be achieved.