Bi-substituted iron garnet polycrystalline thin film is a promising erasable, high density magneto-optic recording material. The magneto-optical properties of the film are changed according to sputtering conditions and annealing techniques. In our experiments, various Bi-substituted iron garnet thin films are prepared by RF sputtering. The garnet thin films are crystallized by using two method, 1) the multiple annealing and 2) the multi-step rapid thermal annealing method (MSRTA). The small Faraday rotation angle was measured by a polarization modulation scheme using phase sensitive detection techniques and a Faraday rotator. In the case of multiple annealing, Faraday rotation angles of the films are measured as functions of substrate temperature, sputtering rate, multiple annealing temperature and time, thickness of the films. The measured Faraday rotation angle is about $2^\circ /\mu m$ and the crystallization temperature of the multiple annealed iron garnet film is $600\,^\circ\!C$. Total annealing time and heating time of the multiple annealing for crystallization are longer than 1 hour and 3 minutes, respectively. In order to improve the microstructure of sputtered Bi-substituted iron garnet films, a MSRTA is proposed and demonstrated. The MSRTA method introduces the concepts of the incubation time and the relaxation time in the heating-cooling cycle. By heating the sample long enough for the seed formation but not too long for the crystal to grow, grains of minimum size are expected to be created. At the same time, by cooling the sample long enough but not too long, one can further force the grain size to be as small as it can be. This optimized MSRTA leads to a finer grain size of less than $0.1 \mu m$ and better surface morphology with high grain density than a conventional annealing method. The crystallized samples show good squarenesses and large remnant Faraday rotations.
