In order to develop a new probe sensor for high frequency electromagnetic field generated around integrated electric circuits, we have synthesized thermally nonequilibrium Bi3Fe5O12 (BIG) and Bi3Fe5-xGaxO12 [(BIGG), x 0.2, 0.5, 1.0] films from metalorganic decomposition technique. The BIG materials are famous for the largest magneto-optical (MO) effect in visible light region. Ga substitutions are expected to increase the sensitivity of the MO effect to magnetic fields. For the preparation of the films, metalorganic solutions were spin coated on the garnet single-crystal substrates and then were annealed in the furnace. The optimized annealing temperature for BIG and BIGG is determined to be around 480°C. The BIG film showed the largest Faraday rotation of approximately 6°/μm at 630 nm. The Ga substitution increases the change ratio of the Faraday rotation to the magnetic field from approximately 5 to 11 × 10-3 deg/mOe. The half width of the ferromagnetic resonance (FMR) line shape of the BIG was approximately 200 Oe. Due to the Ga substitution, the minimum half width became 28 Oe. The sharp resonance will be an advantage for MO imaging using FMR phenomena.