Yttrium iron garnets, which are typically used as magneto-optical isolators, are not suitable for monolithic integration with photonic circuits. We experimentally demonstrated an ultracompact microring optical isolator using an aluminum-substituted cobalt ferrite (CAFO) thin film directly deposited on silicon waveguides. The device footprint was only 43×71µm². The isolator can operate without an external magnet owing to the strong remanent magnetization of the CAFO film and achieved a maximum isolation ratio of 24.7 dB at a wavelength of 1545.7 nm. Our study could facilitate integration of nonreciprocal photonic devices on Si platforms.