Optical isolators are required to block undesired reflections in many photonic integrated circuits (PICs), but the performance of on-chip isolators using the magneto-optic effect has been limited due to high loss of such materials. Moreover, they require precise positioning of a permanent magnet close to the chip, increasing footprint and impeding packaging. In this paper, we propose an optical isolator on the silicon-on-insulator platform with record performance and without the use of any external permanent magnets. A metallic microstrip above the bonded silicon microring (MR) is used to generate the magnetic field required for the nonreciprocal behavior. Simultaneously, the microstrip can be used to provide 0.6 nm of thermal tuning while preserving over 20 dB of isolation. We measure 32 dB of isolation near 1555 nm with only 2.3 dB excess loss in a 35 μm radius MR. The tunability, compactness, and lack of permanent magnets suggest this device is a major step towards integration in PICs.