On-chip integration of nonreciprocal photonic devices is considered a "fundamental challenge" and the "missing link" for silicon photonics. Hybrid integration of magneto-optical (MO) materials in silicon photonic integrated circuits (PICs) shows promising potential to solve this challenge. In the past decade, the performance of silicon-integrated isolators and circulators based on hybrid integrated MO thin films has been significantly improved in terms of low insertion loss, high isolation ratio, and compact footprint. Accordingly, novel integrated magneto-photonic devices have emerged, including nonvolatile MO switch/memory, on-chip magnetometers, quantum nonreciprocal photonic devices, and topologically nontrivial lasers. In this paper, we review recent progress of on-chip nonreciprocal photonic devices based on hybrid integration of MO thin films on silicon. The configuration, operation principle, and key metrics of different types of integrated MO nonreciprocal photonic devices are summarized. In addition, the progress of on-chip electromagnets and novel MO devices are discussed.
