Optical switching of a moiré Chern ferromagnet

Optical control offers a non-contact, high-precision and ultrafast route to manipulating quantum material properties^1-5. Fractional Chern ferromagnetic states in moiré superlattices are a promising platform by which to pursue topological quantum computing^6-10, but an effective optical control protocol has remained elusive. Here we demonstrate robust optical switching of integer and fractional Chern ferromagnets in twisted molybdenum ditelluride (MoTe₂) bilayers using continuous-wave circularly polarized light. Highly efficient optical manipulation of spin orientations in the topological ferromagnet regime is realized at zero field using a pump light power as low as 28 nW µm^-2. Using this optically induced transition, we also demonstrate magnetic bistate cycling and spatially resolved writing of ferromagnetic domain walls. This work establishes a reliable and efficient optical control scheme for moiré Chern ferromagnets, paving the way for dissipationless spintronics and quantized Chern junction devices.