Spin-orbit-driven ferromagnetism at half moiré filling in magic-angle twisted bilayer graphene.

Strong electron correlation and spin-orbit coupling (SOC) can have a profound influence on the electronic properties of materials. We examine their combined influence on a 2-dimensional electronic system at the atomic interface between magic-angle twisted bilayer graphene and a tungsten diselenide crystal. Strong electron correlation within the moiré flatband stabilizes correlated insulating states at both quarter and half filling, and SOC transforms these Mott-like insulators into ferromagnets, evidenced by robust anomalous Hall effect with hysteretic switching behavior. The coupling between spin and valley degrees of freedom is demonstrated through the control of the magnetic order with an in-plane magnetic field, or a perpendicular electric field. Our findings establish an experimental knob to engineer topological properties of moiré bands in twisted bilayer graphene and related systems.