Orbital Chern insulator states in twisted monolayer-bilayer graphene and electrical switching of topological and magnetic order

We experimentally investigate narrow and topologically nontrivial moiré minibands hosted by van der Waals heterostructures consisting of a graphene monolayer rotationally faulted with respect to a Bernal-stacked bilayer. At fillings ν= 1 and 3 electrons per moiré unit cell within these bands, we observe quantized anomalous Hall effects with R_xy≈h/2e², indicative of spontaneous polarization of the system into a single valley-projected band with Chern number C= 2. Remarkably, we also observe the evidence of symmetry broken Chern insulator states at ν= 1.5 and 3.5. At ν= 3 we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential. This curious effect arises from the magnetization contribution due to topological edge states, which drive a reversal of the total magnetization and thus a switch of the favored magnetic state. Remarkably, we find that this switch is hysteretic, which we use to demonstrate non-volatile electric-field-induced reversal of the magnetic state. Voltage control of magnetic states can be used to electrically pattern nonvolatile magnetic domain structures hosting chiral edge states, with applications ranging from reconfigurable microwave circuit elements to ultra-low-power magnetic memory.