Tunable charge transfer in graphene on chromium trihalides

The proximity effect in van der Waals (vdW) heterostructures has been extensively investigated as a controllable method to engineer new materials properties. Interfacing graphene with vdW materials featuring ferromagnetic ordering and strong spin-orbit coupling has been a longstanding goal, owing to predictions of emergent gapped topological states. Here, we investigate heterostructures of monolayer graphene resting on various atomically-thin CrX₃ layered magnets (X=I/Br). We find that charge transfer is ubiquitous in these systems owing to a large work function mismatch between the graphene and CrX₃. In graphene on CrI₃, transport is strongly hysteretic as the gate voltage is swept back and forth. The hysteresis vanishes upon doping the system into the CrI₃ band gap, suggesting that it arises due to a kinetic barrier for transferring charge between graphene and CrI₃. The hysteretic behavior is concomitant with a highly nonlinear relationship between the gate voltage and the charge doping in the graphene, leading to very extended quantum Hall plateaus. Curiously, however, we do not observe any detectible signatures of magnetic exchange coupling in graphene despite achieving a clean interface with the magnetic CrX₃ substrate.