▎ 摘　　要

Stacking a monolayer and bilayer of graphene, with a small twist angle between them, creates a tunable platform where the physics of both twisted bilayer graphene and twisted double bilayer graphene can be realized. Twisted van der Waals heterostructures with flat electronic bands have recently emerged as a platform for realizing correlated and topological states with a high degree of control and tunability. In graphene-based moire heterostructures, the correlated phase diagram and band topology depend on the number of graphene layers and the details of the external environment from the encapsulating crystals. Here, we report that the system of twisted monolayer-bilayer graphene (tMBG) hosts a variety of correlated metallic and insulating states, as well as topological magnetic states. Because of its low symmetry, the phase diagram of tMBG approximates that of twisted bilayer graphene when an applied perpendicular electric field points from the bilayer towards the monolayer graphene, or twisted double bilayer graphene when the field is reversed. In the former case, we observe correlated states that undergo an orbitally driven insulating transition above a critical perpendicular magnetic field. In the latter case, we observe the emergence of electrically tunable ferromagnetism at one-quarter filling of the conduction band, and an associated anomalous Hall effect. The direction of the magnetization can be switched by electrostatic doping at zero magnetic field. Our results establish tMBG as a tunable platform for investigating correlated and topological states.