  Twisted van der Waals heterostructures with flat electronic bands have
recently emerged as a platform for realizing correlated and topological states
with an extraordinary degree of control and tunability. In graphene-based
moir\'e heterostructures, the correlated phase diagram and band topology depend
strongly on the number of graphene layers, their relative stacking arrangement,
and 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 which 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, with a large associated anomalous Hall effect.
Uniquely, the magnetization direction can be switched purely with electrostatic
doping at zero magnetic field. Our results establish tMBG as a highly tunable
platform for investigating a wide array of tunable correlated and topological
states.
