  The atomic thickness of two-dimensional (2D) materials provides a unique
opportunity to control material properties and engineer new functionalities by
electrostatic doping. Electrostatic doping has been demonstrated to tune the
electrical and optical properties of 2D materials in a wide range, as well as
to drive the electronic phase transitions. The recent discovery of atomically
thin magnetic insulators has opened up the prospect of electrical control of
magnetism and new devices with unprecedented performance. Here we demonstrate
control of the magnetic properties of monolayer and bilayer CrI3 by
electrostatic doping using a dual-gate field-effect device structure. In
monolayer CrI3, doping significantly modifies the saturation magnetization,
coercive force and Curie temperature, showing strengthened (weakened) magnetic
order with hole (electron) doping. Remarkably, in bilayer CrI3 doping
drastically changes the interlayer magnetic order, causing a transition from an
antiferromagnetic ground state in the pristine form to a ferromagnetic ground
state above a critical electron density. The result reveals a strongly
doping-dependent interlayer exchange coupling, which enables robust switching
of magnetization in bilayer CrI3 by small gate voltages.
