  The challenge of controlling magnetism using electric fields raises
fundamental questions and addresses technological needs such as low-dissipation
magnetic memory. The recently reported two-dimensional (2D) magnets provide a
new system for studying this problem owing to their unique magnetic properties.
For instance, bilayer chromium triiodide (CrI3) behaves as a layered
antiferromagnet with a magnetic field-driven metamagnetic transition. Here, we
demonstrate electrostatic gate control of magnetism in CrI3 bilayers, probed by
magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near
the metamagnetic transition, we realize voltage-controlled switching between
antiferromagnetic and ferromagnetic states. At zero magnetic field, we
demonstrate a time-reversal pair of layered antiferromagnetic states which
exhibit spin-layer locking, leading to a remarkable linear dependence of their
MOKE signals on gate voltage with opposite slopes. Our results pave the way for
exploring new magnetoelectric phenomena and van der Waals spintronics based on
2D materials.
