  Atomically thin chromium triiodide (CrI3) has recently been identified as a
layered antiferromagnetic insulator, in which adjacent ferromagnetic monolayers
are antiferromagnetically coupled. This unusual magnetic structure naturally
comprises a series of anti-aligned spin filters which can be utilized to make
spin-filter magnetic tunnel junctions with very large tunneling
magnetoresistance (TMR). Here we report voltage control of TMR formed by
four-layer CrI3 sandwiched by monolayer graphene contacts in a dual-gated
structure. By varying the gate voltages at fixed magnetic field, the device can
be switched reversibly between bistable magnetic states with the same net
magnetization but drastically different resistance (by a factor of ten or
more). In addition, without switching the state, the TMR can be continuously
modulated between 17,000% and 57,000%, due to the combination of spin-dependent
tunnel barrier with changing carrier distributions in the graphene contacts.
Our work demonstrates new kinds of magnetically moderated transistor action and
opens up possibilities for voltage-controlled van der Waals spintronic devices.
