  Spin-orbit torque (SOT)-induced magnetization switching shows promise for
realizing ultrafast and reliable spintronics devices. Bipolar switching of
perpendicular magnetization via SOT is achieved under an in-plane magnetic
field collinear with an applied current. Typical structures studied so far
comprise a nonmagnet/ferromagnet (NM/FM) bilayer, where the spin Hall effect in
the NM is responsible for the switching. Here we show that an
antiferromagnet/ferromagnet (AFM/FM) bilayer system also exhibits a SOT large
enough to switch the magnetization of FM. In this material system, thanks to
the exchange-bias effect of the AFM, we observe the switching under no applied
field by using an antiferromagnetic PtMn and ferromagnetic Co/Ni multilayer
with a perpendicular easy axis. Furthermore, tailoring the stack achieves a
memristor-like behaviour where a portion of the reversed magnetization can by
controlled in an analogue manner. The AFM/FM system is thus a promising
building block for SOT devices as well as providing an attractive pathway
towards neuromorphic computing.
