  Antiferromagnets are outstanding candidates for the next generation of
spintronic applications, with great potential for downscaling and decreasing
power consumption. Recently, the manipulation of bulk properties of
antiferromagnets has been realized by several different approaches. However,
the interfacial spin order of antiferromagnets is an important integral part of
spintronic devices, thus the successful control of interfacial
antiferromagnetic spins is urgently desired. Here, we report the high
controllability of interfacial spins in antiferromagnetic / ferromagnetic /
heavy metal heterostructure devices using spin-orbit torque (SOT) assisted by
perpendicular or longitudinal magnetic fields. Switching of the interfacial
spins from one to another direction through multiple intermediate states is
demonstrated. The field-free SOT-induced switching of antiferromagnetic
interfacial spins is also observed, which we attribute to the effective
built-in out-of-plane field due to unequal upward and downward interfacial spin
populations. Our work provides a precise way to modulate the interfacial spins
at an antiferromagnet / ferromagnet interface via SOT, which will greatly
promote innovative designs for next generation spintronic devices.
