Interfacial origin of unconventional spin-orbit torque in Py/$γ-$IrMn$_{3}$

Abstract: Angle-resolved spin-torque ferromagnetic resonance measurements are carried out in heterostructures consisting of Py (Ni${81}$Fe${19}$) and a noncollinear antiferromagnetic quantum material $\gamma-$IrMn${3}$. The structural characterization reveals that $\gamma-$IrMn${3}$ is polycrystalline in nature. A large exchange bias of 158~Oe is found in Py/$\gamma-$IrMn${3}$ at room temperature, while $\gamma-$IrMn${3}$/Py and Py/Cu/$\gamma-$IrMn${3}$ exhibited no exchange bias. Regardless of the exchange bias and stacking sequence, we observe a substantial unconventional out-of-plane anti-damping torque when $\gamma-$IrMn${3}$ is in direct contact with Py. The magnitude of the out-of-plane spin-orbit torque efficiency is found to be twice as large as the in-plane spin-orbit torque efficiency. The unconventional spin-orbit torque vanishes when a Cu spacer is introduced between Py and $\gamma-$IrMn$_{3}$, indicating that the unconventional spin-orbit torque in this system originates at the interface. These findings are important for realizing efficient antiferromagnet-based spintronic devices via interfacial engineering.