Interfacial origin of unconventional spin-orbit torque in Py/${\hat{I}}^3-$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 ${\hat{I}}^3-$IrMn${}_3$. The structural characterization reveals that ${\hat{I}}^3-$IrMn${}_3$ is polycrystalline in nature. A large exchange bias of 158~Oe is found in Py/${\hat{I}}^3-$IrMn${}_3$ at room temperature, while ${\hat{I}}^3-$IrMn${}_3$/Py and Py/Cu/${\hat{I}}^3-$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 ${\hat{I}}^3-$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 ${\hat{I}}^3-$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.