Giant facet-dependent spin-orbit torque and spin Hall conductivity in the triangular antiferromagnet IrMn₃

There has been considerable interest in spin-orbit torques for the purpose of manipulating the magnetization of ferromagnetic elements for spintronic technologies. Spin-orbit torques are derived from spin currents created from charge currents in materials with significant spin-orbit coupling that propagate into an adjacent ferromagnetic material. A key challenge is to identify materials that exhibit large spin Hall angles, that is, efficient charge-to-spin current conversion. Using spin torque ferromagnetic resonance, we report the observation of a giant spin Hall angle θ^eff_SH of up to ~0.35 in (001)-oriented single-crystalline antiferromagnetic IrMn₃ thin films, coupled to ferromagnetic permalloy layers, and a θ^eff_SH that is about three times smaller in (111)-oriented films. For (001)-oriented samples, we show that the magnitude of θ^eff_SH can be significantly changed by manipulating the populations of various antiferromagnetic domains through perpendicular field annealing. We identify two distinct mechanisms that contribute to θ^eff_SH: the first mechanism, which is facet-independent, arises from conventional bulk spin-dependent scattering within the IrMn₃ layer, and the second intrinsic mechanism is derived from the unconventional antiferromagnetic structure of IrMn₃. Using ab initio calculations, we show that the triangular magnetic structure of IrMn₃ gives rise to a substantial intrinsic spin Hall conductivity that is much larger for the (001) than for the (111) orientation, consistent with our experimental findings.