Harmonic Analysis of Spin Torques in Antiferromagnet/Heavy Metal Heterostructures*

Harmonic analysis is a powerful technique to quantify current-induced torques acting on magnetic materials, but so far it has never been applied to antiferromagnets. We formulate an analytical theory of the first and second harmonic responses of an antiferromagnet/heavy metal (HM) heterostructure driven by current-induced torques and other concomitant effects, which can well be confirmed by numerical simulations. By rotating a magnetic field of variable strength in the xy, yz, and xz planes, we are able to clearly distinguish the contributions of the field-like torque, the damping-like torque, and the thermal effects through analyzing the second harmonic signals. Using Fe₂O₃/HM and NiO/HM heterostructures, we demonstrate the validity of our theory. Our theory provides direct and general guidance to current and future experiments.