Quantifying Spin-Orbit Torques in AFM/HM Heterostructures by Harmonic Hall Measurements*

Modifying the magnetic order of insulating antiferromagnets (AFMs) with spin currents is of fundamental interest and can enable new applications of AFMs. Toward this goal, characterizing the type and amplitude of the spin-orbit torques (SOT) associated with AFM/Heavy metal interfaces is important. In this study, we report the full angular dependence of harmonic Hall voltage measurements in a predominantly easy-plane AFM, epitaxial c-axis oriented α-Fe₂O₃, with an interface to Pt. Hall measurements are conducted while rotating an applied field in 3-orthogonal planes. By modeling the harmonic resistance signals together with the α-Fe₂O₃ magnetic parameters, such as its exchange field, magnetic anisotropy and DMI, we determine the amplitudes of field-like and damping-like SOTs. Out-of-plane field scans are shown to be essential to determining the damping-like torques. Our results indicate that the amplitude of the field-like torques are significantly larger than the damping-like torques and also consistent with a small angle tilt of the α-Fe₂O₃ hard axis with respect to the c-axis.