Unidirectional Spin Hall Magnetoresistance in Antiferromagnetic Heterostructures*

Unidirectional spin Hall magnetoresistance (USMR) has been widely reported in heavy metal/ ferromagnet bilayer systems. We observe the USMR in Pt/α-Fe2O3 bilayers where the α-Fe2O3 is an antiferromagnetic (AFM) insulator. Systematic field and temperature-dependent measurements confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven by the imbalance of creation and annihilation of AFM magnons by spin-orbit torque due to the thermal random field. However, unlike its ferromagnetic counterpart, theoretical modeling reveals that the USMR in Pt/α-Fe2O3 is determined by the antiferromagnetic magnon number with a non-monotonic field dependence. Our findings extend the generality of the USMR which paves the way for the highly sensitive detection of AFM spin state.