Interfacial coupling and negative spin Hall magnetoresistance in Pt/NiO/YIG

Inserting an antiferromagnetic layer of NiO between Pt and Y3Fe5O12 (YIG) changes the positive sign of the spin Hall magnetoresistance (SMR) in Pt/YIG to a negative sign at low temperature. Here, we use polarized neutron reflectometry to explore the coupling between NiO and YIG to understand the mechanism of the negative SMR. A weak uncompensated magnetic moment is observed in the NiO and the direction of this moment is perpendicular to YIG. Therefore, we infer that the spin axis of NiO is perpendicular to YIG. This result directly supports the explanation that the negative SMR results from the pure spin current reflected back by the NiO layer and the spin-flop coupling between NiO and YIG.