Observation of terahertz spin-Hall magnetoresistance in the model system $Y_3Fe_5O_{12}|Pt$

We report the observation of spin Hall magnetoresistance (SMR) in prototypical bilayers of ferrimagnetic yttrium iron garnet (YIG) and platinum in the frequency range from 0 THz to as high as 1.5 THz. The effect exhibits a strong low-pass behavior: It decreases by approximately 75% from 0 THz to 0.2 THz and vanishes at 1.5 THz. Based on a dynamic model of SMR, we can consistently explain the measured frequency dependence by a competition of the transverse coherent and longitudinal incoherent spin-torque contributions to the spin current. Our analysis suggests that the nonzero response time of the magnon chemical potential is responsible for the increased out-flow of incoherent magnons through the YIG|Pt interface at high frequencies and, consequently, for the dramatic spectral decay of the SMR. These results establish THz SMR as a powerful probe of ultrafast interfacial spinmagnon coupling.