The half-period terahertz field E THz generates electrical currents j in the Pt and Ta layers. The spin Hall effect converts the charge currents into respective spin currents, leading to a spin accumulation at the Py interfaces. As Ta and Pt have spin Hall angles of opposite signs, the polarization of the accumulated spins is identical at both interfaces (blue arrows). When the sample is flipped around the x axis (keeping the direction of M constant) the spin accumulation changes sign (red arrows). This results in an opposite direction of the field-like SOT effective field H FL in these two geometries and, accordingly, opposite torques on M . The z -axis-projected dynamic magnetization components have antiphase time evolution at the interfaces (due to their opposite torques, marked by green arrows), which can be detected by comparing the SWR signals for the two configurations.
