Coexistence of low-frequency spin-torque ferromagnetic resonance and unidirectional spin Hall magnetoresistance

We investigate the DC voltage under an applied microwave current for platinum(Pt)/cobalt(Co), tantalum(Ta)/Co, and tungsten(W)/Co bilayers, where the microwave-induced alternating magnetic field (${B}_{\mathrm{rf}}$) is parallel to an external static magnetic field (${B}_{\mathrm{ext}}$). Whereas spin torque ferromagnetic resonance (ST FMR) signals do not appear because of the parallel configuration of ${B}_{\mathrm{rf}}$ and the magnetization of the Co layer, we recently found a clear hysteresis signal around ${B}_{\mathrm{ext}}=0\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ only when the microwave frequency (${f}_{\mathrm{MW}}$) is low, typically less than 10 GHz. This low-frequency ST FMR (LFST FMR) signal enables us to detect magnetization switching induced by spin-orbit torque (SOT) with high sensitivity. In this study, we found an additional background (BG) signal superimposed on the LSFT FMR signal. The additional BG signal also has hysteresis behavior and appears even at high ${f}_{\mathrm{MW}}$, where the LSFT FMR signal completely disappears. The sign of the BG signal is changed by changing the nonmagnetic material from Pt to Ta or W. By measuring ${f}_{\mathrm{MW}}$, the microwave current, the temperature, and the magnetic field angle dependences, we conclude that the BG signal is induced by spin-dependent unidirectional spin Hall magnetoresistance (SD USMR), which is generated by a spin current induced by the spin Hall effect of nonmagnetic metals and spin-dependent electron mobility in ferromagnetic metals. The SD USMR signal appears in wide ranges of ${f}_{\mathrm{MW}}$ and ${B}_{\mathrm{ext}}$ because the SD USMR originates from the nonlinearity of current-dependent resistance and is not related to magnetization dynamics. From the magnitude of the BG signal, the spin Hall angles of Pt and Ta are calculated to be 0.026 \ifmmode\pm\else\textpm\fi{} 0.006 and \ensuremath{-}0.042 \ifmmode\pm\else\textpm\fi{} 0.006, respectively. In addition, we demonstrate SOT magnetization switching using the BG signal.