Fully Symmetric Line-shape in Spin-Torque Ferromagnetic Resonance by Impedance Matching

Spin-torque ferromagnetic resonance (ST-FMR) has been widely used to quantify the spin-charge conversion efficiency of various materials such as heavy metals, topological insulators, van de Waals materials, and antiferromagnets. However, the ST-FMR signal may include other contributions such as the influence of the dielectric substrates, leading to the over-estimation of spin Hall angle. In this work, we show by tuning the wiring of the waveguide, fully symmetric dc voltage peak can be observed in Py/Al bilayers, which would deduce an unphysical large spin Hall angle. It indicates the impedance induced phase in the spin rectification plays crucial role in the line-shape of the dc voltage, rather than the conversion of pure spin current. The current-induced linewidth modification can also be explained by taking account of the impedance induced phase in the spin rectification, rather than spin-orbit torques. Our results suggest that ST-FMR may not be reliable to quantify the spin-charge conversion efficiency.