Substrate-induced spin-torque-like signal in spin-torque ferromagnetic resonance measurement

Oxide thin films and interfaces with strong spin-orbit coupling have recently shown exceptionally high charge-to-spin conversion, making them potential spin-source materials for spintronics. Epitaxial strain engineering using oxide substrates with different lattice constants and symmetries has emerged as a mean to further enhance charge-to-spin conversion. However, high relative permittivity and dielectric loss of commonly used oxide substrates, such as ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$, can cause significant current shunting in substrates at high frequency, which may strongly affect spin-torque measurement and potentially result in an inaccurate estimation of charge-to-spin conversion efficiency. In this study, we systematically evaluate the influence of various oxide substrates for the widely used spin-torque ferromagnetic resonance (ST-FMR) measurement. Surprisingly, we observed substantial spin-torque signals in samples comprising only ferromagnetic metal on oxide substrates with high relative permittivity (e.g., ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ and ${\mathrm{KTaO}}_{3}$), where negligible signal should be initially expected. Notably, this unexpected signal shows a strong correlation with the capacitive reactance of oxide substrates and the leakage rf current within the substrate. By revising the conventional ST-FMR analysis model, we attribute this phenomenon to a 90\ifmmode^\circ\else\textdegree\fi{} phase difference between the rf current flowing in the metal layer and in the substrate. We suggest that extra attention should be paid during the ST-FMR measurements, as this artifact could dominate over the real spin-orbit torque signal from high-resistivity spin-source materials grown on substrate with high relative permittivity.