Voltage-circuit analysis in spin-torque ferromagnetic resonance

Spin–orbit torque (SOT) enables efficient control of magnetization via spin–orbit coupling effects, so that accurate quantification of its efficiency is crucial for the development of spin–orbit devices. Among various characterization techniques, spin-torque ferromagnetic resonance (ST-FMR) provides a powerful method to quantify SOT. However, conventional analysis methods—such as voltage ratio analysis and DC modulation—often suffer from the limited accuracy due to uncertainties in RF current estimation. In this study, we propose a voltage-based circuit analysis that directly utilizes the measured ST-FMR signals, including parasitic effects, such as RF power loss, modulation factors, and parasitic shunting. Self-consistent checks and measurements on various heterostructures confirm that our approach yields SOT efficiencies consistent with those obtained from conventional methods. Furthermore, substrate-dependent variations are resolved by applying parasitic correction, validating the robustness of our analysis. This work provides an accurate and practical methodology for evaluating SOT efficiency of spin–orbit materials.