Highly accurate evaluation of spin-torque efficiency by measuring in-plane angular dependence of spin-torque ferromagnetic resonance

Spin-torque ferromagnetic resonance (ST-FMR) is a powerful tool to evaluate spin-torque efficiency (ξ_FMR), the efficiency of current-induced torque in metallic bilayers consisting of ferromagnetic and nonmagnetic materials. In general, ξ_FMR can be evaluated from the amplitude ratio between symmetric and anti-symmetric components of the ST-FMR spectrum. However, the ratio is also affected by an unnecessary magnetic field owing to the asymmetric alternating current flow used to excite the ST-FMR. In this article, we demonstrate that ξ_FMR can be precisely evaluated from the full-angular dependence of the ST-FMR spectrum even though the distribution of alternating current flow is highly asymmetric.