Theory of spin-torque ferrimagnetic resonance

We report a theory of spin-torque-induced magnetic resonance in collinear ferrimagnets, i.e., spin-torque ferrimagnetic resonance (ST-FiMR). We find that the rectified DC voltage of ST-FiMR has several distinct features as compared to spin-torque ferromagnetic resonance (ST-FMR) signals. The most important feature of the ST-FiMR signal is that its magnitude due to dampinglike spin-orbit torque is almost linearly proportional to the net spin density, and thus its sign changes near the angular momentum compensation condition. Our result suggests that the conventional ST-FMR line-shape analysis is unable to correctly estimate the magnitude and sign of spin-orbit torque for compensated ferrimagnets. Therefore, the analysis based on the ST-FiMR theory, of which a complete analytic expression is provided in this work, must be applied.