Spin-torque-driven magnetization dynamics of ferrimagnet Co100−xGdx alloy

Electrically induced spin-torque phenomena in rare-earth–transition-metal ferrimagnetic alloys have been extensively studied. However, the key question of which channel absorbs the spin angular momentum has been debated, which highlights the need for direct measurement of the magnetization dynamics, especially under short-pulsed spin injection. In this work, we investigate the spin-torque-driven magnetization dynamics of ferrimagnet Co100−xGdx, employing optical spin generation in Pt. We observe a sign reversal in the initial tilting direction of magnetization dynamics between Co-rich samples and Gd-rich samples. In addition, the amplitude of magnetization dynamics exhibits a close correlation with the net magnetization. Our results present strong evidence that the net magnetization of ferrimagnet primarily absorbs the spin currents.