Spin-Orbit-Torque Exchange Resonance in Magnetic Topological Insulators*

Recent developments in magnetic topological insulators (MTI) opened new scenarios of electric control of magnetization dynamics. We formulate the adiabatic spin-orbit torque (SOT) arising from the topological electrons in MTI as well as its reciprocal effect— topological charge pumping. Because the SOT acts oppositely on two adjacent magnetic layers, an applied AC electric field can drive the resonance of the exchange mode, whose frequency is much higher than the ferromagnetic resonance. Owing to the topological charge pumping, the SOT-induced exchange resonance manifests as a sharp peak in the effective optical admittance, which entails a remarkably weaker energy dissipation compared to the ferromagnetic and antiferromagnetic resonance driven by ordinary SOT. Our result introduces novel functionalities of MTI-based devices.