Unconventional spin-orbit torques and high-frequency exchange resonance in 2D antiferromagnetic topological insulators

Recent developments in magnetic topological insulators (MTI) such as antiferromagnetic MnBi2Te4 opened new arenas for electric control of magnetization dynamics. PI-Cheng’s group formulated the spin-orbit torque (SOT) arising from the topological electrons in antiferromagnetic MTI as well as its reciprocal effect—topological charge pumping. For undoped MnBi2Te4, the SOT originates from the adiabatic current of valence electrons, which does not incur Joule heating as ordinary current-induced torques. The proposed SOT acts oppositely on two neighboring magnetic layers, whereby an applied AC electric field can drive the resonance of the exchange mode with a frequency lying in the sub-THz regime. The SOT-induced exchange resonance manifests as a sharp peak in the effective optical conductivity, entailing a remarkably weaker energy dissipation compared to ordinary ferromagnetic and antiferromagnetic resonances driven by conventional SOT. In trilayer antiferromagnetic MnBi2Te4, a hitherto unknown exchange resonance lying in the sub-THz regime is predicted, which does not react to microwave radiations but can only be driven by the unconventional SOT in MTI. This works opens new pathways to manipulate MnBi2Te4 and similar antiferromagnetic MTIs by electrical means.

This theoretical work has been published in Physical Review B and is available here.