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Magnon-Mediated Dzyaloshinskii-Moriya Torque in Homogeneous Ferromagnets

In thin magnetic layers with structural inversion asymmetry and spin-orbit coupling, a Dzyaloshinskii-Moriya interaction arises at the interface. When a spin wave current ${\bf j}_m$ flows in a system with a homogeneous magnetization {\bf m}, this interaction produces an effective field-like torque on the form ${\bf T}_{\rm FL}\propto{\bf m}\times({\bf z}\times{\bf j}_m)$ as well as a damping-like torque, ${\bf T}_{\rm DL}\propto{\bf m}\times[({\bf z}\times{\bf j}_m)\times{\bf m}]$ in the presence of spin-wave relaxation (${\bf z}$ is normal to the interface). These torques mediated by the magnon flow can reorient the time-averaged magnetization direction and display a number of similarities with the torques arising from the electron flow in a magnetic two dimensional electron gas with Rashba spin-orbit coupling. This magnon-mediated spin-orbit torque can be efficient in the case of magnons driven by a thermal gradient.