Nonlinear phononic rectification of magnons

Ultrashort electromagnetic pulses are able to control the electronic and structural properties of solids on timescales of pico- and femtoseconds. Pulses in the terahertz and mid-infrared regime enable resonant excitations of optical phonons, by coupling the electric field component of light to the electric dipole moment of IR-active phonon modes. When the vibrational amplitudes become large enough, the excited phonons couple nonlinearly to other collective excitations through ionic Raman scattering. A particular feature of ionic Raman scattering is nonlinear phononic rectification, where the coherently excited IR-active phonons act as a unidirectional force that transiently displaces the atoms along the eigenvectors of a coupled Raman-active phonon mode. Here, we show that an analog process is possible for spin waves: Coherently excited chiral IR-active phonon modes act as a unidirectional effective magnetic field that transiently displaces the spins along the eigenvectors of a coupled magnon. We use a phenomenological model based on Landau-Lifshitz-Gilbert equations and nonlinearly driven oscillators with input from first-principles calculations to describe the response of the coupled spin-lattice dynamics to the excitation with an ultrashort terahertz pulse. We show that the spins can be transiently distorted in an analog way to Raman-active phonons, which provides a novel route towards ultrafast control of magnetism.