Magnons and Phonons Driven out of Equilibrium in a Magnetic Insulator*

We investigate magnons and phonons in a bulk Y$_{3}$Fe$_{5}$O$_{12~}$(YIG) under a large temperature gradient created by laser radiation. YIG is a good playground to study the interaction between phonons and magnons. Because of its absence of itinerant electrons, energy transport is only carried by magnons and phonons. Understanding the coupling between them is a key to the thermally driven spin transport such as Spin Seebeck Effect. We use Brillouin light scattering technique to measure phonon and magnon temperature. We found that they can be driven out of equilibrium under a large temperature gradient. We numerically simulate the phonon and magnon temperatures using two-temperature model. Our results suggest a lower bound of magnon phonon relaxation time in YIG.