Understanding the interaction between magnons and phonons is the key for magnonic applications. In this work, we develop an optical reflectometry to investigate the population of magnons and phonons. Unlike the Brillouin light scattering (BLS) spectroscopy 1, 2, where the sophisticated tandem Fabry-Perot interferometer and single frequency laser with sharp line width are necessary, we use a simple laser-based reflectometry to measure the Kerr rotation and reflectance which are sensitive to the magnon and phonon, respectively.
Using this optical technique, we study the spatial distribution of magnons and phonons in a thulium iron garnet (TmIG) under the temperature gradient generated by current induced Joule
heating. Both the magnon and phonon populations are decaying exponentially as a function of distance from the electrical heating line. We find that the characteristic decay length of the
population of magnons and phonons are different, indicating the non-equilibrium between magnons and phonons in TmIG. Moreover, the characteristic length of magnon population
decreases as increasing the heating power or decreasing the magnetic field, while that of phonon population is almost constant. The different characteristic length of magnon and phonon
population is attributed to the enhanced non-linear magnon scattering processes at high magnon population regime. Our work revealing the magnon-phonon non-equilibrium in TmIG will
motivate the magnonic researches where the role of the magnon-phonon interaction is crucial 3.
1 M. Agrawal., et al., Phys. Rev. Lett. 111, 107204 (2013). 2 K. An., et al., Phys. Rev. Lett. 117, 107202 (2016). 3 G.-H. Lee., et al., Appl. Phys. Lett. 119, 152406 (2021).