First-principles study of magnon-phonon interactions in gadolinium iron garnet
We obtained the spin-wave spectrum based on a first-principles method of exchange constants, calculated the phonon spectrum by the first-principles phonon calculation method, and extracted the broadening of the magnon spectrum, $\Delta \omega$, induced by magnon-phonon interactions in gadolinium iron garnet (GdIG). Using the obtained exchange constants, we reproduce the experimental Curie temperature and the compensation temperature from spin models using Metropolis Monte Carlo (MC) simulations. In the lower-frequency regime, the fitted positions of the magnon-phonon dispersion crossing points are consistent with the inelastic neutron scattering experiment. We found that the $\Delta \omega$ and magnon wave vector $k$ have a similar relationship in YIG. The broadening of the acoustic spin-wave branch is proportional to $k^{2}$, while that of the YIG-like acoustic branch and the optical branch are a constant. At a specific $k$, the magnon-phonon thermalization time of $\tau_{mp}$ are approximately $10^{-9}$~s, $10^{-13}$~s, and $10^{-14}$~s for acoustic branch, YIG-like acoustic branch, and optical branch, respectively. This research provides specific and effective information for developing a clear understanding of the spin-wave mediated spin Seebeck effect and complements the lack of lattice dynamics calculations of GdIG.