In this work, we study the ferromagnetic resonance (FMR) spectra of lanthanum-doped yttrium-iron-garnet (La:YIG) film as a function of temperature and frequency. The experimental results show a significant increase in the number of spin-wave resonance modes with increasing temperature. The FMR linewidth is nonlinear as a function of frequency at low temperatures but linear at room temperature. This nonlinear behavior at low temperature indicates the relaxation of the magnetization by impurities (La3+ dopant) or defects at the surfaces. Most importantly, the results reveal that temperature-dependent Gilbert damping in La:YIG film originates from two-magnon scattering, whereas the enhanced Gilbert damping at low-temperature (≤100K) is due mainly to spin-lattice relaxation of La3+ dopants.
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