  We investigate the temperature dependent microwave absorption spectrum of an
yttrium iron garnet sphere as a function of temperature (5 K to 300 K) and
frequency (3 GHz to 43.5 GHz). At temperatures above 100 K, the magnetic
resonance linewidth increases linearly with temperature and shows a
Gilbert-like linear frequency dependence. At lower temperatures, the
temperature dependence of the resonance linewidth at constant external magnetic
fields exhibits a characteristic peak which coincides with a non-Gilbert-like
frequency dependence. The complete temperature and frequency evolution of the
linewidth can be modeled by the phenomenology of slowly relaxing rare-earth
impurities and either the Kasuya-LeCraw mechanism or the scattering with
optical magnons. Furthermore, we extract the temperature dependence of the
saturation magnetization, the magnetic anisotropy and the g-factor.
