Spin dynamics study in layered van der Waals single-crystal Cr2Ge2Te6

We study the magnetization dynamics of a bulk single crystal ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT), by means of broadband ferromagnetic resonance (FMR), for temperatures from 60 K down to 2 K. We determine the Kittel relations of the fundamental FMR mode as a function of frequency and static magnetic field for the magnetocrystalline easy---and hard---axis. The uniaxial magnetocrystalline anisotropy constant is extracted and compared with the saturation magnetization, when normalized with their low temperature values. The ratios show a clear temperature dependence when plotted in the logarithmic scale, which departs from the predicted Callen-Callen power law fit of a straight line, where the scaling exponent $n$ , ${K}_{u}(T)\ensuremath{\propto}{[{M}_{s}(T)/{M}_{s}(2\phantom{\rule{4pt}{0ex}}\mathrm{K})]}^{n}$, contradicts the expected value of 3 for uniaxial anisotropy. Additionally, the spectroscopic $g$ factor for both the magnetic easy---and hard---axis exhibits a temperature dependence, with an inversion between 20 K and 30 K, suggesting an influence by orbital angular momentum. Finally, we qualitatively discuss the observation of multidomain resonance phenomena in the FMR spectras, at magnetic fields below the saturation magnetization.