  Ferromagnetic resonance (FMR) was used to investigate the static and dynamic
magnetic properties of carbon-doped Mn5Ge3 (C$_{0.1}$ and C$_{0.2}$) thin films
grown on Ge(111). The temperature dependence of magnetic anisotropy shows an
increased perpendicular magneto-crystalline contribution at 80K with an
in-plane easy axis due to the large shape contribution. We find that our
samples show a small FMR linewidth (corresponding to an intrinsic magnetic
damping parameter $\alpha$=0.005), which is a measure of the spin relaxation
and directly related with the magnetic and structural quality of the material.
In the perpendicular-to-plane geometry, the FMR linewidth shows a minimum at
around 200K for all the samples, which seems to be not correlated to the
C-doping. The magnetic relaxation parameters have been determined and indicate
the two-magnon scattering as the main extrinsic contribution. We observe a
change in the main contribution from scattering centres in Mn5Ge3C0.2 at low
temperatures, which could be related to the minimum in linewidth.
