Electron spin resonance and microwave magnetoresistance in Ge : Mn thin films

We report a study of resonant and nonresonant microwave absorption in germanium thin films (t=120 nm) implanted with manganese to a concentration of x=2-8 at. %. The germanium matrix contains Mn(5)Ge(3) precipitates, Mn-rich ferromagnetic nanoclusters, and diluted Mn ions. Electron spin resonances (ESR) observed below 60 K are attributed to collective spin waves in the whole film, while at higher temperatures resonances due to ferromagnetic Mn(5)Ge(3) precipitates are detected. The high-frequency magnetoresistance (MR) exhibits a nonmonotonic field behavior with a minimum around H=1-4 kOe. The orientation dependence of the MR is explained by dimensional effects in the thin film geometry and inhomogeneous distribution of magnetic centers. A phase relaxation length of band carriers in Ge:Mn films is determined. It varies from 70-350 nm with decreasing temperature and exceeds the mean intercluster distance in the whole temperature range. This implies that the intrinsic conductivity of the nanoclusters does not generally influence the MR, and the main contribution to the microwave MR originates from charge carriers.