Clustering in a precipitate free GeMn magnetic semiconductor

We present the first study relating structural parameters of precipitate free Ge0.95Mn0.05 films to magnetisation data. Nanometer sized clusters - areas with increased Mn content on substitutional lattice sites compared to the host matrix - are detected in transmission electron microscopy (TEM) analysis. The films show no overall spontaneous magnetisation at all down to 2K. The TEM and magnetisation results are interpreted in terms of an assembly of superparamagnetic moments developing in the dense distribution of clusters. Each cluster individually turns ferromagnetic below an ordering temperature which depends on its volume and Mn content.