Magnetic and transport polaron percolation in diluted GeMn films

Magnetic and electronic transport properties of Mn-doped Ge films have been studied as a function of Mn content. The films exhibit a long-range ferromagnetic behavior and a Mn dilution-dependent Curie temperature TC. Resistivity shows an insulator-like character with two distinct activation energies below about 80 K, while the Hall coefficient evidences a strong contribution from the anomalous Hall effect, in a p-type material. At a characteristic temperature TR, resistivity experiences a sudden reduction and the Hall coefficient reverses its sign from positive to negative. Moreover, around TR, any residual remanence and coercivity disappear. The transport and magnetic results are deeply related and can be qualitatively explained by a percolation model based on bound magnetic polarons due to localized holes in the GeMn alloy lattice.