Magnetic and structural properties of Ge_xMn_1−x films: Precipitation of intermetallic nanomagnets

We present a comprehensive study relating the nanostructure of Ge_0.95Mn_0.05 films to their magnetic properties. The formation of ferromagnetic nanometer-sized inclusions in a defect-free Ge matrix fabricated by low-temperature molecular beam epitaxy is observed down to substrate temperatures T_S as low as 70 °C. A combined transmission electron microscopy and electron energy-loss spectroscopy analysis of the films identifies the inclusions as precipitates of the ferromagnetic compound Mn₅Ge₃. The volume and amount of these precipitates decrease with decreasing T_S. Magnetometry of the films containing precipitates reveals distinct temperature ranges: Between the characteristic ferromagnetic transition temperature of Mn₅Ge₃ at approximately room temperature and a lower, T_S-dependent blocking temperature T_B the magnetic properties are dominated by superparamagnetism of the Mn₅Ge₃ precipitates. Below T_B, the magnetic signature of ferromagnetic precipitates with blocked magnetic moments is observed. At the lowest temperatures, the films show features characteristic of a metastable state.