Magnetic and structural properties of GexMn1-x films:: Precipitation of intermetallic nanomagnets

We present a comprehensive study relating the nanostructure of Ge0.95Mn0.05 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 degrees 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.