Magnetic and structural properties of GeMn films: precipitation of intermetallic nanomagnets

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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 Tₛ as low as 70 deg. Celsius. A combined transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) analysis of the films identifies the inclusions as precipitates of the ferromagnetic compound Mn₅Ge₃. The volume and amount of these precipitates decreases with decreasing Tₛ. 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, Tₛ dependent blocking temperature TB the magnetic properties are dominated by superparamagnetism of the Mn₅Ge₃ precipitates. Below TB, the magnetic signature of ferromagnetic precipitates with blocked magnetic moments is observed. At the lowest temperatures, the films show features characteristic for a metastable state.