Magnetic and structural properties of Gex Mn1-x films: Precipitation of intermetallic nanomagnets

Abstract

We present a comprehensive study relating the nanostructure of Ge0.95 Mn0.05 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°C. A combined transmission electron microscopy and electron energy-loss spectroscopy analysis of the films identifies the inclusions as precipitates of the ferromagnetic compound Mn5 Ge3. The volume and amount of these precipitates decrease with decreasing TS. Magnetometry of the films containing precipitates reveals distinct temperature ranges: Between the characteristic ferromagnetic transition temperature of Mn5 Ge3 at approximately room temperature and a lower, TS -dependent blocking temperature TB the magnetic properties are dominated by superparamagnetism of the Mn5 Ge3 precipitates. Below TB, 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. © 2006 The American Physical Society.