Magnetic properties of amorphous Co2−xFexGe alloys synthesized by mechanical alloying

Amorphous Co2−xFexGe (0.0≤x≤0.30) alloys were synthesized by direct mechanical alloying of the elemental powders in a SPEX 8000 mixer mill, and their micro-structure and magnetic properties were investigated using X-ray diffraction (XRD), Mössbauer spectroscopy and dc magnetization measurements. The as-milled alloys showed a maximum in magnetization at substantially high temperatures of about 200–240 K and field dependent irreversibility effects, which could be attributed to a superparamagnetic blocking of uncoupled magnetic clusters in a non-magnetic matrix. When the samples were subjected to a heat treatment at 440°C for 1 h, the maxima shifted to very low temperatures in the region of 40–45 K. A broadening of the transition as well as shift to lower temperature was observed with increase in the field. This behavior was consistent with a spin glass-like freezing taking place at this temperature, and in agreement with the observations of Zhou and Bakker who synthesized amorphous Co2Ge by mechanical milling of crystalline Co2Ge for long durations of about 240 h. There is also a systematic increase in the spin glass freezing temperature with Fe concentration. Our results imply that the as-milled alloys are superparamagnetic, the heat treatment carried out on the as-milled samples is resulting in homogenous amorphous alloys, and the magnetic behavior is changing from superparamagnetic to spin glass.