Amorphization and magnetic properties ofCo2Ge during mechanical milling

The structural features and magnetic properties of the intermetallic compounds α-${\mathrm{Co}}_2$Ge (low-temperature phase, LTP) and β-${\mathrm{Co}}_2$Ge (high-temperature phase, HTP) upon mechanical milling were investigated by x-ray diffraction, differential scanning calorimetry (DSC), and magnetic measurements. It turns out that starting from both the ordered LTP with ${\mathrm{Co}}_2$Si-type orthorhombic structure and the ordered HTP with ${\mathrm{Ni}}_2$In-type hexagonal structure mechanical milling generates atomic disorder during the early stage of milling and transforms the materials to an amorphous state after long-time milling. Both the LTP and the HTP are ferromagnetic at lower temperatures with Curie temperatures of 46.4 K and 6 K, respectively. The magnetic moment per Co atom at 4.2 K is 0.113${\mathrm{\mu }}_{\mathit{B}}$ in the LTP and 0.103${\mathrm{\mu }}_{\mathit{B}}$ in the HTP. The average magnetization at 4.2 K and the Curie temperatures of the LTP and the HTP continuously increase with increasing milling time in the early stage of milling. During the intermediate stage of milling, a discontinuous decrease in magnetic-ordering temperature (a mixture of two magnetic phases) is observed in the LTP, which strongly indicates the formation of the amorphous phase. In DSC scans the exothermic heat effects are evident, which correspond to the atomic reordering process of the disordered compounds and to the crystallization of the amorphous phase and the subsequent growth of nanometer-scale crystallites. All physical parameters measured in the present investigation tend to become constant after prolonged periods of milling. The good agremeent of all the experimental results obtained by different techniques proves that by mechanical milling well-defined metastable states in ${\mathrm{Co}}_2$Ge are generated. The amorphous phase as the final state shows spin-glass behavior: a transition at 43 K from the paramagnetic state to the spin-glass state is clearly observed upon cooling from room temperature to liquid-helium temperature.