Atomically disordered nanocrystalline Co2Si by high-energy ball milling

Abstract

The initially ordered intermetallic compound Co2Si with orthorhombic structure was mechanically milled in a high-energy ball mill. The structural changes with milling time were followed by X-ray diffraction, high-field magnetization measurements and differential scanning calorimetry (DSC). Ball milling results in a continuous increase of the magnetization at 4.2 K in a field of 21 T with milling time. This gives evidence that by mechanical milling anti-site disorder is generated in Co2Si, similar to the case of orthorhombic Co2Ge. X-ray diffraction reveals that the number of diffraction peaks decreases with increasing milling time and that all diffraction peaks broaden. The absence of a large number of diffraction lines supports the creation of atomic disorder. The average crystallite size derived from the fine width shows a drastic decrease in the early stage of milling and tends to become constant with a value of 8-10 nm upon further milling. The material remains in the same structure as the starting compound even after prolonged periods of milling. Exothermic heat effects due to both atomic reordering and nanocrystallite growth are evident in the DSC scans of ball-milled samples. Well defined atomically disordered nanocrystalline Co2Si is produced by ball milling. Since the amount of heat released in reordering and grain growth is comparable, it can be concluded that both anti-site disorder and grain boundaries are the important sources of energy storage in Co2Si during high-energy ball milling.