Local structure and magnetic properties of mechanical alloyed Co–C compositions

Abstract

Formation of nanocrystalline Co particles embedded in an amorphous C matrix produced by the mechanical alloying process was studied. The formation of the Co5C95 alloy was examined by the x-ray diffraction (XRD) and extended x-ray absorption fine structure methods. XRD analysis displayed the decrease in the Co grains size, the transformation from the face-center-cubic (fcc)+hexagonal close-packed (hcp) phases to the hcp phase and an amorphization of carbon matrix with milling time. At the early stage of milling, the coercivity, HC, increased because of the higher hcp phase HC than that of the fcc phase. Partial HC increase could be also caused by the decrease of Co particles size. The subsequent powder milling led to the decrease in the HC due to thermal process and/or due to the transformation of hcp-Co to a random close-packed phase under a long-term milling. Magnetization of the compositions decreased gradually with milling. The size of single domain particles after the 60 h of milling was about 10 nm. The Co–C and Co–Co bond distances, deduced by pair distribution function analysis, were equal to 1.92±0.02 Å and 2.48±0.02 Å, respectively.