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

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, H-C, increased because of the higher hcp phase H-C than that of the fcc phase. Partial H-C increase could be also caused by the decrease of Co particles size. The subsequent powder milling led to the decrease in the H-C 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 A and 2.48+/-0.02 A, respectively. (C) 2004 American Institute of Physics.