SUPERPARAMAGNETISM IN CARBON-COATED CO PARTICLES PRODUCED BY THE KRATSCHMER CARBON-ARC PROCESS

A process based on the Kratschmer-Huffman carbon arc method of prepari ng fullerenes has been used to generate carbon-coated cobalt and cobal t carbide nanocrystallites. Magnetic nanocrystallites are extracted fr om the soot with a gradient field technique. For Co/C composites, stru ctural characterization by x-ray diffraction and high-resolution trans mission electron microscopy reveals the presence of a fcc Co phase, gr aphite, and a minority Co2C phase. The majority of Co nanocrystals exi sts as nominally spherical particles, 0.5-5 nm in radius. Hysteretic a nd temperature-dependent magnetic response, in randomly and magnetical ly aligned powder samples frozen in epoxy reveals fine-particle magnet ism associated with monodomain Co particles. The magnetization exhibit s a unique functional dependence on H/T, and hysteresis below a blocki ng temperature, T(B) congruent-to 160 K. Below T(B), the temperature d ependence of the coercivity is given by H(c) = H(ci)[1 -(T/T(B))1/2], with H(ci) congruent-to 450 Oe.