Superparamagnetism in carbon-coated Co particles produced by the Kratschmer carbon arc process

A process based on the Kratschmer-Huffman carbon arc method of preparing fullerenes has been used to generate carbon-coated cobalt and cobalt carbide nanocrystallites. Magnetic nanocrystallites are extracted from the soot with a gradient field technique. For Co/C composites, structural characterization by x-ray diffraction and high-resolution transmission electron microscopy reveals the presence of a fcc Co phase, graphite, and a minority Co₂C phase. The majority of Co nanocrystals exists as nominally spherical particles, 0.5--5 nm in radius. Hysteretic and temperature-dependent magnetic response, in randomly and magnetically aligned powder samples frozen in epoxy reveals fine-particle magnetism associated with monodomain Co particles. The magnetization exhibits a unique functional dependence on H/T, and hysteresis below a blocking temperature, TB{}160 K. Below TB, the temperature dependence of the coercivity is given by Hc=Hci[1-(T/TB{)}1/2$], with ${H}ci${}450 Oe.