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

Abstract

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 ${\mathrm{Co}}_{2}$C 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, ${\mathit{T}}_{\mathit{B}}$\ensuremath{\simeq}160 K. Below ${\mathit{T}}_{\mathit{B}}$, the temperature dependence of the coercivity is given by ${\mathit{H}}_{\mathit{c}}$=${\mathit{H}}_{\mathit{c}\mathit{i}}$[1-(T/${\mathit{T}}_{\mathit{B}}$${)}^{1/2}$], with ${\mathit{H}}_{\mathit{c}\mathit{i}}$\ensuremath{\simeq}450 Oe.