Synthesis and magnetic properties of ε-cobalt nanoparticles

Monodisperse cobalt nanoparticles are synthesized via a high‐temperature thermal decomposition method in the presence of oleic acid and triphenylphosphine. The as‐synthesized nanoparticles are stable against further deep oxidation when they are kept in heptane (C7H16). Time‐dependent XPS studies indicate that oxidation of the as‐synthesized cobalt nanoparticles in air is slow. The valence change of cobalt from the nanoparticle sample is not observed after it is kept in heptane under air for 90 days. The cobalt nanoparticles have a β‐manganese‐type structure (also called ε‐Co). Annealing the nanoparticles at 500°C under Ar (95%) + H2 (5%) converts these particles from ε‐Co to fcc‐Co. Two‐dimensional and three‐dimensional self‐assembled superlattices of the passivated cobalt nanoparticles are formed by slow evaporation of the carrier solvent. The magnetic properties of the cobalt nanoparticles in different forms are compared, which provides helpful information on the magnetostatic interaction of the nanoparticles. Copyright © 2004 John Wiley & Sons, Ltd.