Self-assembled monodispersed ferromagnetic fcc cobalt nanoparticles with sizes varying from 6 to 24 nm were synthesized by reductive thermal decomposition of cobalt-oleyl amine complex in the presence of various surfactants such as oleic acid, polyvinyl pyrollidone and trioctylphosphine. The magnetic measurements at 5 and 300 K established the single to multi-domain behavior. The highest coercivity for the nanoparticles was found to be of the values 622 and 210 Oe at 5 and 300 K respectively. This is due to the coherent rotation of spins. (C) 2008 Elsevier B. V. All rights reserved.