Weak ferromagnetism and exchange biasing in cobalt oxide nanoparticle systems

Cobalt oxide nanoparticle systems have been prepared by wet chemical processing involving the encapsulation of the nanoparticles by an organic ligand shell (oleic acid and oleylamine). CoO nanoparticles were easily prepared by this method, while the synthesis of the CoPt/CoO nanocomposites was achieved using a two step polyol process. CoPt nanoparticles were first obtained by simultaneous reduction of cobalt acetate and platinum acetylacetonate and then used as seeds for the growth upon them of cobalt oxide using a second polyol process. The antiferromagnetic CoO nanoparticles, when field cooled to temperatures below 200 K, show displacement of the magnetization curves (along the magnetization axis) characteristic of weak ferromagnetism phenomena that are attributed to the uncompensated surface magnetic moments. The transition temperature of the particles is considerably lower than the Neel temperature of CoO and it is followed by an upswing at low temperatures, which is attributed to spins that are loosely coupled to the antiferromagnetic core. In the CoPt/CoO nanocomposites, magnetic measurements show the appearance of increased coercivity with respect to the as-prepared CoPt particles and unidirectional anisotropy (loop shift of H-eb=1125 Oe) at temperatures below 20 K, as a result of exchange coupling between CoO and CoPt.