Exchange anisotropy of monodispersed Co/CoO cluster assemblies

Monodispersed Co/CoO cluster assemblies with the mean cluster size of 13 nm have been prepared using a plasma-gas-condensation-type cluster beam depos ition apparatus. The structural analysis and magnetic measurement indicate that the Co cluster is covered by an oxide shell composed of CoO. The effec t of the oxygen gas flow rate during deposition and that of temperature on the coercivity and hysteresis loop shift induced by field cooling were meas ured. The effect of the CoO shell on the loop shift and the temperature dep endence of the exchange anisotropy are discussed. The unidirectional anisot ropy is negligible above 200 K for the present assemblies. This is ascribed to the rapid decrease of the anisotropy of the antiferromagnetic interfaci al layers near the inter-face of the Co cores and CoO shells.