Monodispersed Co/CoO cluster assemblies with the mean cluster sizes of 6 and 13 nm have been prepared by a plasma-gas condensation type cluster beam deposition apparatus. We measured the effects of the oxygen gas flow rate during deposition, temperature, and cluster size on the coercivity and hysteresis loop shift induced by field cooling. The large exchange bias field (10.2 kOe) and coercivity (5 kOe) were observed at 5 K for the monodispersed Co/CoO cluster assembly with $d=6\mathrm{}\mathrm{nm}.$ The correlations between unidirectional anisotropy and uniaxial anisotropy, training effect and magnetic relaxation can be interpreted by the hypothesis of a spin disorder in the interfacial layer between the antiferromagnetic CoO shell and the ferromagnetic Co core.

• Received 28 July 1999

DOI:https://doi.org/10.1103/PhysRevB.61.3103