We compare the magnetic properties of Co cluster assembled films with different degrees of oxidation. Clusters with grain size $(2.3\pm 0.7)\mathrm{nm}$ are produced in a laser vaporization cluster source and soft-landed in ultrahigh vacuum conditions, forming highly porous nanogranular films. After exposure to air for different periods of time, the Co clusters oxidize and the sample may be considered as a thin antiferromagnetic Co oxide matrix containing ferromagnetic Co clusters. Magnetization measurements were performed in a temperature range from 300 down to $5\mathrm{K}$, at applied magnetic fields up to $30\mathrm{kOe}$. The exchange bias value at $5\mathrm{K}$ for the strongly oxidized sample is $4.8\mathrm{kOe}$ against the value of $0.75\mathrm{kOe}$ for the less oxidized sample. The mean values of the thicknesses of the Co oxide layers are estimated to be 0.6 and $0.3\mathrm{nm}$ for the more and less oxidized sample, respectively. We propose a method of measuring the exchange bias inducing temperature, i.e., the temperature at which exchange anisotropy is established. We determined the mean inducing temperatures for both samples, which are 55 and $25\mathrm{K}$, respectively, for the more and less oxidized samples. Both temperatures are well below the bulk CoO Néel temperature of $292\mathrm{K}$. A low value of the inducing temperature of the Co oxide layer is a consequence of its subnanometer thickness, while a large exchange bias value is a consequence of different dimensionality of Co clusters and Co oxide matrix.

• Received 2 December 2005

DOI:https://doi.org/10.1103/PhysRevB.73.245416