We have fabricated CoO-coated monodispersive Co cluster assemblies with the mean cluster size of 13 nm at various oxygen gas-flow rate $R_{{\mathrm{O}}_2}$ by a plasma-gas-condensation-type cluster beam deposition technique, and studied their electrical conductivity, σ, and magnetoresistance. For $R_{{\mathrm{O}}_2}<0.24\mathrm{}\mathrm{SCCM}$ (sccm denotes cubic centimeter per minute), the resistivity revealed a minimum and showed $\ln T$ dependence at lower temperatures, probably due to the weak localization of conduction electrons owing to presence of thin oxide shells covering Co cores. A small negative magnetoresistance was observed in this regime. For $R_{{\mathrm{O}}_2}>0.3\mathrm{}\mathrm{SCCM},$ tunnel-type temperature dependence of σ in the form of ln σ vs $1/T$ was observed between 7 and 80 K. This differs from the well-known temperature dependence of ln σ vs ${1/T}^{1/2}$ for disordered granular materials. The magnetoresistance ratio, $\left({\rho }_{\mathrm{H}=30\mathrm{}\mathrm{}\mathrm{kOe}}-{\rho }_0\right)/{\rho }_0,$ is negative and its absolute value increases sharply with decreasing temperature below 25 K: from 3.5% at 25 K to 20.5% at 4.2 K. This marked increase, by a factor of 6, is much larger than those observed for conventional metal-insulator granular systems. These results are ascribed to a prominent cotunneling effect in the Coulomb blockade regime, arising from the uniform Co core size and CoO shell thickness in the present monodispersed cluster assemblies.

• Received 19 January 1999

DOI:https://doi.org/10.1103/PhysRevB.60.2093