Characteristic transport properties of CoO-coated monodispersive Co cluster assemblies

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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, \ensuremath{\sigma}, and magnetoresistance. For ${R}_{{\mathrm{O}}_{2}}l0.24\mathrm{SCCM}$ (sccm denotes cubic centimeter per minute), the resistivity revealed a minimum and showed $\mathrm{ln}T$ 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 ${R}_{{\mathrm{O}}_{2}}g0.3\mathrm{SCCM},$ tunnel-type temperature dependence of \ensuremath{\sigma} in the form of ln \ensuremath{\sigma} vs $1/T$ was observed between 7 and 80 K. This differs from the well-known temperature dependence of ln \ensuremath{\sigma} vs ${1/T}^{1/2}$ for disordered granular materials. The magnetoresistance ratio, $({\ensuremath{\rho}}_{\mathrm{H}=30\mathrm{}\mathrm{kOe}}\ensuremath{-}{\ensuremath{\rho}}_{0})/{\ensuremath{\rho}}_{0},$ 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.