Characteristic transport properties of CoO-coated monodispersive Co cluster assemblies

We have fabricated CoO-coated monodispersive Co cluster assemblies with the mean cluster size of 13 nm at various oxygen gas-flow rate R_O₂ by a plasma-gas-condensation-type cluster beam deposition technique, and studied their electrical conductivity, {σ}, and magnetoresistance. For R_O₂<0.24SCCM (sccm denotes cubic centimeter per minute), the resistivity revealed a minimum and showed lnT 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_O₂>0.3SCCM, 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/T1/2 for disordered granular materials. The magnetoresistance ratio, (ρH=30kOe-ρ₀)/ρ₀, 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.