Characteristic tunnel-type conductivity and magnetoresistance in a CoO-coated monodispersive Co cluster assembly

We have studied electrical conductivity, sigma, and magnetoresistance in a CoO-coated monodispersive Co cluster assembly fabricated by a plasma-gas-ag gregation-type cluster beam deposition technique. The temperature dependenc e of sigma is described in the form of log sigma vs 1/T for 7<T <80 K. The magnetoresistance ratio (rho(0)=rho(3T))/rho(0) increases sharply with decr easing temperature below 25 K: from 3.5% at 25 K to 20.5% at 4.2 K. This ma rked increase (by a factor of 6) is much larger than those observed for con ventional metal-insulator granular systems. These results are ascribed to t he Coulomb blockade effect in the monodispersed cluster assemblies. (C) 199 9 American Institute of Physics. [S0003-6951(99)01901-4].