Temperature and voltage dependence of the resistance and magnetoresistance in discontinuous double tunnel junctions

We have studied the electric and magnetotransport properties of $\mathrm{Co}//{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{Co}}_{\mathrm{I}}/{\mathrm{Al}}_2{\mathrm{O}}_3//(\mathrm{Co}or\mathrm{Cu})$ tunnel double junctions, in which the intermediate ${\mathrm{Co}}_I$ layer is either granular or percolated clusters and the top electrode is either Co or Cu. In granular junctions, spin-dependent tunneling with Coulomb-blockade effect was observed. The temperature dependence of the resistance can be attributed to two conduction channels in parallel between electrodes via granules and via other channels such as metallic impurities in the insulating layer. Voltage and temperature dependences of the tunnel magnetoresistance are discussed. The Coulomb blockade and the improvement of the antiparallel alignment at low temperature both enhanced the TMR effects.