Spin-dependent Coulomb blockade in ferromagnet/normal-metal/ferromagnet double tunnel junctions

We study theoretically the spin-dependent transport in ferromagnet/normal-m etal/ferromagnet double tunnel junctions by special attention to cotunnelin g in the Coulomb blockade region. The spin accumulation caused by cotunneli ng squeezes the Coulomb blockade region when the magnetizations in the ferr omagnetic electrodes are antiparallel. Outside the squeezed Coulomb blockad e region, we propose an anomalous region where the sequential tunneling in one of the spin bands is suppressed by the Coulomb blockade and that in the other is not. In this region, the tunnel magnetoresistance oscillates as a function of bias voltage. The temperature dependences of the tunnel magnet oresistance and the magnitude of the spin accumulation are calculated. [S01 63-1829(99)08209-0].