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

We study theoretically the spin-dependent transport in ferromagnet/normal-metal/ferromagnet double tunnel junctions by special attention to cotunneling in the Coulomb blockade region. The spin accumulation caused by cotunneling squeezes the Coulomb blockade region when the magnetizations in the ferromagnetic electrodes are antiparallel. Outside the squeezed Coulomb blockade 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 magnetoresistance and the magnitude of the spin accumulation are calculated.