Spin imbalance and magnetoresistance in ferromagnet/superconductor/ferromagnet double tunnel junctions

We theoretically study the spin-dependent transport in a ferromagnet/superc onductor/ferromagnet double tunnel junction. The tunneling current in the a ntiferromagnetic alignment of the magnetizations gives rise to a spin imbal ance in the superconductor. The resulting nonequilibrium spin density stron gly suppresses the superconductivity with increase of bias voltage and dest roys it at a critical voltage V-c. The results provide a new method not onl y for measuring the spin polarization of ferromagnets but also for controll ing superconductivity and tunnel magnetoresistance by applying a bias volta ge.