Spin injection and magnetoresistance in ferromagnet-superconductor-ferromagnet tunnel junctions

We theoretically study the spin-dependent transport in a ferromagnet-superc onductor-ferromagnet double barrier tunnel junction. The spin-polarized tun neling currents give rise to spin imbalance in the superconductor. The resu lting nonequilibrium spin density suppresses the superconductivity with inc rease of the tunneling currents. We focus on the effect of asymmetry in the double tunnel junction, where the barrier height of the tunnel junction an d the spin polarization of the ferromagnets are different, on spin injectio n, and discuss how the superconductivity is suppressed in the asymmetric ju nction. Our results explain recent experimental results on the critical cur rent suppression in high-T-c superconductors by spin injection. (C) 2000 Am erican Institute of Physics. [S0021-8979(00)61708-5].