Effect of Coulomb Blockade on Magnetoresistance in Ferromagnetic Tunnel Junctions

We study spin-dependent electron tunneling in ferromagnetic junctions containing small metallic islands. The tunneling matrix elements depend on the relative direction of magnetization of the island and electrodes. The dependence of the matrix elements amplifies the cotunneling in the Coulomb blockade regime. We show that in single-electron ferromagnetic transistors the magnetoresistance is strongly enhanced by the Coulomb blockade. The results provide a theoretical basis for recent experiments on ferromagnetic single-electron transistors, ferromagnetic double tunnel junctions, and ferromagnetic granular materials.