Tunnel magnetoresistance in ferromagnetic junctions: Tunneling through a single discrete level

Tunnel magnetoresistance in a double-barrier junction with ferromagnetic electrodes, and a quantum dot (or a single atom) as the central part, is analyzed theoretically in the sequential-tunneling regime. The magnetoresistance is due to the rotation of magnetic moments of external electrodes from antiparallel to parallel alignment. The considerations are restricted to the case of a single discrete level, with Coulomb correlations and spin-flip transitions included. The tunneling current and occupation numbers are calculated for both magnetic configurations. It is shown that electron correlations at the dot can enhance the magnetoresistance effect, and give rise to a diodelike behavior. Spin-flip processes, on the other hand, suppress the magnetoresistance, and reduce the magnetically induced asymmetry in the current-voltage characteristics with respect to the bias reversal.