Using non-equilibrium Green functions we calculate the spin-polarized current and shot noise in a ferromagnet-quantum-dot-ferromagnet (FM-QD-FM) system. Both parallel (P) and antiparallel (AP) magnetic configurations are considered. Coulomb interaction and coherent spin-flip (similar to a transverse magnetic field) are taken into account within the dot. We find that the interplay between Coulomb interaction and spin accumulation in the dot can result in a bias-dependent current polarization ℘. In particular, ℘ can be suppressed in the P alignment and enhanced in the AP case depending on the bias voltage. The coherent spin-flip can also result in a switch of the current polarization from the emitter to the collector lead. Interestingly, for a particular set of parameters it is possible to have a polarized current in the collector and an unpolarized current in the emitter lead. We also found a suppression of the Fano factor to values well below 0.5.

We study spin-polarized transient transport in a quantum dot coupled to two ferromagnetic leads subjected to a rectangular bias voltage pulse. Time-dependent spin-resolved currents, occupations, spin accumulation, and tunneling magnetoresistance ͑TMR͒ are calculated using both nonequilibrium Green function and master equation techniques. Both parallel-and antiparallel-lead magnetization alignments are analyzed. Our main findings are a dynamical spin accumulation that changes sign in time, a short-lived pulse of spin polarized current in the emitter lead ͑but not in the collector lead͒, and a dynamical TMR that develops negative values in the transient regime. We also observe that the intradot Coulomb interaction can enhance even further the negative values of the TMR.