Transient charging and discharging of spin-polarized electrons in a quantum dot

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 (TMR) 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 lead), 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.