  We report a study of spin dependent transport in a system composed of a
quantum dot coupled to a normal metal lead and a ferromagnetic lead (NM-QD-FM).
We use the master equation approach to calculate the spin-resolved currents in
the presence of an external bias and an intra-dot Coulomb interaction. We find
that for a range of positive external biases (current flow from the normal
metal to the ferromagnet) the current polarization
$\wp=(I_\uparrow-I_\downarrow)/(I_\uparrow+I_\downarrow)$ is suppressed to
zero, while for the corresponding negative biases (current flow from the
ferromagnet to the normal metal) $\wp$ attains a relative maximum value. The
system thus operates as a rectifier for spin--current polarization. This effect
follows from an interplay between Coulomb interaction and nonequilibrium spin
accumulation in the dot. In the parameter range considered, we also show that
the above results can be obtained via nonequilibrium Green functions within a
Hartree-Fock type approximation.
