  Using the transfer matrix method and the effective-mass approximation, the
effect of resonant states on spin transport is studied in
ZnSe/ZnMnSe/ZnSe/ZnMnSe/ZnSe structures under the influence of both electric
and magnetic fields. The numerical results show that the ZnMnSe layers, which
act as spin filters, polarize the electric currents. Variation of thickness of
the central ZnSe layer shifts the resonant levels and exhibits an oscillatory
behavior in spin current densities. It is also shown that the spin polarization
of the tunneling current in geometrical asymmetry of the heterostructure where
two ZnMnSe layers have different Mn concentrations, depends strongly on the
thickness and the applied bias.
