  We have studied the effects of Mn concentration on the ballistic
spin-polarized transport through diluted magnetic semiconductor
heterostructures with a single paramagnetic layer. Using a fitted function for
zero-field conduction band offset based on the experimental data, we found that
the spin current densities strongly depend on the Mn concentration. The
magnitude as well as the sign of the electron-spin polarization and the tunnel
magnetoresistance can be tuned by varying the Mn concentration, the width of
the paramagnetic layer, and the external magnetic field. By an appropriate
choice of the Mn concentration and the width of the paramagnetic layer, the
degree of spin polarization for the output current can reach 100% and the
device can be used as a spin filter.
