  We study I-V characteristics of an all-II-VI semiconductor resonant tunneling
diode with dilute magnetic impurities in the quantum well layer. Bound magnetic
polaron states form in the vicinity of potential fluctuations at the well
interface while tunneling electrons traverse these interface quantum dots. The
resulting microscopic magnetic order lifts the degeneracy of the resonant
tunneling states. Although there is no macroscopic magnetization, the resulting
resonant tunneling current is highly spin polarized at zero magnetic field due
to the zero field splitting. Detailed modeling demonstrates that the local spin
polarization efficiency exceeds 90% without an external magnetic field.
