Paramagnetic ion-doped nanocrystal as a voltage-controlled spin filter

A theory of spin injection from a ferromagnetic source into a semiconductor through a paramagnetic ion-doped nanocrystal is developed. Spin-polarized current from the source polarizes the ion; the polarized ion, in turn, controls the spin polarization of the current flowing through the nanocrystal. Depending on voltage, the ion can either enhance the injection coefficient by several times or suppress it. Large ion spins produce stronger enhancement of spin injection.