Resonant tunneling via spin-polarized barrier states in a magnetic tunnel junction

Resonant tunneling through states in the barrier of a magnetic tunnel junction has been analyzed theoretically for the case of a spin-polarized density of barrier states. It is shown that for highly spin-polarized barrier states, the magnetoresistance due to resonant tunneling is enhanced compared to the magnetoresistance due to direct tunneling between the ferromagnetic electrodes. Moreover, it is predicted that whenever the barrier states have a nonzero polarization, tunneling magnetoresistance exists even if only one electrode is magnetic, while the other is not. The above effects have been studied for arbitrary location of the states. In particular, we consider the case of a thin sheet of magnetic dopants in close proximity to and coupled ferromagnetically or antiferromagnetically to one of the electrodes. Depending on the dopant polarization, the distance to the electrode and the type of coupling, the total magnetoresistance can be enhanced or reduced, and may be reversed for antiferromagnetic coupling.