Criteria for the occurrence of negative resonant magnetoresistance in magnetic tunnel junctions

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.