Transport in magnetically doped magnetic tunnel junctions

We report striking transport behavior of magnetic tunnel junctions which are engineered to incorporate localized magnetic impurities inside the tunnel barrier. Such junctions exhibited zero-bias anomalies (giant conductance dip or equivalently, giant resistance peak) in dynamic conductance curves, and striking bias and temperature dependencies of tunneling magnetoresistance (TMR), i.e., suppression of TMR at zero-bias and the decrease of TMR with decreasing temperature. Logarithmic dependencies of the conductance on bias and temperature agree well with Applebaum´s theory describing Kondo-type spin flip scattering between tunneling electrons and impurity spins. The systematic variation of the transport anomalies with oxidation time confirmed that an over-oxidation of the tunnel barrier leads to diffusion of Co (Fe) ions from the bottom electrode into the barrier, which is consistent with the Cabrera-Mott oxidation model.