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.
展开▼
