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We report on spin polarization reduction by incoherent tunneling in realisticsingle crystal Co2FeAl/MgO/Co50Fe50 magnetic tunnel junctions (MTJ) compared toreference Fe/MgO/Fe. A large density of misfit dislocations in the Heuslerbased MTJs has been insured by a thick MgO barrier and its 3.8% latticemismatch with the Co2FeAl electrode. Our analysis implicates a correlatedstructural-transport approach. The crystallographic coherence, in the realspace, is investigated using High Resolution Transmission Electron Microscopyphase analysis. The electronic transport experiments in variable temperature,fitted with a theoretical extended-Glazman-Matveev model, address differentlevels of the tunneling mechanisms from direct to multi-center hopping. Wedemonstrate a double negative impact of dislocations, as extended defects, onthe tunneling polarization. Firstly, the breaking of the crystal symmetrydestroys the longitudinal and lateral coherence of the propagating Blochfunctions. This affects the symmetry filtering efficiency of the Delta_1 statesacross the (100) MgO barriers and reduces the associated effective tunnelingpolarization. Secondly, dislocations provide localized states within the MgOgap. This determines temperature activated spin-conserving inelastic tunnelingthrough chains of defects which are responsible for the one order of magnitudedrop of the tunnel magnetoresistance from low to room temperature.

Reduction of spin polarization by incoherent tunneling in Co2FeAl/MgO/CoFe magnetic tunnel junctions with thick MgO barriers