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

We report on spin polarization reduction by incoherent tunneling in realistic single crystal Co2FeAl/MgO/Co50Fe50 magnetic tunnel junctions (MTJ) compared to reference Fe/MgO/Fe. A large density of misfit dislocations in the Heusler based MTJs has been insured by a thick MgO barrier and its 3.8% lattice mismatch with the Co2FeAl electrode. Our analysis implicates a correlated structural-transport approach. The crystallographic coherence, in the real space, is investigated using High Resolution Transmission Electron Microscopy phase analysis. The electronic transport experiments in variable temperature, fitted with a theoretical extended-Glazman-Matveev model, address different levels of the tunneling mechanisms from direct to multi-center hopping. We demonstrate a double negative impact of dislocations, as extended defects, on the tunneling polarization. Firstly, the breaking of the crystal symmetry destroys the longitudinal and lateral coherence of the propagating Bloch functions. This affects the symmetry filtering efficiency of the Delta_1 states across the (100) MgO barriers and reduces the associated effective tunneling polarization. Secondly, dislocations provide localized states within the MgO gap. This determines temperature activated spin-conserving inelastic tunneling through chains of defects which are responsible for the one order of magnitude drop of the tunnel magnetoresistance from low to room temperature.