Structural defects analysis versus spin polarized tunneling in Co₂FeAl/MgO/CoFe magnetic tunnel junctions with thick MgO barriers

We report on spin polarization reduction by incoherent tunneling in single crystal Co₂FeAl/MgO/Co₅₀Fe₅₀ magnetic tunnel junctions (MTJs). A large density of misfit dislocations in the Heusler based MTJs has been provided 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 has been investigated by 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 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 Δ₁ states across the (001) 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. - Highlights: • The paper focuses on tunneling transport in realistic single crystal magnetic tunnel junctions involving the full Heusler alloy Co₂FeAl. • Via correlated structural-transport analysis approach, the paper demonstrates the role of extended structural defects in a single crystal epitaxial barrier on the tunneling polarization efficiency. • We illustrate the effect of the crystal symmetry breaking on the spin and symmetry polarized tunneling transport. • The transport results are confronted with a theoretical model considering different orders of tunneling mechanisms, from direct to multi-center hopping. • Combining theoretical and experimental analysis, the paper demonstrates the negative impact of dislocations, as extended defects, on the tunneling polarization