Effect of partial oxidization on the transport in Fe/MgO/Fe tunnel junctions

The performance of Fe/MgO/Fe-based magnetic tunnel junctions is critically limited by interfacial oxidation; however, a complete theoretical framework capturing its complex role has been elusive. We developed an<i>ab initio</i>quantum transport approach that integrates the coherent potential approximation with vertex corrections to model substitutional disorder at oxidized Fe/MgO interfaces. Our model reveals that oxidation fundamentally reshapes spin-dependent transport, leading to a systematic suppression of the tunnel magnetoresistance (TMR) ratio, emergence of strong bias-voltage asymmetry, and crossover to negative TMR at high bias. Crucially, we demonstrate that the dominant transport mechanism in oxidized junctions is incoherent scattering, which is captured by vertex corrections that create disorder-assisted transport channels. This effect is particularly pronounced in asymmetric interfacial configurations, where it can paradoxically enhance the transmission. Furthermore, we show that increasing the MgO barrier thickness degrades TMR in highly oxidized junctions, as incoherent channels overwhelm coherent tunneling. Our work provides a predictive framework that moves beyond idealized models, establishing that vertex corrections are indispensable for accurately describing quantum transport in realistic disordered spintronic interfaces and guiding their design.