Researchers from CEMES-Toulouse, IJL-Nancy and synchrotron SOLEIL have identified the most important electron states contributing to the tunnel current in Fe_1-xCo_x/MgO/Fe_1-xCo_x magnetic tunnel junctions. They explained the decrease of the magnetoresistance observed for Co-rich electrodes in terms of an interface electron state which starts becoming occupied for Co content above 25%.

The tunnel magnetoresistance of magnetic tunnel junctions with disordered bcc Fe_1-xCo_x electrodes increases with the Co content up to a critical concentration of about 25%. However, the enhancement of the magnetotransport properties decresases at higher Co content.

To understand this strange behaviour, we recorded spin-resolved photoemission spectra at the (001) surface of Fe_1-xCo_x alloys, and we performed ab-initio calculations of the electronic structure to interpret the spectra. We identified a minority spin interface state with D₁ symetry, which start becoming occupied for Co contents above 25%.

         The photoemission spectra and the density of states curves calculated ab-initio have allowed the interpretation of the tunnel junction I(V) curves in terms of majority and minority spin conduction channels corresponding to interface or bulk-like states with Δ₁ or Δ₅ symetry.