First-principles study of Co concentration and interfacial resonance states in Fe_1-xCo_x magnetic tunnel junctions

The optimal Co concentration in Fe_1-xCo_x/MgO magnetic tunnel junctions (MTJs) that maximizes tunneling magnetoresistance (TMR) is still under investigation. We perform a first-principles transport study on MTJs using disordered electrodes modeled using the virtual crystal approximation (VCA) and ordered alloys with various MgO barrier thicknesses. We find that 10-20% Co concentration maximizes TMR using VCA to represent disorder in the electrodes. This TMR peak arises due to a minority d-type interfacial resonance state (IRS) that becomes filled with small Co doping, leading to a decrease in antiparallel conductance. Calculations with ordered Fe_1-xCo _x electrodes confirm the filling of this minority d-type IRS for small Co concentrations. In addition, we construct a 10×10 supercell without VCA to explicitly represent disorder at the Fe_1-xCo _x/MgO interface, which demonstrates a quenching of the minority-d IRS and significant reduction in available states at the Fermi level that agrees with VCA calculations. These results explain recent experimental findings and provide implications for the impact of IRS on conductance and TMR in Fe _1-xCo_x/MgO tunnel junctions.