Large magnetoresistance in bcc Co/MgO/Co and FeCo/MgO/FeCo tunnel junctions

By use of first-principles electronic structure calculations, we predict that the magnetoresistance of the bcc Co(100)/MgO(100)/bccCo(100) and FeCo(100)/MgO(100)/FeCo(100) tunneling junctions can be several times larger than the very large magnetoresistance predicted for the Fe(100)/MgO(100)/Fe(100) system. The origin of this large magnetoresistance can be understood by considering the electrons at the Fermi energy traveling perpendicular to the interfaces. For the minority spins there is no state with Delta(1) symmetry whereas for the majority spins there is only a Delta(1) state. The Delta(1) state decays much more slowly than the other states within the MgO barrier. In the absence of scattering which breaks the conservation of momentum parallel to the interfaces, the electrons traveling perpendicular to the interfaces undergo total reflection if the moments of the electrodes are antiparallel. These arguments apply equally well to systems with other well ordered tunnel barriers and for which the most slowly decaying complex energy band in the barrier has Delta(1) symmetry. Examples include systems with (100) layers constructed from Fe, bcc Co, or bcc FeCo electrodes and Ge, GaAs, or ZnSe barriers.