The dependence of tunneling magnetoresistance and spin-transfer torque in FeCo/MgO/FeCo tunnel junctions on the Co concentration and the bias voltage are investigated ab initio. We find that the tunneling magnetoresistance decreases with the Co conce
ntration in contradiction with previous calculations but in agreement with recent experiments. This dependence is explained from bulk properties of the alloys. By using a realistic description of the disorder in the alloys we can show that even small amounts of disorder lead to a drastic drop in the tunneling magnetoresistance. This provides a quantitative explanation of the difference between calculated and measured values. The spin-transfer torque shows a linear voltage dependence for the in-plane component and a quadratic for the out-of-plane component for all concentrations at small bias voltages. In particular, the linear slope of the in-plane torque is independent of the concentration. For high bias voltages the in-plane torque shows a strong nonlinear deviation from the linear slope for high Co concentrations. This is explained from the same effects which govern the tunneling magnetoresistance.
The theoretical description of modern nanoelectronic devices requires a quantum mechanical treatment and often involves disorder, e.g. form alloys. Therefore, the ab initio theory of transport using non-equilibrium Greens functions is extended to the
case of disorder described by the coherent potential approximation. This requires the calculation of non-equilibrium vertex corrections. We implement the vertex corrections in a Korringa-Kohn-Rostoker multiple scattering scheme. In order to verify our implementation and to demonstrate the accuracy and applicability we investigate a system of an iron-cobalt alloy layer embedded in copper. The results obtained with the coherent potential approximation are compared to supercell calculations. It turns out that vertex corrections play an important role for this system.
