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 concentration, in contradiction to previous calculations but in agreement with recent experiments. This dependence is explained by the bulk properties of the alloys. By using a realistic description of the disorder in the alloys we show that even small amounts of disorder lead to a drastic drop in the tunneling magnetoresistance. This provides an 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 one for the out-of-plane component for all concentrations at low 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 by the same effects which govern the tunneling magnetoresistance.

• Received 24 May 2013

DOI:https://doi.org/10.1103/PhysRevB.88.094421