In this work we investigate the spin(-dependent) transport in FeCo/MgO/FeCo magnetic tunnel junctions (MTJ) using advanced ab initio methods. Here, a special focus lies on the impact of the disordered alloy leads with varying Co concentration.The central effect in these MTJs is the dependence of the tunneling probability of the conduction electrons on their symmetry character (symmetry selection); this is a result of the epitaxial MgO barrier. In combination with the band structure of the ferromagnetic leads, this leads to a giant tunnel magnetoresistance (TMR) and highly spin-polarized currents. The latter are beneficial for the efficient generation of spin-transfer torque (STT) and other applications. These effects are illustrated in detail for the system with pure iron leads. We also discuss other effects that are important, e.g. the interface resonance states. The discussion is then extended to FeCo leads using a realistic description of the disordered alloys, which includes the effects of disorder scattering. We discuss the interplay of the coherent tunneling and the disorder scattering in the leads and also the effects of the band filling and their impact on the (spin) transport. The TMR and STT are investigated in the full concentration range and for a large range of bias voltages. Here, the consistent study of the MTJ with iron leads provides a starting point and a solid basis for the profound understanding of the observed dependencies. These are traced back to the underlying physical effects and contributing states using a combination of advanced techniques.The theoretical prediction of the giant TMR (Butler 2001), i.e. a large increase in the resistance when switching the alignment of the magnetizations in the ferromagnetic leads from parallel to anti-parallel, has inspired a large interest in coherent MTJs. They are now widely used as magnetic field sensors, e.g. in the read heads of hard disk drives. MTJs are also used in MRAM, where the STT provides an efficient switching mechanism. Further, magnetic tunnel contacts are an important element in many proposed spintronic devices. While experiments usually consider FeCo alloys as lead material, the theoretical investigations so far only considered ordered materials.Here, we discuss the impact of the disordered leads of the MTJs based on ab initio calculations. The transport is described using non-equilibrium Green´s functions. For the disordered leads we use the coherent potential approximation, which provides an efficient and accurate description of the alloys including the effects of disorder scattering. These methods (including the necessary vertex corrections for transport calculations and the restricted alloy averages) are derived and discussed in this work. They provide a detailed picture of the transport processes in the presence of disorder.This shows that the disorder scattering interferes with the symmetry selection in the barrier leading to a reduction of the TMR. However, a detailed investigation reveals that the concentration dependence of the TMR (at zero bias voltage) is controlled by a combination of several effects including also band filling and interface resonance states. At higher bias voltages the TMR is increasingly influenced by minority states above the Fermi-energy with a high tunneling probability. For Co leads these lead to a fast decrease of the TMR with increasing bias voltage.The STT is a torque exerted on the magnetizations by electrons which cross the barrier at non-collinear alignment. The component of the STT in the plane spanned by the magnetizations is most important for the switching and can be accurately described by a model in terms of spin currents. This allows us to understand the concentration and bias dependence in terms of effects which are also observed for the TMR. In particular, this explains the observed linear bias dependence at low bias, which is independent of the concentration, and also the strong asymmetric deviations at large bias and high Co concentrations. The out-of-plane component is an even function of the bias voltage and shows a weak concentration dependence.
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