Christian Heiliger1,2 and Mark D. Stiles1
1Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6202
2Maryland NanoCenter, University of Maryland, College Park, MD, 20742



Very high tunneling magnetoresistance (TMR) ratios in crystalline Fe/MgO/Fe tunnel junctions leads to a variety of possible applications e.g. read heads in hard disks and storage elements in magnetic random access memory (MRAM). For the latter the spin transfer torque (SST) is a way to switch the relative orientation of both Fe electrodes to each other to store the desired information. To investigate the spin transfer torque, we calculate the linear response for small currents and voltages.  Our calculations show that the half-metallicity of the Fe with respect to the high symmetry D1 states leads to a strong localization of the spin transfer torque to the interface. As a result, the linear current dependence of the torque in the plane of the two magnetizations is independent of the free layer thickness for more than three monolayers of Fe. For perfect samples we also find a linear current dependence of the out-of-plane component. However, this linear dependence oscillates rapidly with thickness and averages to zero in the presence of structural imperfections like thickness fluctuation, in agreement with experiment.