Simulation of tunneling magneto-resistance in Fe/MgO/Fe junction

The conductivity asymmetry of junctions with insulating MgO barriers embedded between ferromagnetic Fe electrodes has been investigated by the first principles calculation program of Atomistix ToolKit 2008. The conductivity in up-spin electron is larger than that in down-spin electron for the ideal structure. It leads to the large TMR ratio. The defect of O or Fe atoms hardly varies the conductivity in both up and down-spin electrons. The defect of Mg atoms increases the conductivity in down-spin electron, although the up-spin conductivity does not change very much. So the defect of Mg decreases the TMR ratio. The defect of Mg near the interface especially shows this degradation. The derivative conductivity dI/dV vs. V characteristics are asymmetric for the polarity of the bias voltage V according to the location of the atom defect. The quality of the junction in devices can be evaluated by the asymmetry.