Thickness dependence of the tunneling current in the coherent limit of transport

Abstract

We present investigations of the tunneling magnetoresistance (TMR) in planar Fe/MgO/Fe junctions performed by means of ab initio calculations. The electronic and magnetic structures of the junctions are calculated self-consistently in the framework of density-functional theory. The transport properties are investigated as a function of the barrier thickness in the limit of coherent tunneling. Interesting features such as sign reversal of the TMR ratio as a function of the bias voltage and of the interface structure are proven to be stable with increasing barrier thickness. It is shown that at large barrier thicknesses, only a small amount of states contributes to the overall current, but the ${\mathbf{k}}_{\ensuremath{\parallel}}=0$ point is not involved for all Fe/MgO/Fe junctions we considered, in contrast to the general belief supported by simplified parabolic band models. The experimentally observed saturation of the TMR ratio with increasing barrier thickness is confirmed and can be understood only analyzing the complex band structure within the whole Brillouin zone. However, we cannot confirm an oscillating behavior of the TMR ratio depending on barrier thickness as observed experimentally. This means that the measured oscillations are not an intrinsic effect of the coherent tunneling states of the ideal crystalline Fe/MgO/Fe structure.