Fe- and Co-based magnetic tunnel junctions with AlN and ZnO spacers

Abstract

AlN and ZnO, two wide band-gap semiconductors extensively used in the display industry, crystallize in the wurtzite structure, which can favor the formation of epitaxial interfaces to close-packed common ferromagnets. Here we explore these semiconductors as material for insulating barriers in magnetic tunnel junctions. In particular, the ab initio quantum transport code SMEAGOL is used to model the X [111]/Y [0001]/X [111] (X = Co and Fe, Y = AlN and ZnO) family of junctions. Both semiconductors display a valance-band top with p-orbital character, while the conduction-band bottom exhibits s-type symmetry. The smallest complex-band decay coefficient in the forbidden energy-gap along the [0001] direction is associated with the .6.1 symmetry, and connects across the band gap at the I' point in 2D Brillouin zones. This feature enables spin filtering and may result in a large tunneling magnetoresistance. In general, we find that Co-based junctions present limited spin filtering and little magnetoresistance at low bias, since both spin subbands cross the Fermi level with .6.1 symmetry. This contrasts the situation of Fe, where only the minority .6.1 band is available. However, even in the case of Fe the magnitude of the magnetoresistance at low bias remains relatively small, mostly due to conduction away from the I' point and through complex bands with symmetry different than 41. The only exception is for the Fe/AlN/Fe junction, where we predict a magnetoresistance of around 1000% at low bias.