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

TL;DR

This study models Fe- and Co-based magnetic tunnel junctions with AlN and ZnO barriers, revealing that Fe/AlN/Fe junctions can achieve very high magnetoresistance due to favorable symmetry filtering effects.

Contribution

The paper demonstrates the potential of AlN and ZnO as insulating barriers in magnetic tunnel junctions and analyzes their spin filtering and magnetoresistance properties using ab initio quantum transport simulations.

Findings

Fe/AlN/Fe junctions show around 1000% magnetoresistance at low bias.

Co-based junctions exhibit limited spin filtering and low magnetoresistance.

Fe barriers with AlN and ZnO can enable high magnetoresistance due to symmetry filtering.

Abstract

AlN and ZnO, two wide band-gap semiconductors extensively used in the display industry, crystallise in the wurtzite structure, which can favour 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 {\it ab initio} quantum transport code {\it 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 $\Delta_1$ symmetry, and connects across the band gap at the $\Gamma$ point in 2D Brillouin zones. This feature enables spin filtering and may result in a large tunnelling magnetoresistance. In general, we find that Co-based junctions present limited spin filtering and little magnetoresistance at low bias, since both spin sub-bands cross the Fermi level with $\Delta_1$ symmetry. This contrasts the situation of Fe, where only the minority $\Delta_1$ 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 $\Gamma$ point and through complex bands with symmetry different than $\Delta_1$. The only exception is for the Fe/AlN/Fe junction, where we predict a magnetoresitance of around 1,000\% at low bias.