Band-folding-driven high tunnel magnetoresistance ratios in (111)-oriented junctions with SrTiO$_3$ barriers

TL;DR

This study demonstrates that band folding in (111)-oriented Co/SrTiO3/Co and Ni/SrTiO3/Ni magnetic tunnel junctions leads to high TMR ratios, with Co-based junctions exceeding 500%, due to half-metallic band structures and coherent tunneling effects.

Contribution

It reveals how band folding effects induce half-metallic states that significantly enhance TMR ratios in (111)-oriented MTJs with SrTiO3 barriers, providing insights into material-specific differences.

Findings

Co-based MTJs achieve TMR over 500%.

Band folding creates half-metallic $oldsymbol{ m f extLambda_1}$ states.

Differences in TMR are due to $s$-orbital contributions at the Fermi level.

Abstract

We theoretically study the tunnel magnetoresistance (TMR) effect in (111)-oriented magnetic tunnel junctions (MTJs) with SrTiO$_{3}$ barriers, Co/SrTiO$_{3}$/Co(111) and Ni/SrTiO$_{3}$/Ni(111). Our analysis combining the first-principles calculation and the Landauer formula shows that the Co-based MTJ has a high TMR ratio over 500%, while the Ni-based MTJ has a smaller value (290%). Since the in-plane lattice periodicity of SrTiO$_{3}$ is about twice that of the primitive cell of fcc Co (Ni), the original bands of Co (Ni) are folded in the $k_x$-$k_y$ plane corresponding to the $ab$ plane of the MTJ supercell. We find that this band folding gives a half-metallic band structure in the $\Lambda_1$ state of Co (Ni) and the coherent tunneling of such a half-metallic $\Lambda_1$ state yields a high TMR ratio. We also reveal that the difference in the TMR ratio between the Co- and Ni-based MTJs can be understood by different $s$-orbital weights in the $\Lambda_1$ band at the Fermi level.