Large influence of capping layers on tunnel magnetoresistance in magnetic tunnel junctions

TL;DR

This study uses ab initio calculations to investigate how different capping layers affect tunnel magnetoresistance in magnetic tunnel junctions, highlighting tungsten as an optimal material for high TMR ratios.

Contribution

The paper demonstrates that tungsten capping layers significantly enhance TMR in magnetic tunnel junctions through detailed electronic structure analysis.

Findings

Tungsten yields the highest TMR among tested materials.

Resonant tunneling peaks are observed in the minority-spin channel.

Tungsten interface blocks scattering states in anti-parallel configuration.

Abstract

It has been reported in experiments that capping layers which enhance the perpendicular magnetic anisotropy (PMA) of magnetic tunnel junctions (MTJs) induce great impact on the tunnel magnetoresistance (TMR). To explore the essential influence caused by capping layers, we carry out ab initio calculations on TMR in the X(001)|CoFe(001)|MgO(001)|CoFe(001)|X(001) MTJ, where X represents the capping layer material which can be tungsten, tantalum or hafnium. We report TMR in different MTJs and demonstrate that tungsten is an ideal candidate for a giant TMR ratio. The transmission spectrum in Brillouin zone is presented. It can be seen that in the parallel condition of MTJ, sharp transmission peaks appear in the minority-spin channel. This phenomenon is attributed to the resonant tunnel transmission effect and we explained it by the layer-resolved density of states (DOS). In order to explore transport properties in MTJs, the density of scattering states (DOSS) was studied from the point of band symmetry. It has been found that CoFe|tungsten interface blocks scattering states transmission in the anti-parallel condition. This work reports TMR and transport properties in MTJs with different capping layers, and proves that tungsten is a proper capping layer material, which would benefit the design and optimization of MTJs.