Lattice Softening in Metastable bcc CoxMn100-x(001) Ferromagnetic Layers for a Strain-Less Magnetic Tunnel Junction

TL;DR

This paper demonstrates that strain-free metastable bcc CoxMn100-x ferromagnetic layers in magnetic tunnel junctions maintain high TMR ratios, with lattice softening and minimal dislocations contributing to improved spintronic device performance.

Contribution

It introduces a strain-free MTJ design using metastable bcc CoxMn100-x films, revealing their lattice softening and stability across compositions, leading to high TMR ratios at room temperature.

Findings

High TMR ratios (~229%) achieved with x between 66 and 83.

Lattice constants remain consistent across compositions, indicating lattice softening.

Increased dislocations at higher x reduce TMR to 142%.

Abstract

In spintronics, one of the long standing questions is why the MgO-based magnetic tunnel junction (MTJ) is almost the only option to achieve a large tunnelling magnetoresistance (TMR) ratio at room temperature (RT) but not as large as the theoretical prediction. This study focuses on the development of an almost strain-free MTJ using metastable bcc CoxMn100-x ferromagnetic films. We have investigated the degree of crystallisation in MTJ consisting of CoxMn100-x/MgO/CoxMn100-x (x = 66, 75, 83 and 86) in relation to their TMR ratios. Cross-sectional high resolution transmission electron microscopy (HRTEM) reveals that almost consistent lattice constants of these layers for 66 < x < 83 with maintaining large TMR ratios of 229% at RT, confirming the soft nature of the CoxMn100-x layer with some dislocations at the MgO/Co75Mn25 interfaces. For x = 86, on the other hand, the TMR ratio is found to be reduced to 142% at RT, which is partially attributed to the increased number of the dislocations at the MgO/Co86Mn14 interfaces and amorphous grains identified in the MgO barrier. Ab-initio calculations confirm the crystalline deformation stability across a broad compositional range in CoMn, proving the advantage of a strain-free interface for much larger TMR ratios.