Lattice Softening in Metastable bcc Co<i>_x</i>Mn_100-<i>x</i>(001) Ferromagnetic Layers for a Strain-Free Magnetic Tunnel Junction

In spintronics, one of the long standing questions is why the MgO barrier 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. We have developed an almost strain-free magnetic tunnel junction (MTJ) using metastable bcc Co_xMn_100-x (CoMn) ferromagnetic films to reveal the reason. We have investigated the degree of crystallisation in MTJ consisting of CoMn/MgO/CoMn in relation to their TMR ratios. Cross-sectional high resolution transmission electron microscopy reveals that almost consistent lattice constants of the MTJ layers for 66≤x≤83 with maintaining large TMR ratios of 229% at RT, confirming the soft nature of the CoMn layer with some dislocations at the MgO/Co₇₅Mn₂₅ interfaces. For x=86, 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 in the MgO barrier. Ab-initio calculations confirm the crystalline deformation stability across a broad compositional range in CoMn, proving a strain-free interface for larger TMR ratios. Further optimisation can achieve > 1,000% TRM ratio at RT.