  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.
