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

In spintronics, one of the long-standing questions is why the MgO-based magnetic tunnel junction (MTJ) is almost the only option for achieving a large tunneling magnetoresistance (TMR) ratio at room temperature, although this is not as large as the theoretical prediction. This study focuses on the development of an almost strain-free MTJ using metastable bcc CoxMn100-x (Co − Mn) ferromagnetic films. We investigate the degree of crystallization in MTJs consisting of Co-Mn/MgO/Co-Mn in relation to their TMR ratios. Cross-section high resolution transmission electron microscopy reveals that almost consistent lattice constants of these layers for 66≤x ≤83 with large TMR ratios of 229% at room temperature, confirming the soft nature of the Co-Mn layer with some dislocations at the MgO/Co75Mn25 interfaces. Ab initio calculations confirm the crystalline deformation stability across a broad compositional range in Co-Mn, proving the advantage of a strain-free interface for much larger TMR ratios.