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

Abstract

In spintronics, one of the long-standing questions is why the $\mathrm{Mg}\mathrm{O}$-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 ${\mathrm{Co}}_{x}{\mathrm{Mn}}_{100\text{\ensuremath{-}}x}$ ($\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}$) ferromagnetic films. We investigate the degree of crystallization in MTJs consisting of $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}/\mathrm{Mg}\mathrm{O}/\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}$ in relation to their TMR ratios. Cross-section high resolution transmission electron microscopy reveals that almost consistent lattice constants of these layers for 66 \ensuremath{\le} x \ensuremath{\le} 83 with large TMR ratios of 229% at room temperature, confirming the soft nature of the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}$ layer with some dislocations at the $\mathrm{Mg}\mathrm{O}/{\mathrm{Co}}_{75}{\mathrm{Mn}}_{25}$ interfaces. Ab initio calculations confirm the crystalline deformation stability across a broad compositional range in $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}$, proving the advantage of a strain-free interface for much larger TMR ratios.