  Co-rich Co$_{1-x}$Mn$_x$ alloys have hcp or fcc disordered phases and those
ferromagnetic orderings are significantly deteriorated with increasing Mn
concentration $x$ in bulk. On the other hand, those metastable bcc phases show
properties attractive to spintronics, e.g., high tunnel magnetoresistance (TMR)
ratio of more than 200% (600%) at 300 K (10 K) in magnetic tunnel junctions
(MTJs) with the $x$ = 0.25 bcc alloy electrodes [Kunimatsu et al., Appl. Phys.
Express 13, 083007 (2020)]. Here, we report systematic study of structure and
magnetism for epitaxial thin films as well as the TMR effect in
MgO(001)-barrier MTJs with electrodes comprising those bcc films. The single
phase bcc Co$_{1-x}$Mn$_x$(001) films were pseudomorphically grown on Cr(001)
for 0.14 < $x$ < 0.50 with a sputtering technique. The magnetization was larger
than that of pure Co for $x$ = 0.14-0.25 and deceased with further increasing
$x$. This behavior mainly stemmed from the composition dependence of magnetic
moment of Mn that exceeded 2 $\mu _B$ at the maximum, unveiled by X-ray
magnetic circular dichroism. Correspondingly, within the range of 0.25 < $x$ <
0.37, the TMR ratio decreased from 620% (229%) to 450% (194%) at 10 K (300 K)
as $x$ increased. We discussed the relationship between the magnetism and high
TMR ratio with different $x$ with the aid of the ab-initio band structure
calculations.
