Structure and magnetism in metastable bcc Co1-xMnx epitaxial films

Co-rich Co1-xMnx alloys have hcp or fcc disordered phase 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 magnetics and spintronics, as studied recently. Here, we report systematic study of structure and magnetism for epitaxial thin films. The single phase bcc Co1-xMnx (001) films were pseudomorphically grown on Cr(001) for 0.14 < x < 0.50 with a sputtering technique. The saturation magnetization was larger than that of pure Co for the films with x = 0.14 and 0.25 and smaller for the x = 0.34 films, disappearing completely for the x = 0.5 films. X-ray magnetic circular dichroism unveiled that this behavior stemmed from the composition dependence of elemental magnetic moments. Slight enhancement of magnetic moment of Co was observed in bcc alloy films. We also found that magnetic moments of Mn exceeded 2 mu(B) at the maximum and were not as high as theoretical values, approximately 3 mu(B), obtained from ab-initio calculation with coherent potential approximation. This difference is qualitatively understood in terms of microscopic Mn atom pairing with antiferromagnetic coupling, being suggested from the ab-initio band structure calculations based on special quasi-random structure of bcc Co1-xMnx.