Enhancement of the Co magnetic moment in bcc Co$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ on MgO*

Using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (MCD), we show that the elemental Co moment for MBE grown thin films of bcc Co$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ grown on MgO(001) is enhanced by 40{\%} to a maximum value of 2.1 $\mu_{\mathrm{B}}$ at x$=$0.24. The net Mn moment is found to align parallel with Co for all concentrations and remains roughly constant until x$=$0.3, then drops steadily, up to x$=$0.7, where the total moment of the film abruptly collapses to zero. Using a low-concentration Mn moment of 3.0 $\mu _{\mathrm{B}}$, the average magnetization lies directly on the Slater-Pauling (SP) curve for concentrations up to about x$=$.25, where it reaches a maximum moment of 2.3 $\mu_{\mathrm{B}}$ /atom. This peak is slightly shifted and the slope is steeper on the high-Mn concentration side of the peak relative to the standard SP curve. This is in stark contrast to the fcc CoMn and hcp CoCr bulk behavior which shows only a rapid total moment reduction with Mn concentration.