Moment Mapping of bcc Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ Alloy Films on MgO(001)

The magnetic moments of $\sim$ 20 nm single crystal films of compositionally graded Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ films (0.1 $\le $ x $\le $ 0.2) grown on MgO(001) are determined by spatially resolved moment mapping using X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD). RHEED measurements confirmed that the growth of Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ films remained epitaxial and in the bcc phase up to x$=$0.35 but, like Fe growth, is rotated 45 degree with respect to the MgO(001) surface net. This is beyond the bulk bcc stability limit of x$=$0.12. Both magnetometry and XMCD measurements show that the net magnetic moment of these alloy films behave similarly to the bulk behavior, with a gradual moment reduction at low Mn concentrations followed by an abrupt departure from the Slater-Pauling curve and disappearance of the moment at x$=$0.15. By generating a compositional variation around this critical concentration and subsequently using spatially resolved mapping of the X-ray absorption at the Fe and Mn L$_{3}$-edge using linear and circular polarized soft X-rays, the local composition and elemental moments can be simultaneously mapped across the surface of the sample. The Fe moment is found to gradually reduce with increasing Mn content with a very abrupt decline at x$=$0.15. Surprisingly, the Mn moment shows a very small net moment (\textless 0.1 mu$_{\mathrm{B}})$ at all compositions, suggesting a complicated Mn spin structure.