Epitaxial and stoichiometric effects on structural and chemical ordering in Heusler alloys

The Heusler alloys of Co$_{2}$MnGe and Co$_{2}$MnSi have been predicted to be half-metallic, where the minority spin density of states shows a gap at the Fermi Level. However, half-metallicity has not yet been realized owing to its expected sensitivity to atomic disorders associated with off-stoichiometry and epitaxial constraints. Combinatorial epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{z}$ and Co$_{x}$Mn$_{y}$Si$_{z}$ have been grown on Ge (111) substrates in and around the Heusler stoichiometry using molecular beam epitaxy. The structural and chemical ordering of the films has been examined using synchrotron x-ray microbeam techniques, including x-ray diffraction and energy dependent anomalous diffraction. A comprehensive model has been developed for anomalous diffraction, allowing for detection and quantification of various disorders even at small amounts, including site-dependent vacancies and elemental site swapping. The x-ray experiments reveal that the ordering is very sensitive to the Co:Mn atomic ratio and that epitaxial strain can cause a shift in the composition of highest structural ordering away from the Heusler stoichiometry, accompanied by increased chemical disorders. These findings have made it possible to explore spin dependent states as a function of structural and chemical ordering.