Structural and chemical ordering and spin polarization in epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{1-x-y}$ and Co$_{x}$Mn$_{y}$Si$_{1-x-y}$*

The Heusler alloys of Co$_{2}$MnGe and Co$_{2}$MnSi have been predicted to be half-metallic. However, half-metallicity has not been realized owing to its sensitivity to disorders associated with off-stoichiometry and to epitaxial constraints. Combinatorial epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{1-x-y}$ and Co$_{x}$Mn$_{y}$Si$_{1-x-y}$ have been grown on Ge (111) substrates in and around the Heusler stoichiometry using molecular beam epitaxy techniques. The structural and chemical ordering of the films has been examined using x-ray microbeam techniques, including energy dependent anomalous diffraction and diffraction anomalous fine structure. These experiments are shown to be sensitive to various disorders, including stacking faults, and chemical dependent vacancies and site swapping. They reveal that the ordering is very sensitive to the atomic ratio between Co and Mn. Magnetic and spin dependent properties have been examined by magnetoopitc effects and in-situ point contact Andreev reflection spectroscopy, and they exhibit strong correlation with the structural and chemical ordering.