Epitaxial growth and spin dependent states of Co$_{x}$Mn$_{y}$Si$_{z}$ (111) thin films*

Epitaxial growth of Co$_{x}$Mn$_{y}$Si$_{z}$ on Ge (111) substrates has been studied using combinatorial MBE techniques, including that of the Heusler alloy Co$_{2}$MnSi. For Si concentration of 25 at. {\%}, in-situ RHEED and ex-situ X-ray diffraction experiments indicate that the epitaxial growth is coherent for atomic ratio Co:Mn between 1 and 9, while the film is microcrystalline for Co:Mn $<$ 1 and it is rough and of poor crystalline quality for Co:Mn $>$ 9. The crystalline quality is the highest around Co:Mn = 4, whereas it exhibits a plateau around a ratio of 2, i.e. the Heusler alloy, Co$_{2}$MnSi. Within the region of coherent growth, at coverages below 100{\AA}, the growth front is smooth 2D-like. As thickness increases, the surface morphology systematically changes from quasi-2D into 3D. The morphology transition also depends sensitively on composition, i.e. Co:Mn ratio, and temperature. Spin-dependent states as a function of composition at low coverages have been examined by tunneling spectroscopy using Al$_{2}$O$_{3}$ as the tunneling barrier and Fe and Nb as the detector layers.