  GaAs nanowires and GaAs-Fe3Si core-shell nanowire structures were grown by
molecular-beam epitaxy on oxidized Si(111) substrates and characterized by
transmission electron microscopy (TEM) and X-ray diffraction (XRD). Ga droplets
were formed on the oxide surface, and the semiconducting GaAs nanowires grew
epitaxially via the vapor-liquid-solid mechanism as single-crystals from holes
in the oxide film. We observed two stages of growth of the GaAs nanowires,
first the regular growth and second the residual growth after the Ga supply was
finished. The magnetic Fe3Si shells were deposited in an As-free chamber. They
completely cover the GaAs cores although they consist of small grains.
High-resolution TEM micrographs depict the differently oriented grains in the
Fe3Si shells. Selected area diffraction of electrons and XRD gave further
evidence that the shells are textured and not single crystals. Facetting of the
shells was observed, which lead to thickness inhomogeneities of the shells.
