  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. The surfaces of the original GaAs NWs are
completely covered by magnetic Fe3Si exhibiting nanofacets and an enhanced
surface roughness compared to the bare GaAs NWs. Shell growth at a substrate
temperature of T{S} = 200 {\deg}C leads to regular nanofacetted Fe3Si shells.
These facets, which lead to thickness inhomogeneities of the shells, consist
mainly of well pronounced Fe3Si(111) planes. The crystallographic orientation
of core and shell coincide, i.e. they are pseudomorphic. The nanofacetted Fe3Si
shells found in the present work are probably the result of the Vollmer-Weber
island growth mode of Fe3Si on the {110} side facets of the GaAs NWs.
