  X-ray magnetic circular dichroism (XMCD) measurements of iron nano-islands
grown on graphene and covered with a Au film for passivation reveal that the
oxidation through defects in the Au film spontaneously leads to the formation
of magnetite nano-particles (i.e, $Fe_3$$O_4$). The Fe nano-islands (20 and 75
monolayers; MLs) are grown on epitaxial graphene formed by thermally annealing
6H-SiC(0001) and subsequently covered, in the growth chamber, with nominal 20
layers of Au. Our X-ray absorption spectroscopy and XMCD measurements at
applied magnetic fields show that the thin film (20 ML) is totally converted to
magnetite whereas the thicker film (75 ML) exhibits properties of magnetite but
also those of pure metallic iron. Temperature dependence of the XMCD signal (of
both samples) shows a clear transition at $T_{\rm V}\approx 120$ K consistent
with the Verwey transition of bulk magnetite. These results have implications
on the synthesis of magnetite nano-crystals and also on their regular
arrangements on functional substrates such as graphene.
