Magnetite nano-islands on silicon-carbide with graphene

TL;DR

This study demonstrates that iron nano-islands grown on graphene and passivated with gold spontaneously oxidize through defects to form magnetite nanoparticles, with properties confirmed by XMCD and temperature-dependent measurements.

Contribution

It reveals the spontaneous formation of magnetite nano-particles from iron nano-islands on graphene covered with gold, highlighting a novel synthesis pathway.

Findings

Thin iron films convert entirely to magnetite upon gold passivation.

Thicker films show mixed properties of magnetite and metallic iron.

The Verwey transition at ~120 K confirms magnetite formation.

Abstract

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.