Tetragonal-Like Phase in Core-Shell Iron Iron-Oxide Nanoclusters

Two sizes of iron/iron-oxide (Fe/Fe-oxide) nanoclusters (NCs) of 10 and 35 nm diameters were prepared using a cluster deposition technique. Both these NCs displayed X-ray diffraction (XRD) peaks due to body-centered cubic (BCC) Fe-0 and magnetite-like phase. Fe-57 Mossbauer spectroscopy measurements confirmed (a) the core-shell nature of the NCs, (b) the Fe-oxide shell to be nanocrystalline and partially oxidized beyond magnetite, and (c) the Fe-oxide spins are significantly canted. In addition to the BCC Fe and magnetite-like phases, a phase similar to tetragonal sigma-Fe-Cr (8% Cr) was clearly evident in the larger NC, based on XRD. The origin of the tetragonal-like phase in the larger NC could be due to significant distortion of the Fe-0 core lattice planes; subtle peaks due to this phase were also apparent in the smaller NC. Unambiguous evidence for the presence of such a phase was not clear from X-ray photoelectron spectroscopy, vibrating sample magnetometry, X-ray magnetic circular dichroism, or transmission electron microscopy. Fe-57 Mossbauer spectroscopy, however, provided support for a non-BCC Fe-0 phase in the strongest sixth and the first resonance lines of the Fe-0 sextet. To our knowledge, this is the first report of a tetragonal-like phase in the Fe/Fe-oxide core-shell systems.