NO₂-assisted molecular-beam epitaxy of wustitelike and magnetitelike Fe oxynitride films on MgO(100)

In an attempt to obtain wustite Fe_1-xO as epitaxial films on MgO(100), NO₂-assisted molecular-beam epitaxy was applied. At low NO₂ fluxes, the low-energy electron diffraction and reflection high-energy electron diffraction images indeed indicate the formation of a rocksaltlike structure. In addition, Mössbauer spectroscopy provides evidence for the formation of a phase that is paramagnetic at room temperature. However, the layers are not pure oxides but are well-ordered oxynitrides with composition Fe_1-xO_1-yN_y. The nitrogen atoms occupy substitutional sites on the oxygen-anion sublattice. Similarly, at slightly higher NO₂ fluxes, magnetitelike oxynitride films with composition Fe_3+δ O_4-yN_y are obtained. By correlating x-ray photoelectron spectroscopy spectra with the intensity oscillation periods observed during reflection high-energy electron diffraction, it is possible to derive the complete stoichiometry of the films. We propose that the abrupt incorporation of nitrogen atoms only occurs if the atomic oxygen provided by the NO₂ flux is insufficient to form a stoichiometric Fe₃O₄.