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

Abstract

In an attempt to obtain wustite ${\mathrm{Fe}}_{1\ensuremath{-}x}\mathrm{O}$ as epitaxial films on MgO(100), ${\mathrm{NO}}_{2}$-assisted molecular-beam epitaxy was applied. At low ${\mathrm{NO}}_{2}$ fluxes, the low-energy electron diffraction and reflection high-energy electron diffraction images indeed indicate the formation of a rocksaltlike structure. In addition, M\"ossbauer 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 ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{O}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}.$ The nitrogen atoms occupy substitutional sites on the oxygen-anion sublattice. Similarly, at slightly higher ${\mathrm{NO}}_{2}$ fluxes, magnetitelike oxynitride films with composition ${\mathrm{Fe}}_{3+\ensuremath{\delta}}{\mathrm{O}}_{4\ensuremath{-}y}{\mathrm{N}}_{y}$ 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 ${\mathrm{NO}}_{2}$ flux is insufficient to form a stoichiometric ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$