<i>In situ</i>XPS analysis of various iron oxide films grown by<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">NO</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>-assisted molecular-beam epitaxy

We report on a systematic analysis of x-ray photoelectron spectroscopy (XPS) core- and valence-level spectra of clean and well-characterized iron oxide films, i.e., $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3},$ $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3},$ ${\mathrm{Fe}}_{3\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}{\mathrm{O}}_{4},$ and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$ All iron oxide films were prepared epitaxially by ${\mathrm{NO}}_{2}$-assisted molecular-beam epitaxy on single crystalline MgO(100) and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ substrates. The phase and stoichiometry of the films were controlled precisely by adjusting the ${\mathrm{NO}}_{2}$ pressure during growth. The XPS spectrum of each oxide clearly showed satellite structures. These satellite structures were simulated using a cluster-model calculation, which could well reproduce the observed structures by considering the systematic changes in both the Fe $3d$ to O $2p$ hybridization and the $d\ensuremath{-}d$ electron-correlation energy. The small difference in the satellite structures between $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ resulted mainly from changes in the Fe-O hybridization parameters, suggesting an increased covalency in $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ compared to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}.$ With increasing reduction in the $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}{\ensuremath{-}\mathrm{F}\mathrm{e}}_{3}{\mathrm{O}}_{4}$ system, the satellite structures in XPS became unresolved. This was not only due to the formation of ${\mathrm{Fe}}^{2+}$ ions, but also to nonhomogeneous changes in the hybridization parameters between octahedral and tetrahedral ${\mathrm{Fe}}^{3+}$ ions.