Electronic structure of FeO, γ−Fe2O3 , and Fe3O4 epitaxial films using high-energy spectroscopies

Abstract

We study the electronic structure of well-characterized epitaxial films of FeO (wustite), $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ (maghemite), and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (magnetite) using hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption near-edge spectroscopy (XANES), and electron energy loss spectroscopy (EELS). We carry out HAXPES with incident photon energies of 12 and 15 keV in order to probe the bulk-sensitive Fe $1s$ and Fe $2p$ core level spectra. Fe $K$-edge XANES is used to characterize and confirm the Fe valence states of FeO, $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ films. EELS is used to identify the bulk plasmon loss features. A comparison of HAXPES results with model calculations for an $M{\mathrm{O}}_{6}$ cluster provides us with microscopic electronic structure parameters such as the onsite Coulomb energy ${U}_{dd}$, the charge-transfer energy $\mathrm{\ensuremath{\Delta}}$, and the metal-ligand hybridization strength $V$. The results also provide estimates for the ground-state and final-state contributions in terms of the ${d}^{n}, {d}^{n+1}{\underline{L}}^{1}$, and ${d}^{n+2}{\underline{L}}^{2}$ configurations. Both FeO and $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ can be described as charge-transfer insulators in the Zaanen-Sawatzky-Allen picture with ${U}_{dd}\phantom{\rule{4pt}{0ex}}>\phantom{\rule{4pt}{0ex}}\mathrm{\ensuremath{\Delta}}$, consistent with earlier work. However, the $M{\mathrm{O}}_{6}$ cluster calculations do not reproduce an extra satellite observed in Fe $1s$ spectra of $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. Based on simplified calculations using an ${M}_{2}{\mathrm{O}}_{7}$ cluster with renormalized parameters, it is suggested that nonlocal screening plays an important role in explaining the two satellites observed in the Fe $1s$ core level HAXPES spectra of $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$.