We study the electronic structure of well-characterized epitaxial films of FeO (wustite), $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ (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, $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$, 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 $\mathrm{\Delta }$, 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 $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ can be described as charge-transfer insulators in the Zaanen-Sawatzky-Allen picture with $U_{dd}>\mathrm{\Delta }$, consistent with earlier work. However, the $M{\mathrm{O}}_6$ cluster calculations do not reproduce an extra satellite observed in Fe $1s$ spectra of $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ 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 $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4$.

• Received 26 March 2018

DOI:https://doi.org/10.1103/PhysRevB.97.235148