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

We study the electronic structure of well-characterized epitaxial films of FeO (wustite), gamma-Fe2O3 (maghemite), and Fe3O4 (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 1 s and Fe 2p core level spectra. Fe K-edge XANES is used to characterize and confirm the Fe valence states of FeO, gamma-Fe2O3, and Fe3O4 films. EELS is used to identify the bulk plasmon loss features. A comparison of HAXPES results with model calculations for an MO6 cluster provides us with microscopic electronic structure parameters such as the onsite Coulomb energy U-dd, the charge-transfer energy 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)(L) under bar (1) and d(n+2)(L) under bar (2) configurations. Both FeO and gamma-Fe2O3 can be described as charge-transfer insulators in the Zaanen-Sawatzky-Allen picture with U-dd > Delta, consistent with earlier work. However, the MO6 cluster calculations do not reproduce an extra satellite observed in Fe 1 s spectra of gamma-Fe2O3 and Fe3O4. Based on simplified calculations using an M2O7 cluster with renormalized parameters, it is suggested that nonlocal screening plays an important role in explaining the two satellites observed in the Fe 1 s core level HAXPES spectra of gamma-Fe2O3 and Fe3O4.