Electronic structure of FeO, γ-Fe2 O3, and Fe3 O4 epitaxial films using high-energy spectroscopies

We study the electronic structure of well-characterized epitaxial films of FeO (wustite), γ-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 1s and Fe 2p core level spectra. Fe K-edge XANES is used to characterize and confirm the Fe valence states of FeO, γ-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 Udd, the charge-transfer energy Δ, and the metal-ligand hybridization strength V. The results also provide estimates for the ground-state and final-state contributions in terms of the dn, dn+1L 1, and dn+2L 2 configurations. Both FeO and γ-Fe₂O₃ can be described as charge-transfer insulators in the Zaanen-Sawatzky-Allen picture with Udd>Δ, consistent with earlier work. However, the MO6 cluster calculations do not reproduce an extra satellite observed in Fe 1s spectra of γ-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 1s core level HAXPES spectra of γ-Fe₂O₃ and Fe₃O₄.