X-ray emission spectroscopy study of the Verwey transition in Fe₃O₄

Abstract

The temperature-dependent Verwey transition in a 500 Å (111) thin film of Fe3O4 (magnetite) has been studied using soft-x-ray emission spectroscopy at room temperature and below the transition temperature TV. The Fe L2,3 x-ray emission spectra show an increase in the intensity of the L2 emission relative to the L3 emission below TV. This is independent of the excitation energy and is attributed to a metal–insulator transition across TV. Comparison of the Fe L3 emission and O Kα spectra with LDA band structure calculations supports the suggestion of charge ordering in Fe3O4 at low temperature.

Soft X-ray Spectroscopy Study of the Electronic Structure of Oxidized and Partially Oxidized Magnetite Nanoparticles

“…However, in all iron oxides studied by this approach, O K-edge XES detects an energy-dependent spectral weight at and above the VB edge that lowers the apparent band gap. 37,54,55 For magnetite in particular, this effect makes the edge of the VB impossible to locate, in contrast to the clearer determination achieved by VB PES. 45,55 The excess intensity in the O XES data is enhanced as the photon energy is tuned above threshold, but the position of the spectral weight on the emission energy axis is dispersionless (i.e., it does not move with the excitation energy).…”

“…37,54,55 For magnetite in particular, this effect makes the edge of the VB impossible to locate, in contrast to the clearer determination achieved by VB PES. 45,55 The excess intensity in the O XES data is enhanced as the photon energy is tuned above threshold, but the position of the spectral weight on the emission energy axis is dispersionless (i.e., it does not move with the excitation energy). In our previous study of hematite, similar observations were interpreted as representing normal X-ray fluorescence of an indirect gap semiconductor.…”

“…As shown above, photon-in/photon-out X-ray spectroscopy is a versatile approach for studying the electronic properties of both bulk and nanoscale semiconductor materials, including ligand-coated nanoparticles. However, in all iron oxides studied by this approach, O K-edge XES detects an energy-dependent spectral weight at and above the VB edge that lowers the apparent band gap. ,, For magnetite in particular, this effect makes the edge of the VB impossible to locate, in contrast to the clearer determination achieved by VB PES. , The excess intensity in the O XES data is enhanced as the photon energy is tuned above threshold, but the position of the spectral weight on the emission energy axis is dispersionless (i.e., it does not move with the excitation energy). In our previous study of hematite, similar observations were interpreted as representing normal X-ray fluorescence of an indirect gap semiconductor.…”

“…Band gap excitations of transition metal oxide semiconductors involve partially occupied 3d states, and such dd transitions are typically strong features in metal L-edge emission spectroscopy of these materials . However, in recent studies this approach underestimated the band gap for Fe 2 O 3 , and FeO by about 0.1 eV. Since core hole effects do not perturb the low-energy transitions studied by RIXS spectroscopy, this discrepancy is not understood and represents current uncertainty as to the accuracy with which X-ray spectroscopy can determine the excitation energies in transition metal oxides.…”

Electronic structures ofLiFePO4andFePO4studied using resonant inelastic x-ray scattering

“…All XES and XAS spectra have been calibrated with respect to characteristic emission and absorption energies of the metallic Fe reference sample used in Ref. 21.…”