Resonant photoelectron and photoelectron diffraction across the $\text{Fe}{L}_3$ edge of ${\text{Fe}}_3{\text{O}}_4$

We combined measurements of valence-band photoemission and valence-band photoelectron diffraction on ${\text{Fe}}_3{\text{O}}_4$ at the $\text{Fe}2p\text{−}3d$ resonance. The different structures in the valence band of magnetite due to the different sites of iron were identified experimentally. Specifically the structure near the Fermi level is unambiguously attributed only to octahedral ${\text{Fe}}^{2+}$ sites $\left(B{\text{-Fe}}^{2+}\right)$. We showed that tuning the photon energy to the resonance of $B{\text{-Fe}}^{2+}$, the whole valence band is dominated by signal coming from $B$ sites of iron. Moreover this work shows how resonant photoelectron diffraction is a powerful tool for the study of mixed valence oxides.

Figure 1

(Color online) Geometry of the XPD experiment. The $p$-polarized light is impinging on the sample at grazing incidence $\left(6°\right)$. The sample moves with azimuthal angle $\varphi$. The electron analyzer moves with the polar angle $\theta$.

Figure 2

(Color online) (a) experimental absorption spectrum at the $\text{Fe}{L}_3$ edge recorded in partial electron yield during the acquisition of resonant VB; (b) 2D plot of the resonant valence-band spectra recorded on ${\text{Fe}}_3{\text{O}}_4$ in $p$ polarization. The spectra are plotted as a function of the binding energy (horizontal scale) and for the different photon energies across the $\text{Fe}{L}_3$ edge (vertical scale) with the same scale than in (a), the colors indicating the photoemission intensity as a function of these two variables. Vertical dashed lines indicate the position of 1.2, 4.4, and 6.6 eV binding energy. Horizontal lines indicate the position of 707.5, 708.9, and 709.6 eV photon energy.

Figure 3

(Color online) Valence band recorded at different photon energies 707.5 eV (black line), 708.9 eV (red open circles), and 709.6 eV (green squares). Vertical lines indicate the position of 1.2, 4.4, and 6.6 eV binding energy. Inset: absorption curve at the $\text{Fe}{L}_3$ edge. Dashed vertical lines indicate the position of 707.5, 708.9, and 709.6 eV photon energy

Figure 4

(Color online) Bottom part: experimental VB-CIS spectra recorded on ${\text{Fe}}_3{\text{O}}_4$ at the $\text{Fe}{L}_3$ edge for 1.2 eV (black up-triangle), 4.4 eV (red down-triangle), and 6.6 eV (green squares) binding energies. The CIS spectra are normalized to the intensity at 705 eV. Top part: experimental absorption spectrum (blue squares) and calculations of site-resolved contributions to the absorption spectrum of ${\text{Fe}}_3{\text{O}}_4$ from Ref. 24 for $B{\text{-Fe}}^{2+}$ (black solid line), $A{\text{-Fe}}^{3+}$ (red dotted line), $B{\text{-Fe}}^{3+}$ (green dashed line), and total absorption (solid blue line). Vertical lines indicate the position of the maximum of the three CIS spectra.

Figure 5

(Color online) left: experimental VB photoelectron diffraction (between 0 and 8.5 eV binding energy) on ${\text{Fe}}_3{\text{O}}_4$ recorded with photon energy of (a) 692, (b) 707.5, and (c) 709 eV. Black color corresponds to angles where no data was recorded. right: multiple scattering calculations of XPD of ${\text{Fe}}_3{\text{O}}_4$ using EDAC code (see text) for $\text{Fe}3d$ level and (d) for all the iron sites, (e) the $B$ sites of iron, and (f) the $A$ sites of iron. The representation is a projection of diffraction intensity onto the plane of the paper, the scale is $\text{sin}\left(\theta \right)$ and $\varphi =0$ is found at three o’clock and corresponds to $\left[1\overline{1}0\right]$ direction of ${\text{Fe}}_3{\text{O}}_4$.

Figure 6

(Color online) experimental azimuthal scans (black squares) recorded at $\theta =42°$ (left column) and at $\theta =45°$ (right column) with photon energy indicated on the left ($\text{or}=692\text{eV}$, $a=707.5\text{eV}$, and $b=709\text{eV}$). The signal is the integration of the VB absolute intensity (a)–(f) from the Fermi level to 8.5 binding energy and (g)–(j) from Fermi level to 1.5 eV. Comparison with multiple scattering calculations for all sites of iron (green line) for $B$-sites of iron (red circles) and for $A$ sites of iron (black triangles). The best model for each case is indicated on the right of the figure.