Depth-dependent electronic states of a prototypical strongly correlated ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin film across the Verwey transition temperature $(T_{\mathrm{V}}\sim 125\mathrm{K})$ were investigated by hard x-ray photoemission spectroscopy (HAXPES) combined with x-ray total reflection (TR). We found that the Fe $2p$ HAXPES spectral shapes strongly depend on the effective inelastic mean-free-path (${\lambda }_{\mathrm{eff}}$) regardless of temperature ($T$). Particularly, the shoulder structure located at the lower binding energy side of the Fe $2p_{3/2}$ main peak becomes more pronounced with increasing ${\lambda }_{\mathrm{eff}}$ (i.e., increasing bulk sensitivity), indicating a significant difference in the electronic structures between the surface and deeper regions of the film. This trend coincides with the increase of the density of states near the Fermi level $\left(E_{\mathrm{F}}\right)$ for larger ${\lambda }_{\mathrm{eff}}$, suggesting the contribution of nonlocal screening effects. While the valence band spectra show a clear metal-to-insulator transition across $T_{\mathrm{V}}$, the Fe $2p$ core-level HAXPES and the magnetic circular dichroism profiles for large ${\lambda }_{\mathrm{eff}}$ are almost independent on $T$. This behavior suggests that the average valence of the $B$-site Fe species is maintained at 2.5+ across $T_{\mathrm{V}}$. Comparison with the configuration interaction theory based on the cluster model calculations emphasizes the importance of nonlocal screening effects in the Fe $2p$ spectral shapes. Our results demonstrate that HAXPES combined with TR is a powerful technique for probing systematic depth-dependent electronic states. By providing this systematic dataset from the surface to deeper regions, this work serves as a reliable benchmark for the research field.

Close

PrevNext

View figure in article

• Electronic structure
• Magnetic thin films
• Strongly correlated systems
• Hard x-ray photoelectron spectroscopy