Electronic and spin structure of O- and H-adsorbed ${\mathrm{Fe}}_3{\mathrm{O}}_4$(111) surfaces

The adsorption of O and H atoms on the ${\mathrm{Fe}}_3{\mathrm{O}}_4$(111) surface was investigated using ultraviolet photoemission spectroscopy (UPS) and first-principles calculations. On the FeA1-terminated surface, H adsorption decreases the work function without changing the density of states near the Fermi level. The density of states near the Fermi level decreases by O adsorption. On the O/FeA1-terminated surface, H adsorption dramatically increases the density of states near the Fermi level, indicating that the Fe $3d$ state is modified. The first-principles calculations showed that the electronic structure of the surface FeA on the FeA1-terminated surface is half metallic with its spin up forming an isolated spin-polarized conductive layer. By O adsorption, the up-spin band at the Fermi level of the surface FeA site is removed, and the Fermi level of the surface FeA moves to the down-spin $t_{2g}$ band. By subsequent H adsorption, electrons are doped to the FeA1 site, and the surface FeA1 layer becomes semiconducting. These results indicate that the charge and spin structure is modulated by O and H adsorption.

Figure 1

The energy band diagram of the Fe $3d$ orbital in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ with a modulated occupation of the FeA site.

Figure 2

Schematic illustration of the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ (111) surface viewed from the $\left[1\overline{1}0\right]$ direction.

Figure 3

(a) UPS spectra of the FeA1-terminated and the O/FeA1-terminated surfaces. The inset shows the region near the Fermi level. (b) The difference spectrum obtained by subtracting the spectra of the FeA1-terminated surface from the O/FeA1-terminated surface.

Figure 4

(a) UPS spectra of the FeA1-terminated surface with various H+${\mathrm{H}}_2$ exposure, (b) the difference spectra from the FeA1-terminated surface (the dots and lines show the experimental data and the smoothed curves, respectively), and (c),(d) the evolution of the work function and the intensity of the peak at 0.5 eV below the Fermi level with H+${\mathrm{H}}_2$ exposure. The intensity of the peak at 0.5 eV below the Fermi level was estimated by integrating the spectra over $E-E_F$ between 0.5 eV and $-1.0$ eV. Here, 1 $\mathrm{Langmuir}=1.33\times {10}^{-6}\mathrm{mbar}\mathrm{s}$.

Figure 5

(a) UPS spectra of the O/FeA1-terminated surface with various H+${\mathrm{H}}_2$ exposure, (b) the difference spectra from the O/FeA1-terminated surface (dots and lines show the experimental data and the smoothed curves, respectively), and (c),(d) the evolution of the work function and the intensity of the peak near the Fermi level with H+${\mathrm{H}}_2$ exposure. The intensity of the peak near the Fermi level was estimated by integrating the spectra over $E-E_F$ between $-0.5$ eV and $-1.0$ eV.

Figure 6

(a) Schematic illustration of the FeA1-terminated ${\mathrm{Fe}}_3{\mathrm{O}}_4$(111) surface. The adsorption sites for H atoms, FeA1 and O1 sites, are marked by black and green arrows, respectively. (b) Potential energy of an electron relative to the Fermi level of the $\mathrm{Fe}\text{A1}$-terminated surface and H-adsorbed surface. The dashed line shows the position of the topmost FeA1 layer.

Figure 7

The local density of states (LDOS) of (a) the surface $\mathrm{Fe}\text{A1}$ atom and (b) the $\mathrm{Fe}\text{B1}$ atom on the FeA1-terminated surface. The red and blue lines represent the clean and the H-covered $\mathrm{Fe}\text{A1}$-terminated surfaces. We assumed the H atoms are adsorbed on the O1 site.

Figure 8

(a) Schematic illustration of the O/FeA1-terminated ${\mathrm{Fe}}_3{\mathrm{O}}_4$(111) surface. The adsorption sites for H atoms, ${\mathrm{O}}_{\mathrm{top}}$ and O1 sites, are marked by black and green arrows, respectively. (b) and (c) show the local density of states (LDOS) of the surface $\mathrm{Fe}\text{A1}$ atom and the $\mathrm{Fe}\text{B1}$ atom on the O/FeA1-terminated surface, respectively. The red line represent the DOS without H adsorption, and the black and blue lines represent the H adsorption on the O1 and ${\mathrm{O}}_{\mathrm{top}}$ sites.

Figure 9

(a) The spin direction of the conduction electrons of topmost FeA1 and FeB1 layer when the Fermi level at the FeA1 site is located in the up-spin $e_g$ band and (b) in the down-spin $t_{2g}$ band which correspond to the GGA results of the FeA1-terminated surface and the O/FeA1-terminated surface, respectively.