Probing the surface phase diagram of Fe${}_3$O${}_4$(001) towards the Fe-rich limit: Evidence for progressive reduction of the surface

Reduced terminations of the Fe${}_3$O${}_4$(001) surface were studied using scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Fe atoms, deposited onto the thermodynamically stable, distorted B-layer termination at room temperature (RT), occupy one of two available tetrahedrally coordinated sites per ($\sqrt{2}\times \sqrt{2}$)$R$45° unit cell. Further RT deposition results in Fe clusters. With mild annealing, a second Fe adatom per unit cell is accommodated, though not in the second tetrahedral site. Rather both Fe atoms reside in octahedral coordinated sites, leading to a “Fe-dimer” termination. At four additional Fe atoms per unit cell, all surface octahedral sites are occupied, resulting in a FeO(001)-like phase. The observed configurations are consistent with the calculated surface phase diagram. Both XPS and DFT+$U$ results indicate a progressive reduction of surface iron from Fe${}^{3+}$ to Fe${}^{2+}$ upon Fe deposition. The antiferromagnetic FeO layer on top of ferromagnetic Fe${}_3$O${}_4$(001) suggests possible exchange bias in this system.

Figure 1

(a) Inverse spinel bulk unit cell of Fe${}_3$O${}_4$. In the (001) direction planes comprising tetrahedrally coordinated FeA atoms (A layer) alternate with planes containing octahedral FeB atoms and oxygen atoms (B layer). (b) Top view of the distorted B-layer termination of Fe${}_3$O${}_4$(001) as determined by DFT+$U$ calculations. In-plane relaxations perpendicular to the surface FeB(S) row direction lead to a ($\sqrt{2}\times \sqrt{2}$)$R$45° symmetry (black square). The two bulk-continuation FeA(S) sites within the reconstructed unit cell are rendered inequivalent by the reconstruction [sites labeled narrow “$n$” and wide “$w$,” where the O-O distance in the surface is compressed (3.27 Å) and expanded (3.44 Å), respectively]. Two dark blue broken circles indicate the octahedral sites in the oxygen lattice occupied by Fe atoms in the Fe-dimer termination.

Figure 2

Surface phase diagram of Fe${}_3$O${}_4$(001) derived from the DFT+$U$ calculations. For each value of the oxygen chemical potential ${\mu }_{\mathrm{O}}$, the lowest line shows the most favorable surface termination. The distorted B-layer is stable over a broad range of oxygen chemical potentials. In a narrow range towards oxygen poor conditions (low ${\mu }_{\mathrm{O}}$) the B-layer competes with oxygen vacancies (B+V${}_{\mathrm{O}}$) and a 0.5 ML FeA layer, followed by a Fe dimer termination and, finally, by an FeO termination. Chemical potentials beyond the broken lines correspond to formation of metallic Fe (left) and condensation of oxygen (right).

Figure 3

Deposition of Fe on the Fe${}_3$O${}_4$(001) surface at RT. (a) STM image (20 × 20 nm${}^2$, $V_{\mathrm{sample}}$ $=$ $+$1.2 V, $I_{\mathrm{tunnel}}$ $=$ 0.3 nA) after deposition of 0.025 ML Fe at RT. Adsorbed Fe atoms, marked by yellow circles, occupy FeA bulk continuation sites between the FeB rows. The inset shows a high-resolution image (2.7 × 2.7 nm${}^2$, $+$0.9 V/0.36 nA) of an FeA adatom adsorbed in the site labeled $n$ in Fig. 1b. (b) STM image (20 × 20 nm${}^2$, $+$1.2 V/0.3 nA) after deposition of 0.8 ML Fe at RT. FeA adatoms and Fe clusters are observed. The inset shows a high-resolution STM image of the area marked by the orange square, with one FeA indicated by the yellow circle. FeA adatoms occupy positions consistent with ($\sqrt{2}\times \sqrt{2}$)$R$45° symmetry (cyan grid, nodes centered at $w$ sites). (c) Structural model of the 0.5 ML FeA termination derived from DFT+$U$ calculations. The FeA adatom is twofold coordinated to surface O atoms. The O-O distance increases to accommodate the extra Fe atom, which relaxes into the surface by 0.62 Å relative to the bulk spacing.

Figure 4

(a) STM image (20 × 20 nm${}^2$, $V_{\mathrm{sample}}$ $=$ $+$0.91 V, $I_{\mathrm{tunnel}}$ $=$ $+$0.34 nA) following postannealing to 423 K of a surface containing 0.8 ML additional Fe, as also shown in Fig. 3b. In areas of low local Fe coverage (orange rectangle, see inset for high resolution image), isolated Fe adatoms coexist with Fe dimers between the FeB(S) rows. In areas of high local Fe coverage (yellow rectangle, see inset for high resolution image), only Fe dimers are observed between the FeB(S) rows. (b) STM image (20 × 20 nm${}^2$, $+$1.03 V/+0.32 nA) following deposition of 0.7 ML Fe onto the B-layer terminated surface at RT. The surface was postannealed at 573 K for 15 min. Only Fe dimers are observed between the FeB(S) rows. (inset) High resolution STM image of the area contained within the yellow rectangle in Fig. 3b. Note that the atoms of the Fe dimer are located directly between FeB(S) atoms on neighboring FeB rows. (c) DFT+$U$ derived model of the Fe dimer termination (top view).

Figure 5

STM image (20 × 20 nm${}^2$, $V_{\mathrm{sample}}$ $=$ $+$0.65 V, $I_{\mathrm{tunnel}}$ $=$ $+$0.32 nA) following deposition of 2 ML Fe at 573 K. The Fe-dimer termination coexists with an FeO-like termination. This appears as a square lattice as all octahedral interstitial sites in the O(S) lattice are filled (yellow and dark blue in model). The interface between the two regions is smooth, but the FeO-like areas contain many structural defects. (b) DFT+$U$ derived structural model of a FeO(001)-like surface layer on the Fe${}_3$O${}_4$(001) bulk. Note that the Fe${\mathrm{B}}^{*}\left(\mathrm{S}\right)$ row with underlying FeA(S-1) neighbors (dark blue) is raised above the surface plane.

Figure 6

Density of states (DOS) and spin density for the surface terminations shown in Fig. 2. Top and middle panels show the partial density of states (PDOS) of surface and subsurface FeA and FeB 3$d$ states, respectively. The bottom panels show the total DOS. While the surface layer remains insulating for all presented terminations (only for the 0.5 ML FeA structure there is a localized state pinned at $E_{\mathrm{F}}$), half-metallic behavior is observed for all but the B-layer termination in the deeper layers due to incomplete charge localization. (b) Blue and red represent the two different spin orientations. A spherical spin distribution indicates an Fe${}^{3+}$ ion with a 3$d^5$ occupation and deviations from the spherical shape indicates an Fe${}^{2+}$ with a 3$d^6$ occupation. In the B-layer all FeB(S) are Fe${}^{3+}$. With increasing Fe coverage a progressive reduction to Fe${}^{2+}$ takes place. Note that for the “Fe dimer” and FeO terminations the surface layer is completely Fe${}^{2+}$.

Figure 7

XPS for the O 1s (top) and Fe 2p (bottom) data acquired from a pure B-layer termination and a surface containing a high coverage of Fe dimers at both normal and grazing emission. The O 1s peak is reduced in intensity and shifted slightly to higher binding energy (0.2 eV) for the surface with Fe dimers. The Fe $2p_{3/2}$ peak is shifted to lower binding energy and exhibits a satellite peak at 717 eV, consistent with an enhancement in Fe${}^{2+}$ cations. The effect is more pronounced at grazing emission, indicating that the additional Fe${}^{2+}$ is located at or near the surface.