Surface electronic structure of the ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$: Evidence of a half-metal to metal transition

In situ prepared ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ thin films were studied by means of scanning tunneling microscopy (STM) and spin-polarized photoelectron spectroscopy (SP-PES). The atomically resolved $(\sqrt{2}\times \sqrt{2})R45°$ wavelike surface atomic structure observed by STM is explained based on density functional theory (DFT) and ab initio atomistic thermodynamics calculations as a laterally distorted surface layer containing octahedral iron and oxygen, referred to as a modified $B$ layer. The work-function value of the ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ surface extracted from the cutoff of the photoelectron spectra is in good agreement with that predicted from DFT. On the ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ surface both the SP-PES measurements and the DFT results show a strong reduction of the spin polarization at the Fermi level $\left(E_F\right)$ compared to the bulk density of states. The nature of the states in the majority band gap of the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ surface layer is analyzed.

Figure 1

(a) Side view of the inverse spinel ${\mathrm{Fe}}_3{\mathrm{O}}_4$ structure. (b) Top view of the ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ bulk $B$-termination. The black open square in (b) marks the $p(1\times 1)$ bulk unit cell.

Figure 2

(Color online) Fe $L_{2,3}$ edge XAS spectra of an epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ film (upper spectrum) as well as of the epitaxial Fe(100) film (lower spectrum).

Figure 3

A $(\sqrt{2}\times \sqrt{2})R45°$ LEED diffraction pattern of a ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ surface. The $p(1\times 1)$ as well as the $(\sqrt{2}\times \sqrt{2})R45°$ unit cells are marked by 1 and 2, respectively. The main crystallographic axes are shown by arrows. The energy of the electron beam is $107\mathrm{eV}$.

Figure 4

(Color online) $50\times 50{\mathrm{nm}}^2$ STM image of a $(\sqrt{2}\times \sqrt{2})R45°$ reconstructed ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ surface. Tunneling parameters: $U_T=1.1\mathrm{V}$, $I_T=1.2\mathrm{nA}$. A line profile (A) corresponds to the STM height profile along the white line shown in the STM image showing step heights of about $2.1\mathrm{\AA }$.

Figure 5

(Color online) STM image $(11\times 11{\mathrm{nm}}^2)$ of ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ surface showing a wavelike atomic structure which gives rise to a $(\sqrt{2}\times \sqrt{2})R45°$ periodicity: ($U_T=1.1\mathrm{V}$, $I_T=1.2\mathrm{nA}$).

Figure 6

Top (a) and side (b) view of the modified $B$ termination predicted from the DFT calculations, (c) simulated STM image of the modified $B$ termination. The positions of the surface oxygen, octahedral iron, and subsurface tetrahedral iron are marked by white, gray, and black circles, respectively. The black open square in (a) and (c) denotes the $(\sqrt{2}\times \sqrt{2})R45°$ unit cell. The dashed line in (c) is a guide for the eyes to emphasize the displacements of surface ${\mathrm{Fe}}_B$ perpendicular to the [011] direction.

Figure 7

(Color online) Spin-resolved valence band photoemission spectra (spin up: blue open up triangle; spin down: red solid down triangle) and total intensity (black solid circle) of a ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ thin film on MgO(100) as a function of binding energy recorded at RT in normal emission at $42–60\mathrm{eV}$ photon energy.

Figure 8

(Color online) (a) Spin-resolved photoemission spectra (spin up: blue open up triangles; spin down: red solid down triangles; total photoemission intensity: black solid circles) of the ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)∕\mathrm{Mg}\mathrm{O}\left(100\right)$ system recorded with $h\nu =58\mathrm{eV}$ in normal emission, (b) the region near $E_F$, and (c) the spin polarization as function of binding energy of the ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)∕\mathrm{Mg}\mathrm{O}\left(100\right)$ system.

Figure 9

(a) Charge density distribution of the surface states in the majority spin channel from $-0.2\mathrm{eV}$ to $E_F$ in the surface layer. The positions of the surface oxygen, octahedral iron, and subsurface tetrahedral iron are marked by white, gray, and black circles, respectively. Spin-resolved density of states of ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)$ surface terminated by a modified $B$ layer:[24] (b) total DOS; projected DOS of ${\mathrm{Fe}}_B$ in the bulk (c) as well as on the surface in the ideal (d) and modified (e) $B$ layer.