Semiconductor–half metal transition at the ${\text{Fe}}_3{\text{O}}_4\left(001\right)$ surface upon hydrogen adsorption

The adsorption of H on the magnetite (001) surface was studied with photoemission spectroscopies, scanning tunneling microscopy, and density-functional theory. At saturation coverage the insulating $\left(\sqrt{2}\times \sqrt{2}\right)R45°$ reconstruction is lifted and the surface undergoes a semiconductor–half metal transition. This transition involves subtle changes in the local geometric structure linked to an enrichment of ${\text{Fe}}^{2+}$ cations at the surface. The ability to manipulate the electronic properties by surface engineering has important implications for magnetite-based spintronic devices.

Figure 1

(Color online) Modification of an ${\text{Fe}}_3{\text{O}}_4\left(001\right)$ surface with (left) 1 and (right) 8 Hydrogen atoms per surface unit cell. O atoms are red/large, Fe(B) are gold/gray and Fe(A) are blue/light gray. (a) Adsorption geometry. (b) Side view showing the occupation of the minority $t_{2g}$ orbitals at the Fe(B) sites (i.e., ions with ${\text{Fe}}^{2+}$ character); electron density integrated between $-1.3\text{eV}$ and $E_{\text{F}}$. (c) Total density of states (solid black line with yellow filled area) showing the characteristics of half-metallic system. For comparison the DOS of the clean surface (modified B layer; black dashed line), shows a band gap of 0.3 eV.

Figure 2

(Color online) The clean ${\text{Fe}}_3{\text{O}}_4\left(001\right)$ $\left(\sqrt{2}\times \sqrt{2}\right)R45°$ surface: (a) STM image ($V_{\text{sample}}=+1.7\text{V}$, $I_{\text{tunnel}}=0.14\text{nA}$) and (b) LEED pattern $\left(E_{\text{el}}=90\text{eV}\right)$. (c) and (d) Consecutive STM images (${\Theta }_{\text{H}}\sim 0.07\text{ML}$, $4\text{nm}\times 3.5\text{nm}$, 1.4 V, 0.14 nA) at low atomic H coverage. The arrows mark the motion of a H-induced bright double protrusion to a neighboring row. (e) STM image (${\Theta }_{\text{H}}\sim 0.25\text{ML}$, $4\times 4{\text{nm}}^2$, 0.76 V, 0.21 nA). One row of Fe(B) atoms is bright and straight (dashed blue line), an uncovered section, marked by the green arrow, displays the characteristic Fe(B) “wiggle.” (f) LEED pattern from the H saturated surface displaying $\left(1\times 1\right)$ symmetry.

Figure 3

(Color online) (a) UPS spectra ($h\nu =29\text{eV}$, normal emission) from the clean (black) and H-covered (red/gray) ${\text{Fe}}_3{\text{O}}_4\left(001\right)$ surfaces. The inset shows the region around $E_{\text{F}}$; the blue line shows the metallic Fermi edge acquired from the molybdenum sample plate. (b) $\text{Fe}2p$ XPS spectra ($\text{Al}K\alpha$ photons, emission angle at $60°$ from normal) from the clean (black) and H-covered (red) ${\text{Fe}}_3{\text{O}}_4\left(001\right)$ surfaces. The inset magnifies the shift in the leading edge of the $\text{Fe}2p_{3/2}$ peak to lower binding energy and indicates the expected position for ${\text{Fe}}^{2+}$ and ${\text{Fe}}^{3+}$ cations.