Probing the surface phase diagram of Fe3O4(001) towards the Fe-rich limit: Evidence for progressive reduction of the surface

Abstract

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 ($\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$)$R$45\ifmmode^\circ\else\textdegree\fi{} unit cell. Further RT deposition results in Fe clusters. With mild annealing, a second Fe adatom per unit cell is accommodated, though not in the second tetrahedral site. Rather both Fe atoms reside in octahedral coordinated sites, leading to a ``Fe-dimer'' termination. At four additional Fe atoms per unit cell, all surface octahedral sites are occupied, resulting in a FeO(001)-like phase. The observed configurations are consistent with the calculated surface phase diagram. Both XPS and DFT+$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.