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

Z. Novotný, M. Mulakaluri, Z. Edes, M. Schmid, R. Pentcheva, U. Diebold, G. S. Parkinson

Phys. Rev. B 87 (2013) 195410

Reduced terminations of the Fe₃O₄(001) surface were studied using scanning tunneling microscopy (STM), 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 (√2 × √2)R45° 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(100)-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 Fe³⁺ to Fe²⁺ upon Fe deposition. The antiferromagnetic FeO layer on top of ferromagnetic Fe₃O₄(001) suggests possible exchange bias in this system.

Corresponding author: Gareth S. Parkinson (parkinson).

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