Unconventional properties of nanometric FeO(111) films on Ru(0001): stoichiometry and surface structure

We report on the growth of high quality monocrystalline and stoichiometric FeO(111) films on Ru(0001) by infrared pulsed laser deposition (IR-PLD), in a thickness range from below 1 nm to above 8 nm. The films are characterized by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). Besides the 1 : 1 Fe/O ratio, they show some unexpected properties, such as the lack of Fe3+ sites at the surface, the (1 x 1) surface symmetry and the large lattice expansion. First-principles calculations show that these properties can be understood from the existence of a wurtzite-like environment at the surface region that preserves the bulk-like antiferromagnetism. This extends the validity of stacking-faults as efficient mechanisms to compensate surface polarity, and suggests that surface-induced processes can be tailored to design nanoscaled materials beyond the parent bulk phase diagram.