In-plane structural anisotropy of ultrathin Fe films onGaAs(001)−4×6: X-ray absorption fine-structure spectroscopy measurements

Abstract

Iron films of thickness 2 monolayers (ML) and 5 monolayers prepared on pseudo $4\ifmmode\times\else\texttimes\fi{}6$-reconstructed $\mathrm{GaAs}(001)$ surfaces were studied in situ at the PNC/XOR beamline by polarization-dependent x-ray absorption fine structure methods to investigate structural anisotropy in the plane of the films and compare with theoretical predictions. These two thicknesses are on either side of the transition from island to layer-by-layer growth modes and provide insight into possible structural origins of in-plane uniaxial magnetic anisotropy observed in this magnetic film system. First-principles calculations by [Mirbt et al., Phys. Rev. B 67, 155421 (2003)] have suggested a splitting in the nearest neighbor distances of the iron could be as large as $0.06\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, for 1 ML of arsenic having migrated to the film surface, distorting the structure slightly from body-centered tetragonal along the $⟨110⟩$ and $⟨\ensuremath{-}110⟩$ directions. We observe a splitting of approximately $0.02(1)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ at 2 ML in the nearest-neighbor distances. An electronic anisotropy was detected at 2 ML that could correspond to differences in bonding between gallium and arsenic dangling bonds. An anisotropy in mean-square relative displacement was apparent for the 5 ML film, though no difference in nearest-neighbor distances was observed beyond error.