We present an experimental and theoretical study of the geometric structure of ultrathin BaTiO(3) films grown on Fe(001). Surface x-ray diffraction reveals that the films are terminated by a BaO layer, while the TiO(2) layer is next to the top Fe layer. Cations in termination layers have incomplete oxygen shells inducing strong vertical relaxations. Onset of polarization is observed at a minimum thickness of two unit cells. Our findings are supported by first-principles calculations providing a quantitative insight into the multiferroic properties on the atomic scale.

The geometric structure of O / Fe͑001͒-p͑1 ϫ 1͒ is studied by surface x-ray diffraction ͑SXRD͒. After dosing of about 4 L ͑1 L=10 −6 Torr s͒ of oxygen at room temperature we find approximately one monolayer ͑ML͒ of oxygen in an FeO-like structure forming two layer thick islands covering about 40% of a roughened Fe͑001͒ surface. Subsequent annealing up to 500°C results in surface flattening leading to a highly ordered structure. Very precise SXRD data reveal the presence of 1 ML of oxygen atoms located in fourfold hollow sites at d ͑O-Fe͒ = 0.48Ϯ 0.08 Å above the first Fe layer. The first Fe-Fe-interlayer spacing is expanded to d 12 = 1.66Ϯ 0.02 Å corresponding to an increase of 16% relative to the bulk spacing ͑1.43 Å͒. Density-functional calculations confirm our findings and indicate a strong dependence of the ͑local͒ layer expansion on the oxygen coverage. Our results are important for understanding the surface magnetic properties of the O / Fe͑001͒-p͑1 ϫ 1͒ surface in general.

A solution to the phase problem for the case of surface crystallography is presented. By oversampling a surface diffraction pattern along the continuous crystal-truncation rods, we can iteratively recover the phases of the complex structure factors of an unknown surface atomic geometry. Simple Fourier inversion of these structure factors directly yields a three-dimensional map of the electron density in the surface region with ϳÅ resolution. This model-independent determination of atomic positions can then be used as a starting point for quantitative refinement using conventional means.

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The solid-state reaction of thick ͑ϳ50 nm͒ and thin ͑ϳmonolayer͒ films of Hf with cleaned and oxidized Si͑001͒ substrates was investigated. Upon annealing to 1000°C, films of HfSi 2 were formed after reaction times that depended upon the surface condition of the substrate before deposition. The chemical state of the reacted surfaces was characterized using x-ray photoelectron spectroscopy, and the shifts in binding energy upon silicide formation were recorded. Even for thick films, low-energy electron diffraction (LEED) revealed that the ͑2 ϫ 1͒ pattern of the Si substrate emerged, suggesting that three-dimensional islanding of the HfSi 2 film had occurred. The islanding behavior was investigated for both thick and thin films using LEED, atomic force microscopy, and scanning electron microscopy. Streaking in the LEED patterns for the thick films suggest that the island morphology is influenced by the underlying Si substrate.