No abstract

The ␥-ray holography was recently applied ͓P. Korecki et al., Phys. Rev. Lett. 79, 3518 ͑1997͔͒ to image the three-dimensional structure of ␣-57 Fe with an atomic resolution. A simple theory of the hologram formation based on single-scattering cluster formalism taking into account the polarization effects in the resonant nuclear scattering and absorption processes is presented. The high quality of the real-space reconstruction is demonstrated, however, problems arising from the cancellation of real twin images are revealed. The realistic simulations are in a good agreement with the experiment. A method for elimination of the real and twin images cancellation taking advantage of the strong dependence of the scattering phase on the detuning from the resonance in the nuclear scattering is proposed. ͓S0163-1829͑99͒03109-4͔

The adsorption of oxygen on a pseudomorphic iron monolayer deposited on a W(110) surface was studied experimentally and theoretically. Standard surface characterization methods, such as Auger electron spectroscopy and low energy electron diffraction, and specific nuclear methods, such as conversion electron Mössbauer spectroscopy (CEMS) and nuclear resonant scattering of synchrotron radiation, combined with theoretical calculations based on the density functional theory allowed us to determine the structure of the oxygen adsorbate and the electronic properties of iron atoms with different oxygen coordinations. The oxygen-(3 × 2) structure on the iron monolayer was recognized and was interpreted to be a state with oxygen chemisorbed on the non-reconstructed surface with modest electron transfer from iron to oxygen. A transition from chemisorbed oxygen to the onset of Fe-oxidation is revealed by distinct changes in the CEMS spectra.

We used nuclear resonant scattering (NRS) of synchrotron radiation to investigate the details of the thicknessinduced spin reorientation transition (SRT) in ultrathin epitaxial iron films on W(110), where the thicknesses of the films ranged from 1-5 monolayers. During growth, the magnetization of the Fe film, which was probed by the hyperfine magnetic field, changes from a noncollinear configuration with an out-of-plane magnetic component to the homogeneously magnetized state with the in-plane [1-10] easy direction. The fast acquisition of the experimental NRS spectra combined with the high sensitivity of this technique to the orientation of the hyperfine magnetic fields allowed us to study the magnetic evolution during SRT in detail. Our results reveal the complex character of this transition, which has been intensively studied in the past. The noncollinear magnetic structure appears in the system of the mono-, double-, and trilayer areas that coexist due to deviation from the layer-by-layer growth of iron on W(110). We also report the observation of out-of-plane magnetic anisotropy in the double-layer areas at temperatures as high as 300 K. By comparing the experimental results with density functional theory calculations, we conclude that surface magnetic moments are enhanced by 25%.