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We report on the molecular beam epitaxial growth of single-crystalline, stoichiometric Fe 3 O 4 and ␥-Fe 2 O 3 films on MgO͑100͒, using NO 2 as the oxidizing agent. Mössbauer spectroscopy on 57 Fe probe layers is used to determine accurately the stoichiometry of the films. It is found that also all intermediate nonstoichiometric Fe 3Ϫ␦ O 4 phases can be obtained. The formation of the metastable compound ␥-Fe 2 O 3 clearly demonstrates the large oxidizing power of NO 2 . Although the shape anisotropy dictates that the zero-field magnetization direction should lie entirely in the plane of the film, this is never observed. Stoichiometric Fe 3 O 4 has large out-of-plane components and only in the case of highly oxidized Fe 3Ϫ␦ O 4 does the magnetization approach the film plane. Upon further oxidation to stoichiometric ␥-Fe 2 O 3 , however, it rotates back, and finally becomes almost completely perpendicular to the plane of the film. Furthermore, in the case of ͑near-͒ stoichiometric Fe 3 O 4 , the magnetizations of the A and B sublattices are not completely coupled antiparallel. On average, the magnetization of the B site ions is 4°closer to the film plane than the magnetization of the A site ions. All the as-grown films exhibit a (ͱ2ϫͱ2)R45°surface reconstruction, independent of the stoichiometry. Using simple electrostatic considerations, we propose three possible surface terminations: a half-filled A layer, a B layer with oxygen vacancies and a B layer with hydroxyl groups. Upon annealing, the (ͱ2ϫͱ2)R45°reconstruction irreversibly transforms to a 3ϫ1 reconstruction, caused by Mg outdiffusion from the substrate. Strong reflection high-energy electron diffraction intensity oscillations give direct, unambiguous evidence that Fe 3 O 4 has a two-dimensional layer-by-layer growth mode over the entire temperature range studied, i.e., from 273 to 723 K, guaranteeing atomically flat surfaces and interfaces in multilayer structures. The largest oscillations are obtained on ex situ cleaved, UHV-annealed MgO͑100͒ substrates, or on in situ annealed Fe 3 O 4 /MgO(100) films. Deposition above ϳ700 K is accompanied by rapid Mg outdiffusion.

Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Recent studies show that the magnetic properties of epitaxial thin films of magnetite (Fe 3 O 4 ) deviate strongly from bulk behavior: it is difficult to saturate thin films, ultrathin films may become super paramagnetic, their saturation magnetization drops to zero, and the local magnetic moments are oriented out of plane in zero field. The possible relationship between this anomalous behavior and the occurrence of anti-phase boundaries ͑APBs͒ is discussed. Transmission electron microscopy images confirm that APBs are present in our Fe 3 O 4 films grown by molecular beam epitaxy on MgO͑100͒. Only APBs with out-of-plane shift vectors are visible. The much higher APB density found in sputtered films suggests that preparation conditions are important. To explain the deviating saturation and the superparamagnetic behavior of thin Fe 3 O 4 films at the same time, the magnetic coupling over the APB must be dramatically reduced due to spin disorder along the boundaries.

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The microscopic structures of hydrogen-antimony,-tellurium, and-tin complexes in silicon have been studied using first-principles total-energy calculations, in order to obtain a more definitive understanding of the various dopant-hydrogen complexes in n-type crystalline silicon. We find that for neutral SbH, TeH, and SnH complexes, the lowest-energy configurations are similar and of the type AB-Si (the H is located at the antibonding site of a Si atom that is adjacent to the impurity). The reaction SbH + H~SbH2 turns out to be exothermic. The results are consistent with recent experimental results using Mossbauer spectroscopy. For SbH2 various configurations are found that difFer only slightly in energy. The lowest-energy configuration of SbH& complexes exhibits electrical properties similar to those of substitutional Sb. This suggests that the formation of SbH2 not only competes with that of SbH and H2, but may also electrically activate the sample. MICROSCOPIC STRUCTURES OF Sb-H, Te-H, AND Sn-H.. . 8865 atoms in the complexes are calculated. The results indicate that the formation of SbHz is indeed exothermic, in agreement with the experiment. Furthermore, we find that SbH2 exhibits electrical properties similar to those of substitutional Sb. This suggests that the formation of SbH2 not only competes with that of SbH and H2*, but may also electrically activate the sample. Microscopic structures of the various impurity-H complexes are proposed. We find a different lowest-energy configuration for SbHz (and PHz) complexes than that proposed by Korpas, Corbett, and Estreicher. Knowledge about impurity sites and precise positions of the crystal nuclei in the neighborhood of defects in semiconductors is important in order to achieve a better understanding of the impurity electronic structure, the hyperfine interactions, diffusion properties, formation and reaction energies, solubilities, and the interchange of atoms at heterojunctions. Therefore, we have also performed calculations for the relaxations of the host crystal around a fourfold-coordinated substitutional (Sn, Sb, and Te} impurity in c-Si, which is of interest in itself and also helpful for further calculations where the H atoms are taken into account. This paper is organized as follows. The calculational details are described in Sec. II. In Sec. III, we present the results for fourfold-coordinated substitutional impurities, i.e. , Sn, Sb, and Te. Then the results for impurity-onehydrogen and impurity-two-hydrogen complexes are given in Sec. IV and Sec. V, respectively. In Sec. VI, we calculate the binding energies of various complexes. In Sec. VII we try to understand the invisible fraction in Mossbauer experiments based on the theoretical results. Conclusions are drawn in Sec. VIII. 'Reference 15. Reference 16. 'Reference 17. Reference 18.