The Mössbauer spectra of amorphous iron, prepared by using sonochemical methods, exhibit a broad magnetic hyperfine sextet at both 78 and 295 K. The spectra do not change with time if the amorphous iron is not exposed to oxygen or moisture. An analysis of the spectra with the method of Lines and Eibschütz yields average magnetic hyperfine fields of 29.1 and 25.9 T at 78 and 295 K, respectively. The corresponding moments of 1.9 B and 1.7 B agree well with values obtained from earlier magnetization studies and, further, provide strong experimental support for earlier calculations of the magnetic moments in amorphous iron. The observed average isomer shifts of 0.27 and 0.14 mm/s obtained at 78 and 295 K, respectively, correspond to a decrease in the s-electron density at the iron-57 nucleus as compared to that of ␣-iron, a decrease which is consistent with the decreased coordination number of amorphous iron. The similarity of the 295 K iron K-edge x-ray-absorption spectrum of amorphous iron and ␣-iron, up to 7130 eV, indicates that the d-electron density of states just above the Fermi level is similar in both forms of iron. The absence of structural details above 7130 eV in the spectrum of amorphous iron indicates, in agreement with multiple-scattering calculations, that long-range order does not extend beyond the third shell of neighbors in amorphous iron. Greatly reduced extended x-ray-absorption-fine-structure scattering is observed at the iron K edge of amorphous iron as compared to ␣-iron. An analysis of the weak observed scattering reveals both a decrease in the average coordination number from 14 in ␣-iron to 10 in amorphous iron, and an asymmetric radial distribution function of the iron neighbors in the first shell. This asymmetric distribution yields for amorphous iron a minimum iron-iron distance of 2.40 Å and an average iron-iron distance of 2.92 Å. ͓S0163-1829͑98͒08817-1͔

The iron-57 Mössbauer spectra of R3Fe5O12, where R is Y, Eu and Dy, have been measured between 4.2 and 550 K. The substantial quadrupole splittings observed in the paramagnetic spectra confirm that the local symmetry at both the tetrahedral and octahedral iron(III) sites is not cubic. The low temperature Mössbauer spectra of Dy3Fe5O12 clearly confirm the spin reorientation between 10 and 15 K and the 4.2 and 10 K spectra are consistent with the known orientation of the magnetization at 14 K in the cubic Ia3̄d unit cell. The Mössbauer spectra of R3Fe5O12, where R is Y, Eu and Dy, obtained between 45 and 295 K, reveal four different tetrahedral iron(III) Mössbauer spectral components, four components which are inconsistent with a magnetization oriented along the [111] axis of a cubic Ia3̄d unit cell. In contrast, these four components are consistent with a crystal symmetry which is reduced from cubic to rhombohedral R3̄. The temperature dependence of the hyperfine fields in Dy3Fe5O12 indicates a small biquadratic exchange contribution to the magnetic exchange. The temperature dependence of the isomer shifts in Dy3Fe5O12 gives Mössbauer lattice temperatures of 405 and 505 K for the 16a and 24d sites, respectively, values which are in excellent agreement with the Debye temperature measured for Y3Fe5O12.

Abstract:We propose a new family of achromatic phase shifters for infrared nulling interferometry. These key optical components can be seen as optimized Fresnel rhombs, using the total internal reflection phenomenon, modulated or not. The total internal reflection indeed comes with a phase shift between the polarization components of the incident light. We propose a solution to implement this vectorial phase shift between interferometer arms to provide the destructive interference process needed to disentangle highly contrasted objects from one another. We also show that, modulating the index transition at the total internal reflection interface allows compensating for the intrinsic material dispersion in order to make the subsequent phase shift achromatic over especially broad bands. The modulation can be induced by a thin film of a well-chosen material or a subwavelength grating whose structural parameters are thoroughly optimized. We present results from theoretical simulations together with preliminary fabrication outcomes and measurements for a prototype in Zinc Selenide.

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Two phlogopite, two mixed-layer phlogopite-vermiculite, and two vermiculite samples collected from the Palabora Complex of South Africa have been investigated at 295 K by X-ray diffraction, chemical analysis, and Mössbauer spectroscopy. In addition the temperature dependence of the Mössbauer spectra has been measured between 95 and 295 K for one phlogopite and one mixed-layer sample. The results of the chemical analyses and the Mössbauer spectra improve our knowledge of the vermiculitization process in the Palabora Complex. Both techniques indicate oxidation of the Fe ions during the sequence: phlogopite → mixed-layer → vermiculite. Further, the Mössbauer spectra indicate that Fe oxidation occurs mainly in the octahedral sites and suggest that migration and oxidation of the Fe2+ ions from the octahedral sites to the tetrahedral sites may occur during the transformation of phlogopite into a mixed-layer phase. Finally, the vermiculitization process involves both Fe oxidation and loss of K with a concomitant increase in the Mg content.