Search

Selected highly homogeneous powders of Fe3O4 with different particle size on the nanometer scale (10±2 and 3±2 nm) obtained by soft-chemical methods were studied by Mössbauer spectroscopy. The study shows clearly the powerful possibilities of Mössbauer spectroscopy to analyze the surface oxidation of nanostructured powders of magnetite. On the other hand, it is shown that for very small superparamagnetic particles the spectrum of magnetite might be quite similar to that of maghemite, making it difficult to distinguish between both phases. [ABSTRACT FROM AUTHOR]

The processing of high-Si steels remains challenging. The development of ordered phases is the reason for the deteriorated workability which makes not feasible its production. The influence of the hot and the cold work on the order degree in high-Si steel is investigated by TMS (Transmission Mossbauer spectroscopy) and ILEEMS (Integrated low-energy electron MS). The Si-addition favors the creation of an order gradient in the hot rolled sheets; this accounts as an extra strengthening mechanism. In order to have a successful cold rolling the degree of order must be as low as possible; this is difficult to obtain when the Si-content is too high. [ABSTRACT FROM AUTHOR]

We have studied the Morin transition in nanostructured pseudocubic α-Fe2O3 particles of about 1.8 mm side. The preparation was carefully chosen to obtain a system with a very narrow crystallite size distribution and particles of homogeneous morphology. Two samples were studied: one without thermal treatment (α-Fe2O3(ap)) and another annealed at 673 K in air for 12 h (α-Fe2O3(an)). Both were characterized by XRD, SEM, TGA and Mössbauer spectroscopy. The results indicate that the Morin transition is suppressed for α-Fe2O3(ap), however, a-Fe2O3(an) has a TM ≈ 230 K and the transition is completed over a very narrow temperature range. These results are discussed in connection with the crystallite size, the cell parameters, and the presence of OH− groups (hydrohematite) or incorporated water (protohematite). [ABSTRACT FROM AUTHOR]

ILEEMS is the acronym for Integral Low-energy Electron Mössbauer Spectroscopy. In this variant of Mössbauer spectroscopy the low-energy electrons, E < ∼15 eV, emitted by the probe nuclei in the absorber are counted as a function of source velocity. As a consequence of their low energy, the detected electrons' origin lies within a very thin surface layer with thickness of a few nanometers and consequently, ILEEMS is a useful technique to examine surfaces of Fecontaining substances. In a first part of this paper the authors briefly describe the design of a homemade ILEEMS equipment allowing the temperature of the investigated sample to be varied between 77 K and room temperature. The second part of this contribution deals with a selection of results obtained from ILEEMS spectra for various Fe oxides. In particular, the following items are covered: (1) surface versus bulk Morin transition in small-particle and near-bulk hematite, α-Fe2O3; (2) bulk and thin-film magnetite, Fe3O4; (3) ferrihydrite, 5Fe2O3 · 9H20, goethite, α-FeOOH, and lepidocrocite, γ-FeOOH, all to some extent in relation to their morphology. Interesting and intriguing findings concerning the surface properties of these oxides were obtained and it is argued that the results encourage more systematic research in this and related fields using the surface-sensitive ILEEMS technique. [ABSTRACT FROM AUTHOR]

This paper is a review of the current status about the remaining problems that are found in the investigation of the Quaternary Argentine soils and loessic sediments, and the way that Mössbauer studies can assist in solving them. There are two main types of investigations that make use of the magnetic response of the samples to correlate them with information gathered by other methods. On the one hand, there is the stratigraphic and chronological research, which is of importance from the geological and paleontological points of view. On the other hand, the paleoclimatic records, of significance toward a possible model of the past climate, are also studied because of their close relation to the sediments history. However, there is not yet a model that can tell the difference between the modifications due to the climatic conditions at the time when the soils were buried from processes that occurred after burial. Some examples are given that show that Mössbauer studies can be applied with a certain degree of success when cross-checked with magnetic measurements toward understanding the processes that occurred in alluvial B (paleosols) and C horizons (loess) from the eastern part of Buenos Aires Province. Although the application of Mössbauer studies to hydromorphic processes in soils is not straightforward, there are cases in which Mössbauer spectroscopy, if applied properly and correlated with other techniques, is able to characterize the type of iron oxides existing in the materials and thus assist theories about its origin and history. [ABSTRACT FROM AUTHOR]