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The magnetic behavior of nanocrystalline copper spinel ferrites CuxFe3-xO4 (0.42<=x<=1) exhibiting different degrees of crystallinity, have been investigated by means of in-field Mössbauer and dc magnetic measurements. The samples were obtained by a coprecipitation method and were subsequently high-energy ball milled for different times, which induced a progressive improvement of the ferrite crystallinity and cation redistribution. Their magnetism shows some peculiar features; at 5 K, the virgin magnetization curves remain outside the main hysteresis loop for applied fields of ≈1 T and display an S-shaped form. Below 60 K, the field-cooled hysteresis loops are shifted toward the negative field direction. In addition, in the same temperature range, the field-cooled magnetization of the samples with poorest crystallinity decreases sharply due to the spin freezing at the surface. The in-field Mössbauer spectra at 4.2 K show that the magnetic moments of the Fe3+ ions are in a canted spin state for both the tetrahedral and octahedral sites of the spinel structure, and evidence a decrease of the average spin canting with the degree of crystallinity. After calcination at 950 °C, the coprecipitated sample no longer displays the magnetic anomalous and canting features. We explain our results as due to the partially disordered surface within the frame of the core-shell model rather than to the effect of cation redistribution. [ABSTRACT FROM AUTHOR]

A catalyst powder was prepared by adsorption precipitation of iron acetate on a SiO[sub 2] support at a controlled pH. The selective reduction of this powder in a N[sub 2]/C[sub 2]H[sub 2] atmosphere at 700 °C generates multiwall carbon nanotubes. The study by x-ray diffraction and Mössbauer spectroscopy of the catalyst and reduced powders reveals that hematite particles with a 8-20 nm size range are involved in the formation of these carbon nanotubes and end up as Fe[sub 3]C after the catalysis process. Moreover, coalescence upon reduction produces large particles responsible for the formation of a minor proportion of large tubes of the order of 100 nm in diameter. [ABSTRACT FROM AUTHOR]

Alloys with a Si content of 4.2 and 5.4 wt % Si were produced to investigate the effect of increasing the Si on the materials processing and properties and to understand the effect of the order -- disorder phenomenon on its magnetic properties. Different cooling rates after hot rolling were applied: Slow cooling from 780 °C to room temperature in 26 h, air cooling and water quench, followed by cold rolling until 0.5 to 0.7 mm thickness. Magnetic properties were measured after pickling and annealing at 950 °C for 2 h. [SUP57]Fe Mössbauer spectroscopy was used to study the effect of thermomechanical cycles on the ordering phenomena. It was noticed that the quenched samples have the highest values for the magnetic polarization, while slowly cooled samples have the lowest, for the power losses higher values are obtained for the quenched materials. The highest values for the polarization in the quenched samples were explained as a result of a higher B2 ordering. [ABSTRACT FROM AUTHOR]

A series of oil shale samples selected from Abu Gradig oil field (AG-1) in the Egyptian Western Desert have been studied by Mossbauer spectroscopy and Fourier transform infrared spectroscopy. Eleven samples were collected from Baharyia formation at different depths, starting from 10,290 feet to 11,290 feet. The carbon, hydrogen, nitrogen, and sulfur contents in the samples were determined using a Carlo Erba CHNS-OEA1108 elemental analyzer. The Mossbauer spectra were performed at 80 K with a conventional-type mode spectrometer. From the computer analysis of these spectra, it was found that iron was mainly present in pyrite, hematite, and in the carbonates as siderite and ankerite, wherein their presence is considered as an indicator of the hydrocarbon maturity. Maturity is obtained using Fourier transform infrared method whereas the ratio of the aliphatic to aromatic bands as represented by the A-factor (2,922 cm−1+ 2,853 cm−1/2,922 cm−1+ 2,852 cm−1+ 1,642 cm−1) can be calculated. Therefore, the Mossbauer results have been complemented with Fourier transform infrared data in order to demonstrate the correlation between the presence of carbonates (siderite and ankerite) found by MS and the hydrocarbon maturity found in the oil shale samples using Fourier transform infrared. [ABSTRACT FROM PUBLISHER]

To understand the particular properties of thin α-Fe2O3 films, hematite nanoparticles with sizes between 20 and 140 nm have been examined by integral low-energy electron Mössbauer spectroscopy (ILEEMS) and transmission Mössbauer spectroscopy (TMS) at room temperature and 80 K. By comparing the results of both Mössbauer variants, surface effects of the Morin transition are studied. The results clearly indicate that the Morin transition takes place at higher temperature in the interior of the particles as compared to the outer layers. It is found that the lowering of the Morin transition is much stronger at the surface of smaller particles. [ABSTRACT FROM AUTHOR]

Samples of a fly ash and a bottom ash, each before and after ignition, have been investigated by X-ray diffraction and Mössbauer spectroscopy in order to explain the observed negative loss of ignition (LOI). It is shown that the ashes after ignition contain more maghemite resulting from newly formed magnetite. Moreover, the fly ash which contained already magnetite exhibited an increase of hematite after ignition. Both oxidation processes can be responsible for a weight gain which compensates the loss due to the burning of the remaining carbon. The magnetite and the alpha-iron formed after ignition is originated by an unidentified compound represented by a Fe2 + doublet in the Mössbauer spectrum. [ABSTRACT FROM AUTHOR]

Application of iron (Fe) -rich amendments to soils has been proposed as a means of decreasing phosphorus (P) losses from soils. However, anoxic conditions following soil saturation are known to increase Fe and P solubility in soils, thus cancelling out the potential benefits. Our aim was to evaluate the effects of continuous oxic, continuous anoxic and alternating anoxic/oxic conditions on P exchangeability and Fe forms in soil amended with Ca(OH)2 and FeSO4. We incubated amended and unamended soils under these conditions for 8 weeks and measured Fe forms and P exchangeability. Under oxic conditions, addition of Ca(OH)2 and FeSO4 resulted in a strong decrease in P exchangeability and an increase in oxalate-extractable Fe. Mössbauer analyses suggested that an unidentified Fe oxide (D1oxide) with a strong sorbing capacity for P was precipitated. Under continuously anoxic conditions, P exchangeability and oxalate-extractable Fe increased with or without the amendments. Mössbauer analyses suggested that there was a partial dissolution of the D1oxide phase, precipitation of another unidentified Fe oxide (S3) and a reduction of structural Fe3+ in phyllosilicate, thereby increasing soil negative charge. These transformations resulted in a strong increase in rapidly exchangeable P. Alternating anoxic and oxic periods induced the dissolution and precipitation of iron oxides and the increase and decrease in P exchangeability. Implications of the results for limiting P losses from grassland soils are discussed. [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 Mössbauer 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]

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]

We have studied the Morin transition in nanostructured pseudocubic α-Fe2O3 particles of about 1.8 μm 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, α-Fe2O3(an) has a T M ≈ 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]

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]

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 Fe-containing substances. In a first part of this paper the authors briefly describe the design of a home-made 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]

The effect of aluminum content and degree of crystallinity upon the possible values for both the polar coordinates (θ and ϕ) of the magnetic hyperfine field direction with respect to the electric field gradient (EFG) frame of reference and for the asymmetry parameter (η) in synthetic Al-goethites was determined experimentally from their variable temperature Mössbauer spectra. The range of values for these parameters was examined using the ratio R of the quadrupole shift to the quadrupole splitting. It was found that R varies in the range -0.62&les;R&les;-0.55, implying that, considering all the samples, ∼65°&les;θ&les;90°, ∼-1&les;cos 2ϕ&les;-0.25 and ∼0.1&les;η&les;1. Additionally, the principal components of the EFG tensor and η were calculated using the simple point charge model. [ABSTRACT FROM AUTHOR]

A novel method for synthesis of aluminium hematites, based upon the homogeneous precipitation of Fe and Al oxinates in various proportions, is presented. The precursor precipitates are heated in air at 700 °C. X-ray diffraction, thermal analyses, BET, FTIR, optical reflection analysis, TEM and Mössbauer spectroscopy at room temperature and 80 K of the resulting products indicate that single-phase hematites are formed with structural Al substitution of up to 10 at%. Interestingly, the particle size (>100 nm) is not substantially reduced by the Al content. Although it remains difficult to obtain a homogeneously distributed Al substitution in the final hematite, this processing line offers a unique opportunity to separate the effects of grain size and Al substitution on the Morin transition temperature ( TM) of Al hematite. From the comparison between the present hematites and a series of Al-substituted hematites with lepidocrocite as precursor, it could be shown that the effect on TM, associated with a change of a factor 10 in grain size, is about 1/3 of the effect caused by a change of 10 in the degree of substitution. Finally, it is suggested that proper thermal treatments under different conditions of the same precursors are likely to produce spinel phases. [ABSTRACT FROM AUTHOR]

A study of the characteristics of the Morin transition in aluminous hematites, α-(Fe1−xAlx)2O3, produced from thermally transformed lepidocrocites, is reported. Six compositions with Al contents between 0.2 and 10 at% have been considered. It is argued that these samples present the advantage that they contain smaller amounts of hydroxyl and water as compared to hematites obtained by other preparation methods. The samples were characterised by a variety of conventional techniques, including thermal analyses, X-ray diffraction, FTIR, TEM/EDX, BET surface-area measurements and diffuse reflectance spectroscopy. All results indicate that the Al is structurally incorporated in the hematite lattices. Transmission Mössbauer spectra were recorded at various temperatures between 80 K and room temperature in order to precisely determine the Morin-transition region and the spin structure in both the low-temperature antiferromagnetic and weakly ferromagnetic states. It was found that the Morin-transition temperatures are markedly higher as compared to similar hematites made from aluminous goethites and that a transition phenomenon persists to an Al substitution of up to at least 10 at%. This different behaviour is ascribed to lower concentrations of structural hydroxyl groups in these lepidocrocite-based hematites. [ABSTRACT FROM AUTHOR]