Structure and magnetic properties of FeMnAl alloys investigated by Mössbauer spectroscopy and X-ray diffraction

The system (Fe_0.88Mn_{0.12 1–x} Al _x has been investigated in a concentration range from 5 to 14 at.% Al. We applied Mössbauer spectroscopy in the temperature range from 4 up to 900 K and X-ray diffractometry at room temperature. The as-cast samples show a bcc phase for all concentrations and exhibit broadened six-line Mössbauer spectra typical for disordered alloys. The Mössbauer spectra during a high temperature treatment show dramatic changes. These are due to ordering processes appearing at temperatures above 700 K. As an example of the observed changes, we present results obtained for the alloy withx= 14 at.% Al.     

Comparison of ball milling of Fe/Cr powders and Fe-Cr crystalline alloy

Using the mechanical attrition technique (MA), we have prepared a Fe-Cr alloy starting with a mixture of elemental iron and chromium powders with a nominal composition of 28 at% of Fe and 72 at% of Cr. MA was also performed on solid solutions of Fe₂₈Cr₇₂ crystalline alloy. The Mössbauer effect of the mechanically alloyed powder from Cr and Fe metals has been compared with that from crystalline alloy.     

On the Phase Transformations in Cr–FeB and Fe–CrB Systems at High Temperature

The solid state interactions occurring at high temperature in the Cr–FeB and Fe–CrB systems were studied by transmission Mössbauer and X-ray diffraction techniques on samples prepared by powders carefully mixed, cold-compacted and then treated at 1000°C for times up to 16 h. In the Cr–FeB system, iron atoms liberated by the substitutional diffusion of Cr into FeB lattice preferentially destabilize the iron monoboride with formation of Cr-containing Fe₂B. In the Fe–CrB system, chromium atoms liberated by the substitutional diffusion of Fe into CrB lattice interacts with iron forming an Fe–Cr metal alloy. Moreover, zones of Cr-containing FeB and Fe₂B form at the contact between metal iron and chromium monoboride, and tend to disappear as iron is consumed by the alloying process.     

Magnetic composites based on hybrid spheres of aluminum oxide and superparamagnetic nanoparticles of iron oxides

Materials containing hybrid spheres of aluminum oxide and superparamagnetic nanoparticles of iron oxides were obtained from a chemical precursor prepared by admixing chitosan and iron and aluminum hydroxides. The oxides were first characterized with scanning electron microscopy, X-ray diffraction, and Mössbauer spectroscopy. Scanning electron microscopy micrographs showed the size distribution of the resulting spheres to be highly homogeneous. The occurrence of nano-composites containing aluminum oxides and iron oxides was confirmed from powder X-ray diffraction patterns; except for the sample with no aluminum, the superparamagnetic relaxation due to iron oxide particles were observed from Mössbauer spectra obtained at 298 and 110 K; the onset six line-spectrum collected at 20 K indicates a magnetic ordering related to the blocking relaxation effect for significant portion of small spheres in the sample with a molar ratio Al:Fe of 2:1.     

Magnetoresistance in point contacts of the Heusler alloy Co2Cr0.6Fe0.4Al

Following a recent report predicting half-metallicity of Co₂Cr_0.6Fe_0.4Al, we have examined point contacts of this material. Large magnetoresistances (up to 80%) have been observed at room temperature in small fields (10 mT). Magnetostriction in these fields is negligible. A spin polarisation of 81% is inferred. Mössbauer spectra show that 90% of the iron is ordered. The magnetisation of the alloy is 3.65μ_B per formula unit, and its Curie temperature is 665 K.     

A Mössbauer study of samples from an oil well

Seven different samples obtained from the Petrobras Oil Well 1-ICA-1BA from depths down to 1872 m have been studied by Mössbauer spectroscopy, X-ray diffraction and neutron activation analysis. The X-ray diffraction showed the presence of clay minerals, feldspar, calcite, pyrite, quartz, dolomite, andrite and others. The neutron activation analysis has been used to obtain the iron concentration in the samples. The Mössbauer spectra were obtained both at room and liquid nitrogen temperatures. Spectra are complex in nature indicating multiple sites or mixture of iron containing minerals. The spectra at 85 K could be least squares fitted are resolved up to four doublets with typical values of IS(Fe)=1.4, 1.21, 1.16, 0.40 and QS=3.62, 2.82, 2.37, 0.61 mm/s respectively. They have slightly large line widths except for the first doublet. Two samples also showed the presence of hematite.     

Eu and Fe Mössbauer study of mechanically alloyed EuFeO3

EuFeO₃ was prepared by mechanical alloying starting from europium and iron oxides. After 20 h of milling the resulting compound is pure EuFeO₃. Samples were studied as a function of milling period using XRD, Mössbauer, SEM, and magnetic measurements. Mössbauer spectroscopy was used to probe both the transition metal and the rare-earth sites. Results are compared with previous works on EuFeO₃ prepared by different methods.     

Partial Reduction of MgFe2O4 with Al and Mg via Ball Milling

Nanocomposites consisting of metallic Fe particles and a nonmagnetic oxide were prepared by reducing MgFe₂O₄ with Al or Mg in a ball mill. The reaction takes place as a fast self-propagating process sometime between 0.5 and 1 h of milling. Combining XRD and Mössbauer spectroscopy reveals that the bcc Fe phase contains a few percents of dissolved Al, it has a high defect concentration, and the surface tension of the ultrafine (about 15 nm) grains results in local compressive strains. The magnetization is 25% less than expected for pure ferromagnetic Fe.     

Investigation of the Thermal Stability of Laser Nitrided Iron and Stainless Steel by Annealing Treatments

Laser nitriding has revealed to be a very promising and effective treatment to improve the technical properties, like surface hardness and corrosion-wear resistance, of iron and steels. The high nitrogen concentration, the fastness and precision of the treatment and the easy experimental setup make this technique very suitable for applications on industrial scale. Samples of pure iron and austenitic stainless steel have been irradiated with ns laser pulses in the UV radiation range and analyzed by means of Conversion Electron Mössbauer Spectroscopy (CEMS), Resonant Nuclear Reaction Analysis (RNRA), Grazing Incidence X-Ray Diffraction (GXRD) and Microhardness. Mössbauer Spectroscopy, in particular, is capable of detecting the phase composition of the nitrided layer and therefore represents an essential tool for these kind of analysis. The thermal stability of the treated samples have been investigated by subsequent annealings at increasing temperatures in vacuum and in air. For iron samples the annealing treatment at 250°C shows a rather drastic phase transformation from phase (fcc) into (Fe₄N) while a strong depletion of N has been observed for 400°C or higher, regardless of the ambient pressure (atmospheric or vacuum). On the other hand, the stainless steel shows a very good thermal stability up to 500°C, but higher temperatures induce a gradual decrease in the nitrogen concentration which seems to be a common feature for both pure iron and stainless steel. Furthermore, annealing in air leads to the formation of a thin oxide layer on the surface of the iron sample which is easily characterized by Mössbauer spectroscopy.     

Nanostructured Fe50Ni50 alloy formed by chemical reduction

A high purity Fe₅₀Ni₅₀ nanometric alloy was synthesized by ultra rapid autocatalytic chemical reduction of the corresponding transition metal ions in an aqueous solution. The ratio of metal concentration in solution is preserved in the precipitated powder alloy and no metal segregation has been detected. The alloy was characterized as a nanostructured chemically disordered taenite phase by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). Transmission electron microscopy (TEM) showed that the as prepared alloy contained spherical particles with 96 nm mean diameter size. The particles are composed of crystallites (of ∼15 nm size) and a predominant disordered interfacial region. A thermal treatment of 673 K/2 h produced a structural relaxation with a significant narrowing in the XRD and Mössbauer lines with a exothermic flow in the DSC signal and an increase in the crystallite size to 30 nm.     

Evolution of structure, microstructure and hyperfine properties of nanocrystalline Fe50Co50 powders prepared by mechanical alloying

Nanostructured Fe₅₀Co₅₀ powders were prepared by mechanical alloying of Fe and Co elements in a vario-planetary high-energy ball mill. The structural properties, morphology changes and local iron environment variations were investigated as a function of milling time (in the 0-200 h range) by means of X-ray diffraction, scanning electron microscopy (SEM), energy dispersive X-ray analysis and ⁵⁷Fe Mössbauer spectroscopy. The complete formation of bcc Fe₅₀Co₅₀ solid solution is observed after 100 h milling. As the milling time increases from 0 to 200 h, the lattice parameter decreases from 0.28655 nm for pure Fe to 0.28523 nm, the grain size decreases from 150 to 14 nm, while the meal level of strain increases from 0.0069% to 1.36%. The powder particle morphology at different stages of formation was observed by SEM. The parameters derived from the Mössbauer spectra confirm the beginning of the formation of Fe₅₀Co₅₀ phase at 43 h of milling. After 200 h of milling the average hyperfine magnetic field of 35 T suggests that a disordered bcc Fe-Co solid solution is formed.     

Role of Cr ions on superexchange coupling in α-Fe2O3 nanoparticles

Synthesis of nanocomposites of iron oxide & chromium oxide (α-Fe₂O₃–Cr₂O₃) with different concentrations was carried out by a wet-chemical method and the structural, optical and hyperfine properties have been investigated. The prepared nanocomposites were characterized by powder X-ray diffractometry (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV–VIS spectroscopy, Fourier transformed infrared (FTIR) spectroscopy and Mössbauer spectroscopy. XRD measurements confirmed the formation of pure phase composites having particle sizes in nanometer regime. The same has been corroborated by TEM micrographs, which revealed that the formation of monodispersed nanocomposites have the average particle size 44 nm. Mössbauer study of the samples showed the transition of iron oxide from anti-ferromagnetic state to paramagnetic state having a typical relaxation in the spectrum with increasing concentration of Cr₂O₃.     

High-Energy Ball Milling of NiAl(Fe) System

High-energy ball milling was used to promote the solubilization of iron into NiAl powder for an iron concentration range of 10–30 wt.%. The microstructural evolution induced by the intense mechanical deformations, under different milling conditions, was followed by X-ray diffraction and Mössbauer spectroscopy. The Mössbauer spectra are dominated by a magnetic sextet of about 33 T. Increasing the time and the speed of milling gives rise to a non-resolved doublet, having parameters typical of a NiAl compound with Fe atoms in solution. At the same time a reduction of lattice parameter occurs, which can be correlated to composition variations and partial disordering of the NiAl structure. Subsequent annealing modifies the Mössbauer spectra noticeably. In particular, the non-magnetic component becomes a broad singlet. Both diffraction analysis and Mössbauer spectroscopy indicate that a fcc Ni(Al,Fe) solid solution is forming in samples milled in agate. It is observed that the grain size of the milled products remains in the nanometric range even after thermal treatment, which adds interest to possible applications.     

High-Temperature Mössbauer Spectroscopy of Mechanically Milled NiFe2O4

Oxide spinels, in particular those containing iron, often exhibit technically important electrical- and magnetic-properties. We report here on X-ray powder diffraction and Mössbauer studies of nanostructured NiFe₂O₄ particles prepared by high-energy ball milling from bulk NiFe₂O₄, which is an inverse spinel. The Mössbauer spectra were recorded in situ at different temperatures in the range of 300–850 K. The Mössbauer spectra of the milled samples show a broad distribution of magnetic hyperfine fields together with a paramagnetic state at room temperature. Initially, at 700 K the spectrum is mainly paramagnetic, but during the process of annealing, magnetic sextets emerge. The treatment results in a significant change in the B/A area ratio of the ferrite. The Neél temperature of the samples is estimated from the B(T) relation to be in the range of 800–850 K.     

Iron oxide nanoparticles supported on ultradispersed diamond powders: Effect of the preparation procedure

The state of the iron oxide nanoparticles, supported on ultradispersed diamond (UDD) powders is studied by X-ray diffraction, nitrogen physisorption, temperature-programmed reduction, FTIR and Mössbauer spectroscopy. Methanol decomposition to hydrogen and CO is used as a catalytic test. The peculiarities of the iron oxide species strongly depend on the detonation procedure used for the UDD powders preparation as well as on the iron modification procedure.     

Changes in the hyperfine interactions in the Fe80Nb3Cu1B16 metallic glass under tensile loading

The influence of tensile stress on the changes in hyperfine parameters obtained by Mössbauer spectroscopy of the ferromagnetic amorphous NANOPERM-type Fe₈₀Nb₃Cu₁B₁₆ alloy has been investigated. The bulk changes are obtained by Mössbauer spectroscopy in a transmission geometry using γ-rays; the surface properties are studied in a scattering geometry using conversion electrons. The Mössbauer spectra are analyzed by two distributions of hyperfine inductions, which result in high-field and low-field components corresponding with a model of clusters formed by predominantly iron atoms and intermediate phase containing mainly the Nb, Cu and B atoms. The obtained results are completed by the bulk and surface magnetic measurements, with observations of surface morphology by AFM and microstructure by SEM. The investigations yield approximately linear increase of hyperfine parameters and slight deterioration of the bulk as well as surface magnetic characteristics with tensile loading.     

Magnetic properties of mechanochemically prepared iron-wüstite (Fe-FeyO) nanocomposites

In this work, iron-wüstite (Fe-Fe_yO) nanocomposites have been prepared via high-energy ball milling (HEBM), using high-purity hematite (α-Fe₂O₃) and iron (Fe) powders as the raw materials with different Fe/Fe₂O₃ mole ratios (MR)=0.6, 0.9, 1.0, 2.3, 4.9 and 13.6. X-ray diffraction studies of the as-milled powders show that a single-phase wüstite was formed for the lowest mole ratio (MR=0.6) and mixtures with MRs higher than 0.6 result in iron-wüstite nanocomposites, except for MR=13.6 that is dominantly a pure iron phase. The mean crystallite sizes of the iron and wüstite in the nanocomposites have been calculated by Scherrer's formula, which were 9±1 and 7±1 nm, respectively. Using the formula a=3.856+0.478y, for Fe_yO, where “a” is the lattice parameter of wüstite, it is possible to estimate the value of “y” for different nanocomposites and a composition of Fe_0.93O was estimated for the wüstite single phase (MR=0.6). In addition, a gradual decrease in “y” from 0.87 to 0.83 was obtained by increasing MR values from 0.9 to 4.9, respectively. The room-temperature Mössbauer spectrum of the single-phase wüstite shows considerable asymmetry due to two overlapping quadrupole doublets. For higher MRs, room-temperature Mössbauer spectra exhibit sextets, which confirm the existence of iron in the samples. The Mössbauer spectrum of the sample with the highest mole ratio (MR=13.6) shows only a sextet related to α-Fe without any detection of wüstite, which is in agreement with the XRD results. The nanosized prepared wüstite shows ferrimagnetic like behavior, which was interpreted according to spinel-like defect clusters. The M_s values obtained from VSM measurements and those calculated based on the Mössbauer data and chemical reaction are in good agreement. By increasing MR from 0.6 to 2.3, the coercivity (H_c) increases sharply to its maximum value at about MR=2.3, for which the value of Fe content is 45% and then drops off. This behavior is discussed based on α-Fe contents in the nanocomposites and percolation threshold.     

Effect of preparation and metal content on the introduction of Fe in BEA zeolite, studied by DR UV-vis, EPR and Mössbauer spectroscopy

Fe-containing SiBEA zeolites were prepared by a two-step postsynthesis method: creation of vacant T-sites by dealumination of tetraethylammonium BEA zeolite with nitric acid and then impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO₃)₃. X-ray diffraction shows that iron is incorporated in SiBEA at lattice sites. The presence of Fe in its oxidation state +3 and at isolated tetrahedral sites for low metal content, was demonstrated by diffuse reflectance UV-vis, EPR and Mössbauer spectroscopy. For high iron content, diffuse reflectance UV-vis and Mössbauer spectra revealed the additional presence of extra-lattice FeO_x oligomers and superparamagnetic Fe-oxyhydroxide. Mössbauer spectroscopy identified superparamagnetic Fe-oxyhydroxide as the main phase when basic conditions are used for the preparation.     

Synthesis of Fe:Al2O3 thin films by ion beam sputtering: Interface effect

Fe:Al₂O₃ thin films containing 20 at.% iron were deposited at room temperature on different substrates by using ion beam sputtering. Conversion electron Mössbauer spectroscopy, grazing angle X-ray diffraction and transmission electron microscopy have allowed localization of the ferrous ions at the film-substrate interfaces.     

Mössbauer spectra and magnetic properties of iron nitrides

The iron nitrides Fe₃N, Fe₄N and Fe₈N (impure) have been prepared by treating iron in NH₃/H₂ gas mixtures followed by appropriate heat treatment. Fe₈N has also been prepared by mechanical alloying. Mössbauer spectra are analysed in terms of iron sites with different nitrogen environments, and results are compared with magnetization measurements and band-structure calculations. Moments and hyperfine fields on sites with nitrogen neighbours are reduced by p-d hybridization, whereas the sites farthest from nitrogen have the lowest charge and the highest moments. There is no evidence of an exceptionally high magnetization in any of these compounds.