J. A. Tabares

Mössbauer and magnetization evidence of spin-glass phase in the Fe0.45Mn0.25Al0.30 disordered alloy

Abstract A study by Mossbauer spectroscopy and magnetization of the Fe 0.45 Mn 0.25 Al 0.30 disordered alloy is reported. The curves of magnetization versus temperature for zero-filled cooling (ZFC) and field cooling (FC) are different above 60 K. Also, this alloy showed a remanent magnetization and a temporal decrease when the field turns off. This spin-glass behavior was confirmed as a re-entrant spin-glass in the ferromagnetic phase by Mossbauer spectroscopy.

Magnetic properties of the FexMn0.600 −xAl0.400, 0.200 ≤ x ≤ 0.600, disordered alloy series

The magnetic properties of the FexMn0.600 −xAl0.400(0.200 ≤ x ≤ 0.600) disordered alloys were investigated by means of Mossbauer spectrometry, ac magnetic susceptibility, and ac calorimetry measurements. Our results are summarized in a magnetic phase diagram, which includes an antiferromagnetic phase for x ≤ 0.300 at T ≤ 225 K, a ferromagnetic one for x ≥ 0.425 at T ≤ 340 K, and a paramagnetic phase for high temperatures and all the compositions. This phase diagram also shows for the first time both a re-entrant spin-glass–ferromagnetic transition line for x ≥ 0.425 and T ≤ 60 K and a re-entrant spin-glass–antiferromagnetic one for x ≤ 0.300 and T ≤ 40 K. The occurrence of superparamagnetic-like blocked events (for 0.300 ≤ x ≤ 0.600 and at T ≈ 200 K) is associated with the presence of Fe-rich magnetic clusters which can originate from the disordered atomic character of the alloys.

Magnetic properties of FexMn0.3Al0.7-x (0.275 ⩽ x ⩽ 0.525) disordered alloys

We report on the magnetic properties of Fex Mn0.3 Al0.7-x (0.275 ≤ x ≤ 0.525) disordered alloys, which were investigated by means of Mossbauer spectroscopy, ac magnetic susceptibility and magnetization measurements. Our results are summarized in a magnetic phase diagram which includes both a re-entrant spin glass-ferromagnetic transition line for x > 0.425 and a re-entrant spin glass-antiferromagnetic transition line for x < 0.375.

Mössbauer evidence of reentrant spin-glass phase in the Fe0.45Mn0.25Al0.30 disordered alloy

Mössbauer effect measurements in disordered Fe0.45Mn0.25Al0.30 alloy from 15 to 300 K show it to be a reentrant spin-glass with a freezing temperatureTK near 60 K and a Curie temperatureTc near 250 K. The Mössbauer spectra show a paramagnetic site with a relative area that increases withT and this increase is abrupt near 60 K.

Mineral identification in Colombian coals using Mössbauer spectroscopy and X-ray diffraction

Minerals were identified in three Colombian coal samples from the Southwest of the country using Mössbauer spectroscopy and X-ray diffraction. Original and sink separated coal fractions of specific gravity 1.40 and 1.60 with particle size less than 600 µm were used in the study. Using Mössbauer spectroscopy, the minerals identified in the original coal samples were pyrite jarosite, ankerite, illite and ferrous sulfate, whereas by means of X-ray diffraction, minerals identified were kaolinite, quartz, pyrite, and jarosite. Differences in mineral composition were found in the original and sink separated fractions using both techniques. Mössbauer spectra show that the mineral phases in low concentrations such as illite, ankerite and ferrous sulfate do not always appear in the spectra of sink coals, despite of those minerals occurring in the original coal, due to the fact that they are associated with the organic matter and not liberated in the grinding process. X-ray results show that the peak intensity grows as the specific gravity is increased indicating that the density separation method could be an effective process to clean coal.

Mössbauer and XRD Characterization of the Mineral Matter of Coal from the Guachinte Mine in Colombia

The aim of this work was the characterization and differentiation, using Mössbauer spectroscopy (MS) and X-ray diffraction (XRD), of coal samples with different ash and sulfur contents obtained in three places corresponding at cuts in different seams from the Guachinte mine, Valle, Colombia. The mineral phases identified by XRD were in general kaolinite, quartz, pyrite and gypsum, and in particular dolomite and calcite. MS confirms the presence of pyrite and kaolinite, besides evidences the additional presence of jarosite which was not detected by the XRD results due their low amounts. In the high mineral matter ash sample quartz and hematite was identified by XRD, the last one confirmed by MS results. A second phase in this sample was detected by Mössbauer spectroscopy, which could be superparamagnetic hematite. Rietveld refinement for XRD pattern from a sample is reported.

Quantification of Pyritic Sulfur of One Colombian Coal by Mössbauer Spectroscopy

The aim of this work was to identify and quantify by means of Mössbauer spectroscopy the amount of pyritic sulfur in coal samples of the Guachinte mine, Valle, Colombia. For the quantification a calibration curve for pyritic sulfur content vs. relative spectral area ratio of sulfur and pure (99.99%) Fe-powder was obtained, using Mössbauer spectroscopy. The samples used in the calibration were the representative ones of the fractions obtained after one and two hydrocyclonic removal processes of a homogeneous sample of raw coal. A linear relationship was obtained and used to determine the amount of pyritic sulfur of the original coal.

Mössbauer study of Fe-Al disordered alloys near the critical concentration

Disordered bcc Fe1−qAlq alloys in the composition range 0.5≤q≤0.6 were studied by Mössbauer effect measurements. The Mössbauer spectra at 300 K of all the samples consist of two paramagnetic sites, one is a singlet and the other a doublet with quadrupole splitting. The results can be interpreted by considering that the sites of this disordered system are arranged near the configurations of the Fe and Al sites of the Fe-Al ordered system.

Spin glass behavior in some Fe-Mn-Al disordered alloys

In this work, we report an ac susceptibility study of Fe0.5Mn0.2Al0.3, Fe0.5Mn0.1Al0.4, and Fe0.225Mn0.375Al0.40 disordered alloys. All the samples exhibit only one well defined peak near 58, 18, and 31 K, respectively. When the frequency of the ac field increases, the measured intensities of the peaks decrease and the maxima shift to higher temperatures, and when the dc field increases the intensity of the peaks decreases. This dependence of the position and intensity of the peaks on the field and frequency is evidence of the spin glass character of the transitions in these alloys. Mossbauer spectra at different temperatures were performed for the sample with 30 at. % Al and the plot of the adjusted mean hyperfine field versus temperature shows a kink at 53 K which is further evidence that this sample shows spin glass behavior.

A CEMS STUDY OF SURFACE OXIDATION OF FE-NI ALLOYS

A study by Conversion Electron Mössbauer Spectroscopy (CEMS) carried out by using a Parallel Plate Avalanche Counter with samples of Fe--Ni alloys (50 and 65 at.% Fe) is reported. Each sample was analyzed without oxidation and after heating it under an oxygen atmosphere at 200°C. All CEMS measurements were carried out at room temperature. In both samples (50 and 65 at.% Fe), without oxidation and after oxidation, the Mössbauer spectra showed a six line magnetic spectrum according to their ferromagnetic character, with a broad Hyperfine Field Distribution (HFD), according to the disordered character of the alloys. The obtained Mean Hyperfine Field (MHF) for the sample 50 at.% Fe was 30.9 T, meanwhile for the invar composition (65 at.% Fe) was 25.5 T, which is close to values previously reported by Transmission Mössbauer Spectroscopy (TMS). Results from the treated samples (with oxidation at 200°C) showed a difference in the surface composition as a result of this process. In the 50 at.% Fe sample, additionally appeared a doublet that could be assigned to an oxihydroxide of Fe3+. Otherwise, the 65 at.% Fe sample (invar) presented ferromagnetic oxides (α-Fe2O3 and Fe3O4) with a large relative area (82.5%).