A. Bohórquez

Rationalisation of defect structure of tin- and titanium-doped α-Fe2O3 using interatomic potential calculations

Two recently proposed models for the defect structure of tin-doped α-Fe2O3 have been assessed using interatomic potential calculations. The results show that a structure involving tin (or titanium) partially substituting at the octahedral iron sites as well as partially occupying the empty interstitial octahedral sites in corundum-related α-Fe2O3 is more favourable than an alternative model in which the tin (or titanium) ions only occupy the empty interstitial sites.

Magnetic properties of the FexMn0.70−xAl0.30 (0.40⩽x⩽0.58) alloy series

The magnetic properties of the FexMn0.7−xAl0.30 (0.40⩽x⩽0.58) alloy series have been investigated by means of 57Fe Mossbauer spectroscopy and ac magnetic susceptibility measurements. From our data, we propose a magnetic phase diagram for the system which is adequately reproduced by a diluted and random-bond Ising model which was analyzed on the basis of the mean-field renormalization-group method.

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 phase diagram of the FexMn0.6−xAl0.40 alloys series

This work shows preliminary results on the study of Fe x Mn0.6−x Al0.40 alloys. It includes measurements of the temperature dependence of the acsusceptibility, zero field cooled magnetization, and Mossbauer spectroscopy. From the results obtained, the occurrence of reentrant spin glass behavior from a spin glass phase to an antiferromagnetic one for 0.2≤x≤0.35 and from a spin glass phase to a ferromagnetic one for the x=0.55 sample is proposed. Also, a normal spin glasstransition was detected in the Fe range corresponding to 0.35≤x≤0.5. These results allow us to construct a magnetic phase diagram for the alloy series.

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.

Description in a local model of the magnetic field distributions of Fe1−xNix disordered alloys

A local model based on the idea of atoms distributed within a crystalline lattice and following a binomial probability method for describing the hyperfine field distributions obtained from the room temperature Mossbauer spectra of the disordered Fe–Ni system is presented. This model allows one to calculate the mean hyperfine field for each concentration from 5 to 95 at. % Ni, to determine what are the most probable sites and to interpret why the mean hyperfine field initially increases and then decreases with the Ni concentration in the BCC phase. The results are in good agreement with the experimental results except for Invar composition and for 85 at. % Ni which demand a different treatment.

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.

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.