M. Jiménez

NANOCRYSTALLIZATION STUDIES IN CO-RICH AMORPHOUS-ALLOYS

Abstract Nanocrystallization from Vitrovac® amorphous ribbon was studied by Mossbauer spectroscopy, transmission electron microscopy and differential scanning calorimetry. Results are interpreted in terms of a crystallization of Co, Co 2 B, and Co 3 B, leading to an increase in Fe content in the residual amorphous phase, which in turn results in an increase in hyperfine field and Curie temperature.

Site occupancy and Tc degradation in iron substituted YBa2(Cu1−xFex)4O8+δ

Abstract A series of YBa2(Cu1−xFex)4O8+δ samples with x=0.00625, 0.0125, 0.01875, 0.025 and 0.05, were synthesized and characterized by resistance vs. temperature measurements, X-ray diffraction and by Mossbauer spectroscopy. In order to make the site assignment for the Fe atoms, a Rietveld refinement of the X-ray spectra was performed and a point charge calculation of the quadrupole splitting for all the possible oxygen environments around the two Cu sites was carried out, taking into account the two possible ionic states of the iron atoms that occupy those sites. We found that, for low level iron concentrations, the Fe atoms occupy only the Cu(1) sites of the structure in a fivefold pyramidal coordination, with the apical oxygen atom placed along the a-axis between the double chains of the unit cell. The presence of these extra oxygen atoms in the crystal structure could be related to the rapid Tc reduction as iron concentration increases.

Temperature evolution of the internal magnetic hyperfine field of Metglas: amorphous and crystallized phases

We present di!erential scanning calorimetry (DSC), X-ray di!ractometry, in situ MoK ssbauer spectroscopy (MS), and transmission electron microscopy (TEM) studies in Metglas ribbons subjected to di!erent heat treatments. The temperature evolution of the hyperne eld H )& („) and the Curie temperature („ # ) of the amorphous phase are determined. The magnetic eld originally present in the amorphous phase has a &normal’ behavior, in the sense that it can be described by the Weiss molecular eld theory. The total angular momentum of the iron atoms turns out to be 5/2 and this implies Fe3‘ in which the electronic spins are uncoupled. When the samples are maintained near „ # (+673 K), three new magnetic phases are detected in the MoK ssbauer spectra, indicating an onset of a crystallization process well below the rst crystallization temperature („ X1 ), as determined by DSC (+820 K). The magnetic behavior of these phases is also described by universal Brillouin curves with J 1 3 , J 2 5 and J 3 5 . A pre-identication of the crystalline phases is made through the MoK ssbauer parameters of the three observed magnetic phases, and the nal characterization by X-ray and electron di!raction. Two of the three phases were identied as a-FeSi and Fe 2 B. The third phase could be the cubic phase of Fe 3 Si or a solid solution of FeSi. The size of the nanocrystals formed in a sample heated at 673 K during di!erent time intervals were determined by TEM. Even though the number of the nanocrystals increases with time, their average size remains the same. ( 2000 Elsevier Science B.V. All rights reserved.

Site occupancy of Ga3+ in NiGaxFe2-xO4 ferrites

Abstract Mossbauer spectra of the NiGa x Fe 2- x O 4 system were obtained at 11 K and the ratio of line intensities were determined. Results show that a high concentration Ga 3+ does not enter completely in A-sites. The lattice constant decreases linearly with the mean ionic tetrahedral radius. The Curie temperature decreases smoothly as x increases.

Néel temperature and initial susceptibility quenched nickel ferrite

Abstract The Neel temperature T N of nickel ferrite quenched from a temperature T q is a minimum, T N = 568°C for T q = 1000°C, and nearly the same value (587°C) for both extremes, the slowly cooled sample and the T q = 1180°C quenched sample. All the quenched samples regained the Neel temperature of the slowly cooled sample by annealing.

Amorphous to Crystalline Phase Transformation in Metglas® Studied by Mössbauer Spectroscopy

The authors present differential scanning calorimetry (DSC) and in situ Moessbauer spectroscopy results for Metglas ribbons, to which different heat treatments were made. The Curie temperature of the amorphous phase is determined and the evolution of the magnetic field of this phase is studied as a function of temperature.

Magnetic Softening and Hardening in Metglas® 2605SC at the Onset of Nanocrystallization

Moessbauer spectroscopy, coercive field and permeability were used to study the transformations produced by annealing at 375 C as-quenched amorphous ribbons of Metglas{reg_sign} 2605 SC of nominal composition Fe{sub 81}B{sub 13.5}Si{sub 3.5}C{sub 2}. The coercive field showed a decrease (softening) in the 0--30 min range, followed by an increase (hardening) for annealing times above 30 min. Permeability exhibited the opposite behavior, with a maximum for about 30 min. The Moessbauer magnetic hyperfine spectra showed variations in the intensities ratios, which were related with macroscopic measurements in good agreement with the softening-hardening processes.