S. Aburto

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.

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.

Magnetic sublattices in nickel ferrite

Abstract A definition of the concept of “magnetic sublattice” is proposed by the following three steps, i.e.,i) the “site sublattice”, ii) the “cationic sublattice” and iii) the “magnetic sublattice”. The most important feature of this concept is the condition of long range order in the occupancy of the cation sites, including the direction of the magnetic moment. These ideas are applied to the case of nickel ferrite. Under the assumption that an ordered configuration on B sites (alternate rows of Ni and Fe cations) is obtained by slow cooling of samples, an estimation of the Curie temperature of the disordered (quenched) configuration is given. The calculated value (826 K) is in good agreement with the experimental value (838 K).