R. Ridaura

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.

Structural transitions in a TiNiFe shape memory alloy

Results for resistance versus temperature, differential scanning calorimetry (DSC), Moessbauer spectroscopy, and magnetic susceptibility measurements in a Ti[sub 50]Ni[sub 47]Fe[sub 3] sample are presented. The resistance versus temperature curve shows a sharp increase at 245K and then rises slowly until a maximum is attained at [approx]150K. The DSC curve also exhibits a peak at the former temperature. The room temperature Moessbauer spectrum consists of a single line. As the temperature is lowered to 270K, a small asymmetric quadrupole doublet appears. The isomer shift and quadrupole splitting values of this doublet increase with decreasing temperature until almost constant values are achieved at temperatures [approx]100K. The magnetic susceptibility curve shows a sharp decrease that begins at 255K and then an unexpected increase at 90K. The authors relate these observations with a charge density wave-driven premartensitic transition in this system, and with the final martensitic phase.

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.

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.

About the ionic state of iron in the Cu sites of the Nd2−xCexCu2−yFeyO4−δ superconductor

Abstract The substitution of a small fraction of Cu atoms by Fe atoms in the copper oxide superconductors has proven to be useful in sensing, with Mossbauer spectroscopy, the local surroundings of the Cu sites of their structures. In this work we report the results of such substitution in the Nd 1.85 Ce 0.15 Cu 2 O 4−δ system. After annealing in argon at high pressure and temperature, a strong quadrupole doublet is observed indicating that the iron atoms have been incorporated into the superconductor structure.

Iron substitution in the Cu sites of the YBa2Cu4−xFexOδ structure studied by Mössbauer spectroscopy

Abstract The site distribution of 57 Fe in a series of YBa 2 Cu 4−x Fe x O δ superconducting samples ( x = 0.025, 0.05 and 0.075) was determined bu Mossbauer spectroscopy. Resistance vs. temperature studies were also performed in order to determined the influence that increasing concentration of Fe has on T c .

Mössbauer studies on magnetic and electrical properties of La1.85Sr0.15Cu0.99Fe0.01O4−δ☆

Abstract The local magnetic and electric fields in the CuO planes of Fe doped and Sr- doped La 2 Cu 1 O 4 superconductor can be sensed, using Mossbauer spectroscopy, if a small fraction of the Cu atoms is substituted for Fe atoms. In this work we report the results of such substitution in the superconducting La 1.85 Sr 0.15 Cu 0.99 Fe 0.01 O 4−δ system. Mossbauer spectra were obtained at different temperatures ranging from 13 to 300 K. The observed spectra consist of two quadrupole doublets, associates with two inequivalent sites. One site may be associate with a narrow range of x centered at x = 0.15 and the other site with x = 0.

LOW TEMPERATURE MAGNETIC BEHAVIOR OF NI-FE-AL-B SHAPE MEMORY ALLOYS - MAGNETIC SUSCEPTIBILITY AND MOSSBAUER SPECTRA

Magnetic susceptibility measurements and Mossbauer spectra (MS) were performed on a series of Ni x Fe y Al z B shape memory ribbons obtained by a melt spinning technique. The MS at room temperature show quadrupole splittings of two iron sites. At low temperatures (T