Cu doping as a tool for understanding CMR

in Key Engineering Materials [=KEM] (2002), 206-2

Doping at the Mn-site in CMR manganate-based perovskites has been shown to affect strongly the physical properties of those compounds. We study here the effect of copper substitution in the low doping ... [more ▼]

Doping at the Mn-site in CMR manganate-based perovskites has been shown to affect strongly the physical properties of those compounds. We study here the effect of copper substitution in the low doping range on the electrical transport properties of La0.7Ca0.3MnO3. It turns out that the transition temperature decrease observed in doped samples can be drastically reduced by addition of silicon dioxide SiO2. It is shown that copper is trapped in a secondary phase composed of La,Ca,Si,Cu and O. The resultant Mn-site vacancies appear to be less detrimental to the electronic conduction than the likely antiferromagnetic clusters induced by the copper ions in the Mn-O network. [less ▲]

YBa2Cu3O7 tapes prepared by sol-gel deposition techniques: microstructure and structural characterizations

in Physica C: Superconductivity (2002), 372(Part 2), 715-718

We report on the preparation conditions of YBa2Cu3O7 polycrystalline superconducting tapes by a sol-gel deposition technique. We present some discussion on the compatibility between the nature of the ... [more ▼]

We report on the preparation conditions of YBa2Cu3O7 polycrystalline superconducting tapes by a sol-gel deposition technique. We present some discussion on the compatibility between the nature of the substrate, the use of a buffer layer, and the conditions used to prepare appropriate superconducting YBa2Cu3O7,, materials. We report also on the microstructural characterizations performed in order to evaluate the crystallites size, degree of orientation and connectivity. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

Magnetic properties of melt-textured DyBCO single domains

in Materials Research Society Symposium Proceedings (2001), 659

This communication aims at reporting the superconducting properties of several bulk melt-processed DyBa2Cu3O7-x samples determined by using various measuring techniques. The original single domain ... [more ▼]

This communication aims at reporting the superconducting properties of several bulk melt-processed DyBa2Cu3O7-x samples determined by using various measuring techniques. The original single domain material was first cut into 9 similar cubic samples which have been characterized by AC susceptibility, flux profiles and DC magnetization. The results indicate good quality melt-processed (RE)BCO materials with Tc = 89 K. Next, the effect of post oxygen annealing treatments on these samples was studied. The influence of the non-uniformity of the oxygen content on the magnetic property anisotropy is discussed. Finally, additional magnetic measurements were carried out by using a couple of parallel pick-up coils wound on the same sample. From the results we conclude that geometric effects have to be taken into account in order to extract the critical current density from the magnetic properties. [less ▲]

Magnetotransport properties of a single grain boundary in a bulk La-Ca-Mn-O material

in Journal of Applied Physics (2001), 90(11), 5692-5697

Besides the "intrinsic" colossal magnetoresistance effect observed in single crystals, the polycrystalline manganate compounds also exhibit an "extrinsic" magnetoresistance related to the presence of ... [more ▼]

Besides the "intrinsic" colossal magnetoresistance effect observed in single crystals, the polycrystalline manganate compounds also exhibit an "extrinsic" magnetoresistance related to the presence of grain boundaries. We report electrical transport and magnetic measurements carried out on a bigrain sample extracted from a floating zone method-grown rod of calcium doped lanthanum manganate. Electrical resistance was measured both within a grain and across the grain boundary, between 20 and 300 K and from 0 to 8 T. Magnetoresistance values up to 99% are reached within the grain. The temperature dependence of the resistance across the grain boundary displays a "foot-like" feature towards the bottom of the transition. Low field and high field magnetoresistance effects are examined. We compare our results for a "bulk" grain boundary to those obtained by other authors for bicrystal thin films and bulk polycrystalline materials. (C) 2001 American Institute of Physics. [less ▲]

Two peak effect in GMR : a chemical effect ?

in Materials Research Society Symposium Proceedings (2000), 602

We show that the synthesis conditions have a dramatic influence on the resistivity behavior of calcium and sodium doped LaMnO3. Several samples prepared by lowtemperature techniques exhibit a double ... [more ▼]

We show that the synthesis conditions have a dramatic influence on the resistivity behavior of calcium and sodium doped LaMnO3. Several samples prepared by lowtemperature techniques exhibit a double-peaked curve of resistance versus temperature. The model of spin-polarized intergranular tunneling provides a good approach to discuss our experimental results. Grain size and crystallinity are proved to be essential parameters, which are strongly influenced by the preparation process (e.g. the precursors nature and the thermal treatment). [less ▲]

A new fast and non-destructive method for evaluating the superconducting properties of bulk materials

in Obradors, X.; Sandiumenge, F.; Fontcuberta, J. (Eds.) Applied Superconductivity 1999. Volume 1 : Large Scale Applications (1999)

This paper aims at reporting a new, simple and low-cost experimental technique designed for investigating the electrical properties of disk-shaped superconducting samples. The principle is similar to that ... [more ▼]

This paper aims at reporting a new, simple and low-cost experimental technique designed for investigating the electrical properties of disk-shaped superconducting samples. The principle is similar to that of an induction motor. Two neighbouring and 90° out-of-phase AC magnetic fields are applied perpendicularly to the disk to be measured. As a result, the material experiences an electromagnetic torque which depends on its electrical properties. In this paper the experimental system is briefly described and illustrative measurements on different HTS are reported and discussed. [less ▲]

Transport properties of bulk-bicrystal grain boundaries in artificially joined large-grain YBCO

in IEEE Transactions on Applied Superconductivity (1999), 9(2), 2038-2041

A technique for joining large-grain YBCO has been used to produce bulk-bicrystal grain boundaries of different orientations. The behaviour of boundaries nominally of 0 degrees[100], 0 degrees[001] and ... [more ▼]

A technique for joining large-grain YBCO has been used to produce bulk-bicrystal grain boundaries of different orientations. The behaviour of boundaries nominally of 0 degrees[100], 0 degrees[001] and asymmetric 15 degrees[100]-tilt misorientation have been investigated using low frequency transport measurements in fields up to 7T. The boundaries exhibit a metallic normal state and a superconducting transition which broadens with increasing field having an irreversibility line which closely matches that of the adjoining grains. This same qualitative behaviour is seen in all the samples measured for field applied parallel and perpendicular to the c-axis and up to the highest current densities employed (20 Acm(-2)). Variation between the different samples due to microstructural differences are discussed. We interpret these results as strong suggestive evidence that this joining technique can be used to produce strongly-coupled large-grains for bulk-scale engineering applications. [less ▲]

High field behavior of artificially engineered boundaries in melt-processed YBa2Cu3O7-delta

in Applied Physics Letters (1998), 73(1), 117-119

Artificial bulk "zero-angle" boundaries parallel to the c axis have been engineered between large melt-processed YBa2CU3O7-delta (YBCO) grains and observed to carry a transport supercurrent at fields up ... [more ▼]

Artificial bulk "zero-angle" boundaries parallel to the c axis have been engineered between large melt-processed YBa2CU3O7-delta (YBCO) grains and observed to carry a transport supercurrent at fields up to at least 5 T at 77 K. The temperature and angular dependencies of the boundary resistance have exactly the same form as those of the grains, which is evidence that the grains are intimately coupled. The limiting mechanism for current transfer across these boundaries is, therefore, not a simple weak link or Josephson effect. This joining technique is extremely promising for production of macroscopic engineering artifacts. (C) 1998 American Institute of Physics. [less ▲]

AC susceptibility data on Dy2O3 seeded randomly oriented Dy-123 mono domains melt-textured superconductor

in Materials Science & Engineering : B (1998), 53(1-2), 198-202

Dy-123 superconducting ceramics have been grows using small Dy2O3 single-crystals as seeds. Randomly oriented large mono-domains have been observed in the vicinity of the seeding single crystal. These ... [more ▼]

Dy-123 superconducting ceramics have been grows using small Dy2O3 single-crystals as seeds. Randomly oriented large mono-domains have been observed in the vicinity of the seeding single crystal. These domains are one order of magnitude greater in volume than the seed. The AC susceptibility versus temperature curves have been measured in a polycrystalline cubic sample for different orientations of the magnetic field with respect to the sample and for amplitudes up to 20 G. Three measuring coils, each parallel to one side of the sample, were used to investigate the anisotropy of the properties. Electrical resistance versus temperature and flux profiles at 77 K were also explored. The results show good grain connectivity. A non negligible part of the magnetic shielding is due to currents flowing inside the grains. (C) 1998 Elsevier Science S.A. All rights reserved. [less ▲]

Magnetic flux penetration and creep in BSSCO-2223 composite ceramics

in Superconductor Science and Technology (1998), 11(1), 94-100

We have experimentally investigated the magnetic flux penetration through a Bi-2223 polycrystalline superconductor synthesized by a classical solid-state reaction method. Electrical resistance, AC ... [more ▼]

We have experimentally investigated the magnetic flux penetration through a Bi-2223 polycrystalline superconductor synthesized by a classical solid-state reaction method. Electrical resistance, AC susceptibility, the Campbell method and magnetic flux waveform recordings have been analysed and compared in order to separate clearly intergrain and intragrain contributions. The AC susceptibility frequency dependence has been also examined at T = 77 K in a broad field range (0.01 G < B-AC < 100 G) The activation energy as a function of AC applied magnetic field is found to present a pronounced minimum for an induction (8 G) corresponding to full magnetic flux penetration through the intergranular matrix. [less ▲]