Abstract A preliminary study of the magnetic relaxation in (Bi, Pb)-2223 superconducting ceramics doped with ®-Al2O3 nanoparticles is presented, taking as starting point the measurements of the M(t) dependence, both in samples in the form of powder as in pellet. The relaxation of the three types of vortices that can exist inside these materials is analyzed, emphasizing on the intragranular region where the planar defects exist and, consequently, Abrikosov-Josephson vortices emerge. Finally, interesting results of the comparison between the behavior of the samples in the form of powder and in the form of pellet are shown from the estimation of the pinning energy of the vortices.

Amostras policristalinas de La5/8-yPr yCa3/8MnO3, 0 <; y <; 0,625, foram produzidas pelo método da mistura estequiométrica de óxidos e caracterizadas através de medidas resistividade elétrica ro(T, H) como função da temperatura T e do campo magnético aplicado H. Todas as amostras apresentam uma transição de fase do tipo metal-isolante T MI e aquelas com y > 0,35 também mostram uma transição para o estado de ordenamento de carga/orbital T OC. Uma análise das medidas de ro(T, H) sugere a coexistência de pelo menos duas fases distintas nesses materiais: uma ferromagnética-metálica FMM e uma segunda, com ordenamento de carga/orbital e comportamento isolante OCI. Os resultados também permitem concluir que as frações volumétricas dessas fases podem ser manipuladas, alterando-se parâmetros termodinâmicos como T e H. A magnetorresistência MR(T), determinada através das medidas de ro(T, H), foi observada ocorrer em uma larga faixa de temperatura e em todas as amostras pertencentes à série. Amostras ricas em Pr (y > 0,35) revelam um efeito de magnetorresistência colossal amplificado devido à coexistência e competição das FMM e OCI cujas frações volumétricas podem ser alteradas via mudança da temperatura e aplicação de um campo magnético externo.

Polycrystalline samples of La5/8-yPr yCa3/8MnO3; 0 <; y <; 0.625; were synthesized by the solid-state reaction method and studied by several techniques including electrical resistivity rho(T, H) measurements. The occurrence of an insulator-to-metal transition was observed for all samples at a temperature T MI, and for samples with y > 0.35 a transition to a charge and orbital-ordered state was also observed. The rho(T, H) data analyses suggest the coexistence of at least two ordered phases in these samples: a ferromagnetic metallic phase FMM and a charge and orbital-ordered insulating COI phase.The data also indicate that the volume fraction of these phases can be changed by the variation of thermodynamic parameters like temperature T and magnetic field H. The magnetoresistance MR(T) determined from rho(T, H) data were observed in a large temperature range for all samples. In addition, samples with y > 0.35 were found to display a pronounced colossal magnetoresistance effect due to a severe competition between FMM and COI coexisting phases.

We report on the application of the Resistively Shunted Junction (RSJ) model to granular high T C superconductors. Some derived predictions of the RSJ model are applied to a set of superconducting granular samples which can be considered as a network of Josephson junctions. The investigated samples belong to both hole-doped Y1-xPr xBa2Cu3O7-delta (x = 0.0, 0.35, and 0.45) and the electron-doped Sm2-xCe xCuO4-delta (x = 0.18) systems which display the so-called double resistive superconducting transition. We have performed several transport measurements in these compounds including temperature and magnetic field dependence of the electrical resistance, R(T,H), and I-V characteristics. Several aspects of the I-V characteristics were quantitatively well described by the RSJ model. The combined results strongly suggest that dissipation in granular superconducting samples is a natural consequence of the normal current flowing in parallel with the supercurrent current.

We have measured transport properties at low temperatures for the granular Sm1.82Ce0.18CuO4-delta samples belonging to the family of electron-doped superconductors. The effect of applied electrical current on the resistive behavior is investigated. The experimental data are analyzed using a modified form of the theory for a field-tuned superconductor-insulator transition in 2D superconductors. The results suggest a possible electrical-current driven superconductor-insulator transition.

We have studied the effects of the uniaxial compacting pressure on the physical properties of polycrystalline Bi1.65Pb0.35Sr2Ca2Cu3O10+delta (Bi-2223) superconductors. Powders of this material were pressed at different uniaxial compacting pressures ranging from ~ 90 to ~ 600 MPa and heat-treated at the same temperature. A characterization of samples by using Scanning Electron Microscopy and X-ray diffractometry indicated an appreciable improvement of the degree of texture with increasing pressure. The temperature dependence of the electrical resistivity rho(T) exhibits a T-linear behavior at temperatures higher than T<FONT FACE=Symbol>*</FONT> ~ 235 K. The deviation of rho(T) from the linear behavior below T<FONT FACE=Symbol>*</FONT> indicates the opening of the pseudogap, a feature confirmed by magnetic susceptibility measurements performed in powder samples. From linear fittings of the normal-state electrical resistivity we were able to separate contributions to rho(T) arising from both the grain misalignment and microstructural defects. The results suggest that the grain orientation and the connectivity between them are improved with increasing compacting pressure. Also, based on the linearity of the electrical resistivity data both the transport electron-phonon coupling constant, lambdatr, and the mean free path, l, were estimated. We have found that in the sample with the highest degree of texturelambdatr ~ 0.53, a value comparable with the one obtained in Bi-2223 single crystals. However, the result for l ~ 12.7 Å at 300 K, in the same ceramic sample, is close to 3 times lower than the single crystal value. The influence of the intergranular electrical resistivity in determining band-theory parameters was analyzed within the framework of a current conduction model for granular superconducting materials.

We have performed a systematic study of the structural, transport, and magnetic properties of polycrystalline samples of the magnetic superconductors Ru1-xIr xSr2GdCu2O8; x = 0, 0.02, 0.05, 0.10, and 0.15. The samples were prepared by solid state reaction and sintered at 1060 ºC for 72 h under O2 flow. X-ray diffraction analysis shows that all samples are nearly single phase and that the lattice parameters are independent of Ir content. Transport properties measurements revealed that the Ru substitution by Ir results in a decrease of Tc,onset from ~50 K (x = 0) to ~30 K (x = 0.10). Further addition of Ir (x > 0.10) causes an evolution from metallic to nonmetallic behavior of r(T). We have also found that magnetic order develops in the undoped Ru-1212 materials near T M ~ 130 K. This temperature decreases linearly with increasing Ir content at the rate of ~ -1.6 K / Ir at.%, suggesting that Ir effectively substitutes Ru in the RuO planes. A subtle drop in r(T) is observed close to T M, probably due to the suppression of the spin-flip scattering. The magnetoresistivity measurements revealed that the temperature Tc,zero, in which r(T) ~ 0, decreases rapidly for low applied magnetic fields (H < 2 T), and that this drop becomes much less pronounced in higher magnetic fields (2 < H < 18 T). The appreciable broadening of r(T) curves at low magnetic fields is reminiscent of the behavior in high-Tc materials showing granular behavior, as for example in Sm1.85Ce0.15CuO4-y.

Polycrystalline samples of PrBa2Cu3O7– y were prepared through two different routes: (a) mixing Pr6O11, BaCO3, and CuO; (b) sol-gel precursors. The samples were heat-treated in the temperature range 700 <= T <= 930 ºC and under different atmospheres, (Argon and air). Oxidation of these compounds, based on the results of thermogravimetric analysis, TGA, was performed at low temperatures, from 300 to 400 ºC, for time intervals as long as 120 h. The crystal structure was determined by measurements of X-ray powder diffraction and revealed that samples heat-treated at 810 ºC in Ar and for 130 h have a tetragonal structure with lattice parameters c <FONT FACE=Symbol>~</FONT> 11.88 Å. Oxygenated PrBa2Cu3O7-y samples were found to have an orthorhombic structure with lattice parameter c ranging from 11.72 to 11.75 Å. Electrical resistivity rho(T) measurements performed on these compounds revealed a semiconductor-like behavior in the temperature range 77 <= T <= 300 K. The magnetic susceptibility chi(T) data showed a paramagnetic-like behavior from 5 to 300 K and a clear contribution of the crystalline electrical field (CEF) below 150 K. We have also found an antiferromagnetic ordering associated with the Pr sub-lattice at T N <FONT FACE=Symbol>~</FONT> 17 K. Evidence of super-conductivity was not observed on our samples, in agreement with most of the results reported in the literature.