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The physical properties of the La0.6Y0.1Ca0.3MnO3 compound have been investigated, focusing on the magnetoresistance phenomenon studied by both dc and ac electrical transport measurements. X-ray diffraction and scanning electron microscopy analysis of ceramic samples prepared by the sol–gel method revealed that specimens are single phase and have average grain size of ∼0.5 μm. Magnetization and 4-probe dc electrical resistivity ρ(T,H) experiments showed that a ferromagnetic transition at TC ∼ 170 K is closely related to a metal-insulator (MI) transition occurring at essentially the same temperature TMI. The magnetoresistance effect was found to be more pronounced at low applied fields (H ≤ 2.5 T) and temperatures close to the MI transition. The ac electrical transport was investigated by impedance spectroscopy Z(f,T,H) under applied magnetic field H up to 1 T. The Z(f,T,H) data exhibited two well-defined relaxation processes that exhibit different behaviors depending on the temperature and applied magnetic field. Pronounced effects were observed close to TC and were associated with the coexistence of clusters with different electronic and magnetic properties. In addition, the appreciable decrease of the electrical permittivity e′(T,H) is consistent with changes in the concentration of eg mobile holes, a feature much more pronounced close to TC.

A detailed study of the effect caused by the partial substitution of Ru by Ir on the magnetic and superconducting properties of the ruthenocuprate Ru(1-x)Ir(x)Sr2GdCu2o8; 0 = 2 K up to the highest applied magnetic field of 18 T. The resistive upper-critical field Hc2 as a function of temperature has been determined for these polycrystalline samples. Values of Hc2(0) were found to be \~ 52 T, and weakly dependent on the Ir concentration. We have also observed that the superconducting transition width decreases and the slope of the resistive transition increases with increasing Ir doping, a feature which is much more pronounced at high applied magnetic fields. The double-peak structure observed in the derivative of the resistive curves has been related to an inhomogeneous nature of the physical grains which is enhanced due to the Ru substitution by Ir. This indicates that the Josephson-junction-array (JJA) model seems to be appropriated to describe the superconducting state in these ruthenocuprates. The low temperature rho(T) data along with the determined vortex thermal activation energy are consistent with a 2D vortex dynamics in these materials., 26 pages, 7 figures, 2 tables

In this work we present a theoretical study on the magnetic field dependence of the electrical resistance R (Ba) and the transport noise (TN) in a high-Tc polycrystalline superconductors. In the model, we have considered the ceramic superconductor as a series-parallel array of Josephson devices and the intergranular magnetic field is described within the framework of the intragranular flux-trapping model. The obtained results qualitatively reproduce the hysteretic behavior of the R (Ba) dependence in increasing and decreasing applied magnetic fields. We have found that the hysteretic behavior in the R (Ba) dependence changes appreciably if different statistical distributions of the geometric factors of grains are used. In addition, such changes are also reflected in the TN, which is produced by the electric current rearrangement in the array with increasing applied magnetic fields. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have performed a systematic study of the general physical properties of (Bi,Pb) $$_2$$ Sr $$_2$$ Ca $$_2$$ Cu $$_{10+y}$$ (Bi-2223) powders that were ball milled for time intervals of up to $$t_{\mathrm{m}}\,=\,$$ 210 min. X-ray powder diffraction diagrams indicate that the phase composition of the samples seems to be preserved after the ball milling process, but the average grain size of the samples decreases appreciably with increasing milling time $$t_{\mathrm{m}}$$ . For $$t_{\mathrm{m}}\,=\,$$ 120 min, the transmission electron microscopy (TEM) images revealed the presence of Bi-2223 grains in the form of nanorods with a mean diameter of $$d_{\mathrm{g}} \sim$$ 20 nm. The temperature dependence of the magnetic susceptibility, $$\chi (T)$$ , showed the occurrence of superconductivity below $$T_{\mathrm{c}} \sim$$ 108 K in all samples studied and a progressive decrease of the diamagnetic contribution to $$\chi (T)$$ with increasing $$t_{\mathrm{m}}$$ . Such a decrease in the diamagnetic contribution to $$\chi (T)$$ is accompanied by an increase of a paramagnetic contribution that has its origin in a disordered shell of the milled grains. Increasing $$t_{\mathrm{m}}$$ results in a progressive increase of the width of disordered shell further increasing the paramagnetic contribution to $$\chi (T)$$ that shows a very small diamagnetic component in samples subjected to large milling times. We have also found that the occurrence of oxygen vacancies in the disordered shell is responsible for a ferromagnetic contribution to $$\chi (T)$$ at high temperatures. Such a shell–core morphology of the milled grains, comprising a superconducting and diamagnetic core and a disordered and paramagnetic shell, is sufficient for interpreting the magnetic properties of the samples.

16 pages, 9 figures, 3 tables., Polycrystalline Nd1-xEuxNiO3 (0≤x≤0.5) compounds were synthesized in order to investigate the character of the metal-insulator (MI) phase transition in this series. Samples were prepared through the sol-gel route and subjected to heat treatments at ~1000 °C under oxygen pressures as high as 80 bar. X-ray diffraction (XRD) and neutron powder diffraction (NPD), electrical resistivity ρ(T), and magnetization M(T) measurements were performed on these compounds. The NPD and XRD results indicated that the samples crystallize in an orthorhombic distorted perovskite structure, space group Pbnm. The analysis of the structural parameters revealed a sudden and small expansion of ~0.2% of the unit cell volume when electronic localization occurs. This expansion was attributed to a small increase of ~0.003 Å of the average Ni-O distance and a simultaneous decrease of ~-0.5° of the Ni-O-Ni superexchange angle. The ρ(T) measurements revealed a MI transition occurring at temperatures ranging from TMI~193 to 336 K for samples with x = 0 and 0.50, respectively. These measurements also show a large thermal hysteresis in NdNiO3 during heating and cooling processes, suggesting a first-order character of the phase transition at TMI. The width of this thermal hysteresis was found to decrease appreciably for the sample Nd0.7Eu0.3NiO3. The results indicate that cation disorder associated with increasing substitution of Nd by Eu is responsible for changing the first-order character of the transition in NdNiO3., This work was supported by the Brazilian agency Fundaçâo de Amparo à Pesquisa do Estado de Sâo Paulo (FAPESP) under Grant No 05/53241-9. One of us (MTE) acknowledges a FAPESP fellowship under Grant No 97/11369-0 and RFJ the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) fellowship under Grant No 303272/2004-0.

We report on the detection of transport Barkhausen-like noise (TBN) in polycrystalline samples of Bi 1.65 Pb 0.35 Sr 2 Ca 2 Cu 3 O 10 + δ T = 77 K . Measurements of magnetization versus applied magnetic field performed in powder samples at the same temperature indicate that the grains have a Bean-like behavior. From this curve we were able to determine the first critical field of the grains H clg ∼ 80 Oe and the full penetration field of the grains H * ∼ 135 Oe . Based on these results, the sample was subjected to an AC magnetic field with amplitude, B max = 6 mT and frequency, f = 1 Hz in order to avoid the flux penetration within the grains. Then, by using the standard DC four-probe method, the transport Barkhausen-like noise was measured for several values of excitation current. The results reveal that, under certain conditions, a noise in the voltage signal of the sample is clearly identified and related to the transition of the sample to the flux-flow regime. By using the Fast Fourier Transform (FFT) method, the power spectrum of TBN data is calculated and exhibits an 1 / f 2 frequency dependence, suggesting that the transition to the flux-flow regime in these materials is not a smooth process.

We have studied in detail the dependence of the electrical resistivity as a function of temperature of Bi1.65Pb0.35Sr2Ca2Cu3O10+δ((Bi,Pb)-2223) and Bi1.65Pb0.35Sr2CaCu2O8+δ ((Bi,Pb)-2212) ceramic samples. Both types of samples were obtained by a solid-state reaction method and pressed uniaxially at different compacting pressures ranging from 90 to 600 MPa before the last heat treatment. From electrical resistivity measurements as a function of temperature, we were able to separate contributions arising from both the grains' misalignment and microstructural defects, by using a current-conduction model. The results suggest that in both compounds the texture degree and the connectivity between grains are improved with increasing compacting pressures. It was found that in the (Bi,Pb)-2223 samples, even in those obtained at low compacting pressures, the grains exhibit a preferential orientation. A similar effect, but less pronounced, was observed in the (Bi,Pb)-2212 samples. In both cases, the behavior is different to the one reported in YBCO ceramic samples, in which the grains exhibit random orientation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have produced nanocomposite films of Ni:SiO2 by an alternative polymeric precursor route. Films, with thickness of ~ 1000 nm, were characterized by several techniques including X-ray diffraction, scanning electron microscopy, atomic force microscopy, flame absorption atomic spectrometry, and dc magnetization. Results from the microstructural characterizations indicated that metallic Ni-nanoparticles with average diameter of ~ 3 nm are homogeneously distributed in an amorphous SiO2 matrix. Magnetization measurements revealed a blocking temperature T B ~ 7 K for the most diluted sample and the absence of an exchange bias suggesting that Ni nanoparticles are free from an oxide layer.

We have studied the effects of the uniaxial compacting pressure on the physical properties of polycrystalline Bi1.65Pb0.35Sr2Ca2Cu3O10+d (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 r(T) exhibits a T-linear behavior at temperatures higher than T* ~ 235 K. The deviation of r(T) from the linear behavior below T* 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 r(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, ltr, and the mean free path, l, were estimated. We have found that in the sample with the highest degree of textureltr ~ 0.53, a value comparable with the one obtained in Bi-2223 single crystals. However, the result for l ~ 12.7 A 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 studied normal and superconducting transport properties of nearly single phase Bi1.65Pb0.35Sr2Ca2Cu3O10+δ (Bi-2223) ceramic samples. The samples were prepared from the same batch obtained by solid-state reaction method and pressed uniaxially at different compacting pressures ranging from 90 to 600 MPa before the last heat treatment. We have found by using magnetization versus applied magnetic field measurements, performed in powder samples, that both the lower critical field Hclg ∼ 80 Oe and the full penetration field H* ∼ 135 Oe of grains at 77 K are similar in all samples. From electrical resistivity measurements as a function of temperature, we were able to separate contributions arising from both the grain misalignment and microstructural defects. The results suggest that the grain orientation and the connectivity between grains are improved with increasing compacting pressures. It was found that the superconducting critical current density in zero applied magnetic field Jc(0) measured in our samples increases from 58 A/cm2 to 418 A/cm2. The normalized critical current density dependence on applied magnetic field, Jc(Ba)/Jc(0), has showed to be very sensitive to the compacting pressure: it exhibits a clear Josephson-like behavior at low compacting pressure, which changes to a more magnetic field independent behavior at higher compacting pressures. In addition, the flux-trapping curves of samples subjected to different compacting pressure have revealed the presence of three superconducting levels: the superconducting grains, the superconducting clusters, and the weak links. A correlation between the normal and superconducting transport properties of these materials is discussed based on their dissipation mechanisms.

Measurements of the electrical resistivity as a function of temperature, ρ(T), for different values of applied magnetic field, Ba (0 ⩽ Ba ⩽ 50 mT), were performed in samples extracted from different regions of the same Bi1.64Pb0.36Sr2Ca2Cu3Oy pellet in order to evaluate its homogeneity regarding the transport properties. Three pieces in form of slab were cut from the pellet. The first was extracted from the interface where the piston used in the compacting process has a direct mechanical contact with the powders (face A). The second, was taken from the interface parallel to the latter which is located in the base of the cylindrical sample (face B). Finally, the last piece was obtained by cutting an slab along the axis of the cylinder from the face A to face B. We have found appreciable differences in the grain orientation between these pieces by using X-ray diffractometry. Results indicate that the grains of the samples extracted from the interfaces A and B, are preferentially aligned with their c-axis parallel to the compacting direction. However, this feature is practically absent in the sample that was cut along the axis of the pellet, where the grains are more randomly oriented. The ρ(T) dependence at zero applied magnetic field shows a similar metallic-like behavior in the normal-state region in all samples, but the electrical resistivity at 200 K of the samples extracted from the interfaces A and B is ∼16% lower than the observed value for the other sample. In addition, the values of the so-called offset critical temperature, Toff, show an appreciable decrease with increasing applied magnetic field, Ba. In the case of the samples extracted from the faces A and B Toff falls ∼9%, while in the sample cut along the thickness of the pellets was ∼13%. The above transport experimental results are interpreted in terms of changes in the grain alignment and the effective intergranular pinning energy provoked by the inhomogeneous distribution of uniaxial compacting pressure in these ceramic samples.

We report on the synthesis of polycrystalline thin films of PbZr0.3Ti0.7O3 (PZT) by the so-called chemical solution deposition technique. The thin films were deposited on Pt∕Ti∕SiO2∕Si substrates by the spin-coating method, and were heat treateded at 700°C in air and under several oxygen pressures (10

In this work, magneto-resistance and magneto-impedance (MI) data for bulk samples of pure and Y-doped La 0.7 Ca 0.3 MnO 3 manganites are reported. Temperature dependent measurements were carried out sweeping the applied magnetic field H in the − 6 T T range. The measurements were done with dc and ac electrical currents, varying the frequencies f up to 5 MHz, using a four-probe technique. The pure La 0.7 Ca 0.3 MnO 3 sample presented a metal-insulator-transition near the ferromagnetic transition ( T C =245 K). For these samples, magneto-resistance of the order 60% for H =6 T was observed close to T C . The substitution of ions of La by Y (La 0.6 Y 0.1 Ca 0.3 MnO 3 ) reduced T C to 170 K but, on the other hand, increased the magneto-resistance to over 90% near T C . In the measurements with ac currents, the data for both samples presented a large magneto-impedance effect. The MI is more pronounced at high fields and for the Y-doped samples. Moreover, the Y-doped samples showed a strong hysteretic behavior that was not observed in the pure samples.

The electrical resistivity ρ( T ), magnetoresistivity ρ( T , H ), and impedance spectroscopy Z (ω, T ) of polycrystalline La 0.6 Y 0.1 Ca 0.3 MnO 3 compounds with different intergranular surfaces were investigated. The interfaces were modified through controlled addition of pores in samples prepared by the sol-gel technique. X-ray diffraction, scanning electron microscopy, and mercury porosimetry analysis showed that all specimens are single phase having similar microstructures and differing only by the volume fraction of pore. ρ ( T , H ) and Z ( ω , T ) data revealed that all samples undergo a metal-insulator transition at T ∼175 K. In addition, the Z ( ω , T ) data indicate two well defined contributions to the electrical resistivity. The ρ ( T , H ) data exhibit features of colossal magnetoresistivity CMR phenomenon but were found to be independent of porosity. The combined results suggest that the CMR effect is mainly associated with percolation of metallic regions and weakly dependent on sample morphology.

Electrically conductive LaNiO 3− δ (LNO) thin films with typical thickness of 200 nm were deposited on Si (111) substrates by a chemical solution deposition method and heat-treated in air at 700 °C. Structural, morphological, and electrical properties of the LNO thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), field-emission scanning electron microscopy (FEG-SEM), and electrical resistivity ρ ( T ). The thin films have a very flat surface and no droplet was found on their surfaces. The average grain size observed by AFM and FEG-SEM was approximately 100 nm in excellent agreement with XRD data. The ρ ( T ) data showed that these thin films display a good metallic character in a large range of temperature. These results suggest the use of this conductive layer as electrode in the integration of microelectronic devices.

The electrical resistivity ρ(T), magnetization M(T), magnetoresistivity ρ(T, H), and the impedance spectroscopy Z(ω, T) of the polycrystalline La0.6Y0.1Ca0.3MnO3 compound were investigated. X-ray diffraction and scanning electron microscopy were used for microstructural characterization. The results show that the sample is single phase and has average grain size of ∼0.5 μm. ρ(T, H) and Z(ω, T) transport data revealed that the material undergoes a metal–insulator transition at TCR ∼ 175 K. In addition, the deconvolution of Z(ω, T) data indicates two well defined contributions to the electrical resistivity occurring at low and high frequencies. The temperature dependence of the dielectric constant associated with the low frequency component exhibits a percolation-like phase transition at TCR ∼ 175 K. The normalized ρ(T, H) data exhibit features of colossal magnetoresistivity CMR phenomenon. The combined results suggest that the transport properties of these manganites, assessed through macroscopic techniques, can provide some insights regarding the metal–insulator transition, which have been confirmed by spectroscopic measurements. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The hysteretic behavior of the superconducting critical current density Jc(Ba) dependence under low applied magnetic fields (Ba

Au nanoparticles (Au NPs) have gained significant attention as catalysts for the selective oxidation of alcohols; however, the catalytic activity is highly dependent on the presence of a base. Alternatives to the use of strong bases, such as NaOH, are still needed. Here, we explored the basicity of magnesium ferrite/oxide supports to study the catalytic behaviour of supported Au NPs for the oxidation of benzyl alcohol. The presence of Mg2+ ions in the ferrite structure improved the catalytic activity of supported Au NPs to ca. 35% conversion in the absence of an additional base. After modifying the support with MgO, the catalytic activity of supported Au NPs was further improved to ca. 50% conversion, but the catalyst deactivated in successive recycling tests. When the catalysts were tested in the presence of a sub-stoichiometric amount of K2CO3, they became more active and remained stable upon recycling with no loss of activity and selectivity for the preferential production of benzoic acid.

The metal-insulator (MI) phase transition in Nd1-xEuxNiO3, 0 = x = 0.35, has been investigated through the pressure dependence of the electrical resistivity ?(P, T) and measurements of specific heat CP(T). The MI transition temperature (TMI) increases with increasing Eu substitution and decreases with increasing pressure. Two distinct regions for the Eu dependence of dTMI/dP were found: (i) for x = 0.15, dTMI/dP is nearly constant and ~-4.3 K/kbar; (ii) for x = 0.15, dTMI/dP increases with x and it seems to reach a saturation value ~-6.2 K/kbar for the x = 0.35 sample. This change is accompanied with a strong decrease in the thermal hysteresis in ?(P, T) between the cooling and warming cycles, observed in the vicinity of TMI. The entropy change (?S) at TMI for the sample x = 0, estimated by using the dTMI/dP data and the Clausius-Clapeyron equation, resulted in ?S ~ 1.2 J/mol K, a value in line with specific heat measurements. When the Eu concentration is increased, the antiferromagnetic (AF) and the MI transitions are separated in temperature, permitting that an estimate of the entropy change due to the AF/Paramagnetic transition be carried out, yielding ?SM ~ 200 mJ/mol K. This value is much smaller than that expected for a s = 1/2 spin system. The analysis of ?(P, T) and CP(T) data indicates that the entropy change at TMI is mainly due to the electronic delocalization and not related to the AF transition.

The grain boundaries in polycrytalline high-Tc superconductors constitute a limitation for the use of these materials in several devices. In order to verify the effect of the consolidation process in the microstructural features, powders of single-phase Bi1:65Pb0:35Sr2Ca2Cu3O10+ (Bi-2223) were consolidate by the spark plasma sintering (SPS) method at 750 ◦ C. The SPS samples exhibited very high density but are oxygen deficient, as inferred from the electrical resistivity, ( T ), data. The samples were then subjected to a post-annealing heat treatment in air, at 750 ◦ C, and for different times, tpa = 0, 5, 30 min, and 40 hours. From the X-ray diffraction analysis we have found that SPS post-annealed samples for tpa > 5 min show the presence of an extra phase identified as the infinite layer compound with general formula Ca1−xSrxCuO2. Irrespective of the presence of this additional phase, the superconducting properties, including the superconducting critical current at 77 K, of the post-annealed SPS samples are optimized for tpa = 5 min. The combined properties of the samples obtained by the SPS technique make them suitable for practical applications as superconducting fault current limiters (SFCL).

In this work we have studied the effect of NiS doping on the superconducting properties of the Bi-2212 (nominal composition Bi 2.1 Sr 2.1 Ca 0.8 [(CuO) 1− x (NiS) x ] 2 O 8+ δ ), superconductors. Our studies show that NiS doping stabilizes superconductivity in the system but all the NiS doped samples gave a T c lower than the superconducting transition temperature of pure Bi-2212 superconductors. It appears that NiS substitution for CuO planes and modification of the oxygen content due to S substitution on O places jointly produce strong localization of charge carriers and at the same time increase the hole concentration of the system. These combined phenomena occurring in the Bi 2.1 Sr 2.1 Ca 0.8 [(CuO) 1− x (NiS) x ] 2 O 8+ δ system are responsible for the deterioration of superconductivity of NiS doped Bi-2212 superconductors.

The hysteretic behavior of the superconducting critical current density Jc(Ba) dependence under low applied magnetic fields (B a mT ) of polycrystalline Bi1.64Pb0.36Sr2Ca2Cu3Oy samples was investigated. All the samples were prepared from the same powder but subjected to different pressures ranging from 40 to 250 MPa before the last heat treatment. The samples were characterized by scanning electron microscopy and X-ray powder diffraction analysis. The magnetic behavior of the superconducting grains was studied by measuring magnetization versus applied magnetic field in powder samples. The Jc(Ba) data are discussed within the scenario of the three-level superconducting system.

We have performed measurements of electrical resistance, R(T), and magnetoresistance, R(T, H), in applied magnetic fields Hup to 18 T in granular samples of the electron-doped superconductor Sm 1.83 Ce 0.17 CuO 4-y , These samples have a small superconducting volume fraction, bellow the percolation threshold, and can be described by a collection of superconducting islands embedded in a non-superconducting host. All the investigated samples revealed diamagnetic signal below T ci ∼17.5 K. Nevertheless, electrical resistance measurements showed an abrupt increase in the magnitude of R(T) below T ci . The electrical resistance ΔR(T,H) exceeding the normal state resistance value extrapolated to low temperatures was found to decrease with increasing H. We have also found that ΔR(T,H) (T/T ci ) -4 in the temperature range 0.6 < (T/T ci ) < 1, in agreement with the two-fluid theory of superconductivity. However, ΔR(T,H) shows a crossover to a (T/T ci ) -1/4 dependence at lower temperatures (T/T ci ) < 0.6. Such a crossover is discussed within the scenario of high anisotropic layered superconductors.

Polycrystalline samples of the electron-doped high-Tc superconductor Pr1.85Ce0.15CuO4−y were prepared with different starting materials: (1) mixture of simple oxides Pr6O11, CeO2, and CuO; (2) mixture of simple oxides Pr6O11 and CuO, and an intermediate compound PrCeO2; and (3) from a sol–gel precursor. All the samples were sintered in air at temperatures below 1030°C and reduced under Ar atmosphere at 950°C. Observations of X-ray diffraction, electrical resistivity, ρ(T), and magnetization, M(T), indicate the important effects that Ce diffusion and homogeneity have upon the macroscopic properties of this compound. The kinetics of the phase formation was accompanied by measurements of X-ray powder diffraction. These measurements, along with the determination of the lattice parameters a and c of the tetragonal T′-structure of these compounds, revealed that a complete diffusion of Ce into Pr2CuO4−y is only observed in samples prepared through a sol–gel precursor. Electrical resistivity measurements are consistent with the X-ray data and show both higher and sharper superconducting transition temperatures Tci in sol–gel samples. Measurements of magnetization performed on either pellets and powders confirm the latter results and indicate that samples prepared through sol–gel route have larger diamagnetic susceptibility at low temperatures. In addition, qualitative arguments suggesting the importance of Ce stoichiometry in both the normal-state electrical resistance and the superconducting properties of these polycrystalline samples are discussed.

We report on the synthesis and general physical properties of polycrystalline samples of LnNiO3 (Ln=Pr, Nd, Sm). These compounds crystallize in a GdFeO3-type orthorhombically distorted perovskite structure, Pbnm space group symmetry, and reveal a first-order metal–insulator transition in a large range of temperatures varying from ∼130 to 400 K. The samples were prepared through three different methods: (1) the conventional ceramic process by mixing and reacting simple oxides; (2) through precipitation of hydroxides of Ln3+ and Ni3+ and (3) through sol–gel precursors. The samples were subjected to heat treatments at temperatures up to 1000°C and under oxygen pressures as high as 70 bar. Observations of the microstructure and measurements of X-ray powder diffraction XRD, electrical resistivity ρ(T), and differential scanning calorimetric DSC, indicated important effects that precursors from different methods have upon the final properties of these compounds. A comparison between these characterizations revealed that sol–gel precursors resulted in better quality samples of LnNiO3 compounds.

To further study the charge carrier concentration in electron doped cuprate superconductors. a systematic x-ray absorption near edge structure (XANES) measurement has been carried out on Th-doped superconductor system R 2-x Th x CuO 4-δ (R=Nd, Sm, and Gd). The XANES results show that, similar to the Ce-doped compounds, while the intensity of the Cu 1+4pπ feature increase with the increase of the Th doping level x, the intensities of the Cu 2+4pπ and 4pσ features decreases. This clearly indicates that the electrons doped by the Th atoms are injected into the local Cu 3d-orbitals. The normalized Cu 1+4pπ intensity data show that the Cu 1+ concentration in the Th-doped compound series with different R-elements is linearly proportional to the Th doping-level x. The data suggest that both Ce and Th donate the same fraction of electrons into the Cu sites.

We report on intergranular transport properties of polycrystalline samples of Sm 1.82 Ce 0.18 CuO 4− y exhibiting the double resistive superconducting transition. These samples are comprised of small islands which undergo a superconducting transition at a higher temperature T ci ∼21 K and the long-range phase coherence is achieved through Josephson coupling at a lower temperature T cJ ∼10 K. We have performed several transport measurements including magnetic field and temperature dependence of electrical resistance, R ( T , H ), and I – V characteristics in these samples. Such measurements were taken under low applied magnetic fields 0≤ H ≤200 Oe and at temperatures T T cJ ∼10 K in order to investigate the phase correlations of the superconducting order parameter throughout the material. The results of magnetoresistance R ( T , H ) enabled us to conclude that dissipation due to Josephson flux flow is governed by an intergranular activation energy given by U J ( T , H )∝(1− T / T cJ )( H −0.6 ). In addition, we have found an irreversible behavior of R ( H ) curves, which shows a crossover field H = H *∼30 Oe. We also observed that H * is excitation current independent and that I cJ ( H *)≈0. The combination of these results has indicated that the phase of the superconducting order parameter is depressed at intergranular regions or more appropriately at the grain boundaries in these superconducting granular materials.

We have produced polycrystalline samples of Pr1-x Ba x MnO3 (0.0 ≤ x ≤ 0.5) through two different processes: (1) by mixing stoichiometric amounts of Pr oxide and carbonates of Mn and Ba (MO); and (2) through sol-gel precursors (SG). These samples were heat treated in air at several temperatures (500°C ≤ T ≤ 1400°C) and were characterized by X-ray powder diffraction, optical metallography, electrical resistivity ρ(T), and magnetoresistivity ρ(T, H). Results of X-ray powder diffraction revealed that all samples have the pereovskite-like structure and that MO samples are comprised of at least two phase: Pr1-x Ba x MnO3. Metallographic analysis confirmed this last result and revealed, in sample prepared via sol-gel route (SG), uniform morphologies and average grain size < 1 μm. Measurments of electrical resistivity ρ(T) revealed that the magnitude of μ increases with decreasing temperature until reaching a maximum value at T c, which is the long-range ferromagnetic ordering temperature of the system....

We report on synthesis and general physical properties of the solid solutions Nd1-x Ln x NiO3; Ln = Sm, Eu, Gd, Er, Yb; 0 ≤ x ≤ 1. The samples were prepared through sol-gel precursors and were subjected to heat treatments at temperatures T ≤ 1000°C under oxygen pressures as high as 70 atm. From the results of x-ray powder diffraction we have found that all samples studied crystallize in the GdFeO3-type orthorhombically distorted perovskite structure. Structural analysis of Nd1-x Ln x NiO3 compounds revealed that both the tolerance factor t and the Ni—O—Ni bond angle decrease with increasing Ln content. The latter is corrected with the temperature in which the Metal—Insulator (MI) transition develops in these compounds. Characterization of these materials through measurements of electrical resistivity revealed that all samples display the MI transition in a large range of temperature 200 K ≤ T ≤ 450 K. These results have enabled us to mapped out the Mi transition as a function of either the compos...

Using field-dependent (0{le}B{le}7T) noncontact rf surface-resistance measurements, we confirm the onset of granular superconductivity in the high-temperature superconductor Sm{sub 2{minus}x}Ce{sub x}CuO{sub 4{minus}y}, 0.15{le}x{le}0.18 at temperatures for which there is no evidence for a bulk Meissner effect. The surface-resistance data are described with an extended form of the Coffey-Clem theory. For the samples with x=0.15, these measurements also indicate that near the mean-field critical temperature T{sub c}, {minus}dB{sub c2}{sup c}/dT{congruent}0.38T/K, for B{parallel}c axis. {copyright} {ital 1998} {ital The American Physical Society}

The magnetoresistance and the magnetisation of ceramic La 2/3 A 1/3 MnO 3 (A=Sr, Ca) oxides have been studied as a function of the grain size. It is found that these ceramics become magnetically harder when reducing the particle size exhibiting a huge magnetic anisotropy (K I ) of about 10 7 erg/cm 3 that increases when reducing the grain size; thus signalling the relevance of the surface contribution to the magnetic anisotropy. In concomitance with this enhancement of K 1 a gradual increase of the low-field magnetoresistance is also detected. We suggest that both phenomena could be closely related and associated to the existence of some degree of spin disorder at the grain boundaries. We propose that these features are fingerprints of quenched spin disorder at interfaces that can be tuned through bandwidth modulation of the magnetic frustration.

We have studied the magnetic and transport properties of granular samples of the electron-doped superconductor Sm1.83Ce0.17CuO4−y prepared under certain conditions. Measurements of magnetic susceptibility, electrical resistance and magnetoresistance in applied magnetic fields up to 8.8 T reveal that superconductivity develops below Tci ≈ 17.5 K in these compounds. We also observed an abrupt increase of the electrical resistance below the superconducting transition. It was found that the resistance excess, ΔR(T,H), is proportional to (T/Tci)−4 close to Tci, 0.6 < (T/Tci) < 1, and shows a crossover to a (T/Tci)−14 dependence at lower temperatures. We argued that the increase of ΔR close to Tci is due to the depletion of the concentration of normal carriers as predicted by the semi-phenomenological two-fluid theory of superconductivity. The behaviour of ΔR at lower temperatures is also discussed.

We investigated the transport Barkhausen-like noise (TBN) by using nonlinear time series analysis. TBN signals were measured in (Bi,Pb)2Sr2Ca2Cu3O10+δ ceramic samples subjected to different uniaxial compacting pressures (UCP). These samples display similar intragranular properties but different intergranular features. We found positive Lyapunov exponents in all samples, λm≥0.062, indicating the nonlinear dynamics of the experimental TBN signals. It was also observed higher values of the embedding dimension, m9, and the Kaplan-Yorke dimension, DKY2.9. Between samples, the behavior of λm and DKY with increasing excitation current is quite different. Such a behavior is explained in terms of changes in the microstructure associated with the UCP. In addition, determinism tests indicated that the TBN masked determinist components, as inferred by |k[over arrow]| values larger than 0.70 in most of the cases. Evidence on the existence of empirical attractors by reconstructing the phase spaces has been also found. All obtained results are useful indicators of the interplay between the uniaxial compacting pressure, differences in the microstructure of the samples, and the TBN signal dynamics.

We have studied the mixed state Hall effect in YBa2Cu3O7−δ single crystals with unidirectional twins: ϱxy=f(H). An analysis of the Hall conductivity σxy in free flux flow regime reveals that σxy can be successfully described by two terms which are related to the quasiparticle excitations and the motion of free vortices respectively. We have discovered the strong twin dependence of the Hall conductivity σxy in the pinning (TAFF) regime and detected the sign reverse of the Hall conductivity σxy at changing angle α (+45 °→−45 °) between current and twin plane that indicated the presence of backflow of vortices. We have observed that σxy tends to -∞ while approaching the melting line. These results provide the evidence of strong planar pinning influence on the Hall conductivity.

We performed detailed measurements of electrical resistivity ϱ versus temperature T in superconducting Bi2Sr2CaCu2O8±δ compounds prepared by melt-casting followed by recrystallization at higher temperatures under v conditions. Differential thermal analysis (DTA) suggests that the recrystallization process starts near 460°C, and that the superconducting phase starts developing near 780°C. The porosity of the annealed materials depends strongly on the annealing temperature, time, and heat-up rate. Typically a sharp drop in ϱ at the onset of superconductivity is followed at lower T by either a shoulder-like feature, a tail-like feature, or both. The ϱ versus T data, SEM, and porosity analysis suggest that weak links with narrow or broad distribution of sustaining critical current densities Jc are responsible for the latter behaviors, respectively.

The diffusion of Ce in polycrystalline samples of Eu2−xCuO4−y ().0 ≤ x ≤ 0.18) percent prepared from a sol-gel precursor and sintered at several temperatures 150°C ≤ T ≤ 950°C was investigated. The results of X-ray powder diffraction revealed that complete diffusion of Ce into the Eu2CuO4−y matrix occurs in samples sintered at T = 950°C, which is below the eutectic temperature. It was also observed that the Ce diffusion monotonically increases with increasing x, suggesting that the Eu2−xCexCuO4−y (0.0 ≤ x ≤ 0.18) system forms a true chemical solid solution. The solubility limit of Ce in this series was found to be close to x = 0.15. An accurate analysis of the full width at half maximum of the X-ray diffraction peaks reveals an inhomogeneous Ce distribution in samples sintered for short time intervals. An improvement of the homogeneity of Ce was found in samples sintered for longer times. Results of differential thermal analysis and thermogravimetry were found to be in complete agreement with the X-ray diffraction data. We also found that the peritectic temperature Tp increases with increasing Ce content in this series.

Transition from superconductive to normal state and back into superconductive state in bulk single crystal YBaCuO bridges with twins induced by a strong transport current has been investigated. The current-voltage characteristics (CVC), the temperature and magnetic field dependences of resistance [resistive transition (RT)] in the regime of fixed current nearT c were studied in detail. A sharp dynamic superconductive transition or switching over effect, nonlinear CVC, and hysteresis were observed. The CVC and RT characteristics can be explained by the thermal bistability phenomena at strong Joule self-heating of bridges, in particular, in the presence of internal local inhomogeneities in superconductor. Electrical characteristics of bridges which can be used as nonlinear elements for cryogenic electronics were obtained.

The thermal decomposition (TD) methods are among the most successful in obtaining magnetic nanoparticles with a high degree of control of size and narrow particle size distribution. Here we investigated the TD of iron(III) acetylacetonate in the presence of oleic acid, oleylamine, and a series of alcohols in order to disclose their role and also investigate economically attractive alternatives for the synthesis of iron oxide nanoparticles without compromising their size and shape control. We have found that some affordable and reasonably less priced alcohols, such as 1,2-octanediol and cyclohexanol, may replace the commonly used and expensive 1,2-hexadecanediol, providing an economically attractive route for the synthesis of high quality magnetic nanoparticles. The relative cost for the preparation of Fe3O4 NPs is reduced to only 21% and 9% of the original cost when using 1,2-octanediol and cyclohexanol, respectively.

The manganite Nd0.5Ca0.5MnO3 has a CE-type antiferromagnetic-insulating (AFI) ground state at low temperatures. This compound undergoes a charge-ordering (CO) transition at TCO∼250 K followed by a paramagnetic to AFI (orbital-ordered) OO phase transition at TN∼170 K. The ferromagnetic (FM) double-exchange interaction between Mn ions is quenched by such a CO effect. Upon Cr doping, as in single crystals of Nd0.5Ca0.5Mn0.96Cr0.04O3, suppression of the quenched CO-OO state occurs at TC∼140 K and is accompanied by a FM-metal-insulator transition. These Cr-doped materials have a low temperature ground state comprised of a mixture of 20–30 nm domains of the FM phase embedded in the CO-OO matrix, as inferred from Lorentz microscopy. Our results show that these Cr-doped materials undergo a field-dependent magnetic transition in a large range of temperature T

Polycrystalline samples of the electron-doped high- T c superconductor Nd 1.85 Ce 0.15 CuO 4−γ were prepared with different starting materials, sintering temperatures, reducing atmospheres, and cooling rates after reduction. Observations of the microstructure and measurements of X-ray diffraction, electrical resistivity, and magnetic susceptibility indicate the important effects that cationic (Ce) and atomic (O) distributions have upon the properties of this compound. A double resistive superconducting transition, attributed to small superconducting islands within grains and coupling between these islands, is an intrinsic property of this compound. Samples sintered below the eutectic temperature are composed of small grains, are porous, and have an additional phase. An inhomogeneous cationic distribution results, even when an intermediate oxide NdCeO 3.5 is used as a starting material in order to promote the diffusion of Ce into the matrix, as indicated by a large and semiconducting electrical resistivity, a broad resistive transition, and a small diamagnetic susceptibility. For these samples, the oxygen distribution has a pronounced effect on the resistive transition, with a more non-equilibrium distribution resulting in higher transition temperatures. On the other hand, samples sintered above the eutectic temperature, and therefore with a liquid phase present, have larger grains and are dense. Their properties are relatively insensitive to the oxygen distribution, which is inhomogeneous across the entire sample, while the Ce distribution is more uniform, as indicated by a small and nearly metallic electrical resistivity, a sharper resistive transition, and a larger diamagnetic susceptibility. In addition, those samples prepared with the intermediate oxide have higher transition temperatures. Implications of the granularity of electron-doped compounds on the small diamagnetic contribution and also the absence of the peak in the specific heat at the superconducting transition are briefly discussed.

Low-temperature specific heat measurements were performed on polycrystalline samples of (Nd1−xSmx)2CuO4. All samples show an anomaly at low temperatures characterized by a maximum in the specific heat, as previously reported for the pure compounds, which has been associated with a transition to an ordered state of the rare earth ion. The results at zero field show that there are two different mechanisms involved in the rare earth ordering process in the mixed compounds. Measurements on these samples in applied magnetic fields reveal that the temperature at which the maximum occurs decreases slightly with increasing field for Sm-rich samples and increases for Nd-rich samples.

The dynamic magnetic properties of Ni nanoparticles diluted in an amorphous SiO2 matrix prepared from a modified sol-gel method have been studied by the frequency f dependence of the ac magnetic susceptibility \c{hi}(T). For samples with similar average radii ~ 3-4 nm, an increase of the blocking temperature from TB ~ 20 to ~ 40 K was observed for Ni concentrations of ~ 1.5 and 5 wt.%, respectively, assigned to the effects of dipolar interactions. Both the in-phase \c{hi}'(T) and the out-of-phase \c{hi}"(T) maxima follow the predictions of the thermally activated N\'eel-Arrhenius model. The effective magnetic anisotropy constant Keff inferred from \c{hi}"(T) versus f data for the 1.5 wt.% Ni sample is close to the value of the magnetocrystalline anisotropy of bulk Ni, suggesting that surface effects are negligible in the present samples. In addition, the contribution from dipolar interactions to the total anisotropy energy Ea in specimens with 5 wt.% Ni was found to be comparable to the intrinsic magnetocrystalline anisotropy barrier., Comment: 12 pages, 2 figures

We have performed measurements of dc electrical resistivity ρ(T), magnetic susceptibility χ(T), and impedance spectroscopy Z(T, ω) in both single crystals and polycrystalline samples of Nd0.5Ca0.5Mn1−xCrxO3 (0.00⩽x⩽0.07). The pristine compound exhibits a transition to a charge ordered (CO) state at TCO∼250 K. However, no evidence of either metal-insulator (MI) or ferromagnetic (FM) transitions are observed at lower temperatures. The substitution of Mn by Cr partially suppresses the CO transition and induces a FM/MI ordering at lower temperatures TC=TMI∼140 K, which is essentially Cr independent. Both ρ(T) and χ(T) measurements indicate the development of a ferromagnetic (FM) component at low temperatures T

Ni nanoparticles embedded in an amorphous SiO2 matrix were produced by a modified sol–gel method. This method resulted in nanocomposites with a controlled size distribution and good dispersion of the metallic particles. The particle-size distributions were found to have an average radius of ∼3 nm, as inferred from transmission electron microscopy, X-ray-diffraction analysis, and magnetic measurements. Magnetic characterizations revealed that samples exhibit superparamagnetic behavior above the blocking temperature TB, 20 K≤TB≤40 K, and absence of a shift along the field axis on hysteresis loops measured at T≤TB, indicating that the metallic nanoparticles are also free from an oxide layer.

Neutron diffraction on the double perovskite Sr(2)YRuO(6) with a quasi-fcc lattice of Ru moments reveals planar magnetic correlations that condense into a partial long-range ordered state with coupled alternate antiferromagnetic (AFM) YRuO(4) square layers coexisting with the short-range correlations below T(N1) = 32 K. A second transition to a fully ordered AFM state below T(N2) = 24 K is observed. The reduced dimensionality of the spin correlations is arguably due to a cancellation of the magnetic coupling between consecutive AFM square layers in fcc antiferromagnets, which is the simplest three-dimensional frustrated magnet model system.

Few authors have been attempting between mast cells and dermal dendrocytes interactions on urticaria.To describe the extruded mast cell granules and dermal dendrocytes in drug-induced acute urticaria.Seven patients with drug-induced acute urticaria were enrolled in the study. We token skin biopsies of urticaria lesion and perilesional skin. The 14 fragments collected were processed to immunogold electron microscopy using single stains to tryptase and FXIIIa, besides double immunogold labeling with both.Some sections demonstrated mast cells in degranulation process, both in anaphylactic and piecemeal degranulation types. After double immunogold staining, 10 nm (FXIIIa) and 15 nm (Tryptase) gold particles were present together over the granules in mast cells indicating that tryptase and FXIIIa are each localized within the granules of these cells. Interestingly, we found a strong evidence of than the exocytosed mast cell granules contents both FXIIIa and tryptase immunolabeled are phagocytized by dermal dendrocytes.The current observations provide morphological evidence that the exocytosis-phagocytosis mechanisms of mast cell granules represents one pathophysiological example of mast cells-dermal dendrocytes interactions in urticaria.

The synthesis of nickel catalysts for industrial applications is relatively simple; however, nickel oxidation is usually difficult to avoid, which makes it challenging to optimize catalytic activities, metal loadings, and high-temperature activation steps. A robust, oxidation-resistant and very active nickel catalyst was prepared by controlled decomposition of the organometallic precursor [bis(1,5-cyclooctadiene)nickel(0)], Ni(COD)2, over silica-coated magnetite (Fe3O4@SiO2). The sample is mostly Ni(0), and surface oxidized species formed after exposure to air are easily reduced in situ during hydrogenation of cyclohexene under mild conditions recovering the initial activity. This unique behavior may benefit several other reactions that are likely to proceed via Ni heterogeneous catalysis.

It is reported superconductivity in Nb5Ge3C0.3, an interstitial carbide compound. The temperature dependence of the electrical resistivity, ac-susceptibility, and heat capacity (HC) indicate that a bulk type-II superconductivity appears at TC = 15.3 K. Magneto-resistance measurements suggest an upper critical field of BC2 ∼ 10.6 T and a coherence length of ξ ∼ 55 A at zero temperature. Neutron diffraction analyzes locate the carbon atoms at the interstitial 2b site of the Mn5Si3 type-structure. Heat capacity data below TC are well described by BCS theory. The size of the jump at TC is in good agreement with the superconducting volume fraction observed in susceptibility measurements. A Debye temperature and Sommerfeld constant were also extracted from heat capacity data as 343 K and 34 mJ/mol K2, respectively.