Narrow Results

The effect of partial substitution of Ca by x = 0.02 La on the electrical and magnetically properties of two (Bi,Pb):2223 superconductor ceramic S1 and S2 (with different Pb and Sr atomic concentration) were investigated by using electrical resistance and AC magnetic susceptibility measurements. The parameters from linear resistivity data in the normal state and superconducting transition are sensitive to the compounds stoechiomerty. The intergrain critical current density J_cJ(T) dependence near T_c obtained from immaginary AC susceptibility data agrees with the assumption of SNS intergrain junctions.

Superconducting ceramic samples (Bi,Pb):2223 (sample V) and (Bi)(Pb),(Sr,Ba):2223 (samples S1 and S2) were investigated by electrical resistance and AC magnetic susceptibility measurements. The intergrain critical current density J_cJ(T) dependence near T_c obtained from imaginary AC susceptibility data agrees with the assumption of SNS intergrain junctions, and are sensitive to the compounds stoichiomerty and thermal treatments. The crossover from linear to nonlinear dependence of T_p(H_AC) of irreversible line (IL) observed in all samples, was discussed based on the modification of Josephson medium in the Muller model.

Influence of flux creep on ac susceptibility (acs) of a superconductor was examined by simulation. Comparing the acs with and without flux creep, it was found that frequency of ac field shifted the acs with flux creep but did not affect the acs of critical state model. In addition, the amplitude of ac field and dc field affect differently for the two models.

In this paper, the effects of M = Mn and Ru substitutions at the Tl site of Tl${}_{1-x}$M_x(BaSr)CaCu₂O₇ (Tl-1212) on the electrical properties and AC susceptibility were investigated. The X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) methods were used to examine the phase and microstructure, respectively. All samples showed diffraction patterns with Tl-1212 as the major phase and an increase in the Tl-1223 phase with increasing Mn and Ru substitutions. The microstructure showed partial melting and randomly oriented grains in all samples. The electrical resistance measurements showed metallic normal state behavior for all samples. The zero-resistance transition temperature $T_{c-\mathrm{\text{zero}}}$ was the highest for x = 0.4 where $T_{c-\mathrm{\text{zero}}}$ was 107K and 90K for Mn and Ru substitutions, respectively. The superconducting transition, $T_{c\chi \prime }$ obtained from AC susceptibility measurements increased with Mn and Ru substitutions up to x = 0.4. Using Bean’s model, the current density at the peak temperature of the imaginary part of the AC susceptibility, $J_c$($T_p)$ was determined to be between 16 and 22A⋅cm${}^{-2}$ for Mn, and between 15 and 19A⋅cm${}^{-2}$ for Ru-substituted samples. Mn substitution showed a higher transition temperature compared with Ru. This study showed that optimization of the average Cu valence was achieved with Mn and Ru substitutions for x = 0.4 at the Tl site. The ionic radii of the substituting and substituted elements are important factors in enhancing the transition temperature of Tl(BaSr)CaCu₂O₇ superconductor.

Ca-doped (x = 0.00, 0.07 and 0.20) epitaxial Y_1-xCa_xBa₂Cu₃O_7-δ thin films were prepared on SrTiO₃ (100) by PLD technique. The superconducting transition temperature is found to decrease with increasing Ca content. The AC-susceptibility data were used to determine the intergranular critical current density, when varying the field amplitude H_AC. The temperature dependence of critical current density was determined from the imaginary part of the complex susceptibility, χ′′(T), using Xing's relation. The results were analyzed in terms of superconductor-insulator-superconductor (SIS) and superconductor-normal-superconductor (SNS) type models.

The temperature dependence of the harmonic susceptibilities has been measured on a melt textured YBCO for different amplitudes of the AC magnetic field with combination of a DC magnetic field. The model calculations have been performed on the basis of a model which incorporates the effects of the collective creep concept in a critical-state equation. The temperature dependencies of the activation energy and the critical current density chosen as U = [U₀(1 - (T/T_c)⁴)/μ)][(J_c/J)^μ - 1] and J_c = J₀[(1 + (T/T_c)²)]^-1/2[(1 - (T/T_c)²)]^5/2 respectively have been used to explain the DC bias field and temperature dependencies of the harmonic susceptibilities. The predictions of the model for fundamental and higher harmonic susceptibilities were consistent with the experimental data.

Electron-doped Sr_1-xLa_xMnO₃(0≤x≤0.50) and hole-doped (x=0.70 and 0.82) samples have been prepared by solid state route. X-ray diffraction patterns could be refined using P6₃ space group for x≤0.30 and space group for x≥0.40. Temperature variation of AC susceptibility measurements show that all the electron-doped materials exhibit paramagnetic to ferromagnetic transitions with the transition temperatures ranging from 375 to 393 K, followed by low temperature antiferromagnetic transitions. The ferromagnetic maximum susceptibility is found to increase systematically with La doping, i.e. with increase in Mn³⁺ ions. The high temperature resistivity data could be analyzed using the Mott-variable range hopping model. The hole-doped materials exhibit ferromagnetic to paramagnetic transition.

AC susceptibility measurements have been done to determine the intergranular properties for some 123-superconducting ceramics. From the imaginary part of the complex susceptibility, χ″(T), the temperature dependence of the critical current density was determined by using Bean's model. The intergranular current density was experimentally determined from AC susceptibility data by varying the field amplitude. All intragrain critical parameters are gradually degraded with increasing the concentration of the dopant substituting on the rare-earth side and with decreasing the sintering temperature during the sample preparation.

The structural and magnetoelastic properties of polycrystalline samples of Y₃Fe_27.2Cr_1.8 and Ce₃Fe₂₅Cr₄ intermetallic compounds are investigated by means of X-ray diffraction, thermomagnetic, thermal expansion and magnetostriction measurements in the temperature range of 77–500 K under applied magnetic fields up to 1.5 T. The well defined anomalies observed in the linear thermal expansion coefficient curves are associated with the magnetic phase transitions and presence of small amounts of 1:12 phase in the Ce₃Fe₂₅Cr₄ sample; the latter is also confirmed by AC susceptibility measurements. For Y₃Fe_27.2Cr_1.8, saturation behavior is observed in the anisotropic magnetostriction isotherms near the magnetic ordering temperature (T_C), whereas for Ce₃Fe₂₅Cr₄ compound, saturation starts well below T_C. The additional anomalies observed in the volume magnetostriction isotherms of both compounds are explained based on the anisotropy field in the a–b plane and along the c-axis of the unit cell.

We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa₂Cu₃O₇ films with artificial pinning centers, with various impurities, various architectures and thickness, and various techniques of nano-scale pinning engineering: substrate decoration, BaZrO₃ (BZO) nano-inclusions, and (quasi) multilayer architecture. Here we will present the results regarding vortex matter, dynamics, and pinning, in some of our best samples. Magnetic relaxation studies with magnetic field perpendicular to the film show that in both cases the splayed defects and/or nanoscale inclusions help reduce the dissipation in certain field-temperature range, by inhibiting the detrimental double vortex-kink formation. The response of the vortex system to AC excitation is rather complex but extremely useful for the characterization of the vortex dynamics. We have measured AC susceptibility in various DC fields, with various AC fields and frequencies and found that the effective vortex activation energy $U_{\mathrm{\text{eff}}}$ has a logarithmic dependence on the AC-field-induced current density $J$. Synergetic pinning centers assured a low anisotropy of the critical current for various field orientations, a property desired in coil/solenoid applications.

In this paper, the AC susceptibility in superconducting film is studied by the extended exponential critical state model, in which the critical-current density is assumed to be inhomogeneous distributed, namely, space gradient dependent. The results show that it should be noted that existence of the gradient parameter produces a quantitative effect on the form of the dependence of AC susceptibility. The magnetization properties caused by the gradient parameter are also highly nonlinear, which should be useful for the design of thin superconducting film devices.

The magnetic properties of Li_0.5-x/2Co_xFe_2.5-x/2O₄ (where x=0.1,0.2,0.3,0.4,0.5,0.6,0.7) ferrites have been studied. The x-ray diffraction studies reveal the single phase formation of the ferrite samples. The SEM micrographs show that the grain size decreases up to x=0.4 and increases for x>0.4. The grain size for all the samples lies in the range 0.2 μm and 6.7μm. The permeability studies at room temperature show that the values lie in the range of 5.5 to 20.6. The AC susceptibility measurements show a single domain structure. Usually the single domain structure is present in case of grain size < 0.1 μm. Both these variations suggest the presence of non- magnetic grain boundaries (NMGB) which involves deposition/ segregation of non magnetic particles over the surface of grains which reduce effective grain size. An attempt has been made to calculate the effective grain size using the data obtained from the measurements of initial permeability with temperature and the grain size obtained from the SEM micrographs.