The effect of treatment at a temperature of 200°C and the natural aging on the critical parameters of a highly textured yttrium barium cuprate YBa2Cu3O6.9 has been investigated. It has been shown that non-superconducting (at T = 77 K) particles precipitated during phase decomposition of this compound are effective pinning centers. At 200°C, the YBa2Cu3O
 y
 compound interacts with atmospheric moisture. This inter-action results in the formation of stacking faults, which also provide pinning of magnetic vortices. The structural changes occurring during low-temperature annealing and natural aging of the compound lead to an increase in the critical current density and the first critical field. The presence of pinning centers of different nature in the structure causes a synergistic effect, which significantly increases the current-carrying capacity of materials, including those in strong magnetic fields.

Effect of low-temperature annealing on the critical parameters of highly textured YBa 2 Cu 3 O y

Eu–Ba–Cu–O composition was synthesized by solid state reaction technique. To determine optimum growth temperature, heat treatment was examined on the material at 880–1,100 °C. Microstructural evolution, phase formation and elemental distribution depending on heat treatments were examined by using X-ray diffraction, scanning electron microscope, energy dispersive X-ray spectroscope analysis. Optimum fabrication conditions were determined as 1,020 °C for 24 h under oxygen atmosphere and detailed characterization of corresponding compound was performed. The magnetization hysteresis loops are expounded to be the product of superconducting Eu-123 grains and magnetic Eu2+ ions. The peak effect on the magnetization curves was described by the extended critical state model. Scaling of the pinning force was found such that the peak position is proportional to the irreversibility field H
 irr and the maximum pinning force is proportional to H
 irr
 2
 .

Vortex pinning and magnetic peak effect in Eu(Eu,Ba)2.125Cu3Ox

The influence of replacements of yttrium and barium in YBa2Cu3O
 y
 and doping of this compound with CeO2, ZrO2, and Pr2O3 on the capability of absorbing water from the annealing atmosphere at T = 200°C has been studied The complication of the chemical composition of the 123-type compounds leads to an enhancement of their stability to aggressive components of the gaseous phase (H2O, CO2) These compounds have a variable chemical composition both in oxygen and water and can exist in three modifications: the oxygen-ordered ortho-phase, the oxygen-disordered tetra-phase, and the pseudocubic phase with a variable water content, which at T = 200°C separates into an oxygen-deficient water-enriched phase and an oxygen-rich anhydrous phase

Influence of chemical composition on the stability of YBa 2 Cu 3 O y in a humid atmosphere

A method to determine a value of the effective field in the intergrain medium of a granular superconductor is proposed. The space between superconducting grains is considered to be a Josephson medium where passage of the transport current causes dissipation and the effective field is superposition of external field H and the field induced by magnetic moments of superconducting grains. The method proposed is based on the comparison of hysteresis field dependences of magnetoresistance and magnetization and their relaxation at H = const. By the example of granular YBa2Cu3O7, it is shown that, in the region of weak fields, the effective field in the intergrain medium exceeds by far the external field, i.e., compression of a magnetic flux occurs.

Compression of a magnetic flux in the intergrain medium of a YBa2Cu3O7 granular superconductor from magnetic and magnetoresistive measurements

Magnetoresistance of bulk textured Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag ceramics has been studied in the magnetic fields applied parallel and perpendicular to a–b planes of Bi2223 crystallites. Besides well known anisotropy of magnetoresistance of textured superconductors (RH || c > RH || a–b), anisotropic hysteresis of R(H) dependences was investigated. Parameters characterizing hysteretic R(H) curves differ for the cases H || c and H || a–b. This behavior is explained within the model of a granular superconductor where the total magnetic induction in the intercrystallite boundaries is superposition of the external field and the magnetic field induced by dipole magnetic moments of neighbor crystallites.

Magnetoresistance hysteresis of bulk textured Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag ceramics and its anisotropy

The hysteretic behavior of the magnetoresistance R(H) of granular high-temperature superconductors (HTSCs) of the Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and La-Sr-Cu-O classical systems is investigated for transport current densities lower and higher than the critical density (at H = 0). All systems exhibit universal behavior of the width of the magnetoresistance hysteresis loop: independence of transport current under identical external conditions. This means that flux trapping in HTSC grains is the main mechanism controlling the hysteretic behavior of the magnetoresistance of granular HTSCs, while pinning of Josephson vortices in the intragranular medium makes no appreciable contribution to the formation of magnetoresistance hysteresis (when transport current flows through the sample). Experimental data on relaxation of residual resistance after the action of a magnetic field also confirm this conclusion.

Mechanism of the hysteretic behavior of the magnetoresistance of granular HTSCs: The universal nature of the width of the magnetoresistance hysteresis loop

The intragrain pinning in high-Tc superconductor compounds Y1−xRExBa2Cu3O7−δ with low concentration of RE (La, Ce, Pr) was investigated. Magnetic and transport measurements reveal that the pinning is maximal for the concentration of heterovalent RE such that the average distance between the impurity ions in the plane of rare-earth elements is close to the diameter of Abrikosov vortices in YBCO.

Pinning enhancement by heterovalent substitution in Y1−xRExBa2Cu3O7−δ

The intragrain pinning in high-$T_c$ superconductor compounds Y$_{1-x}$RE$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ with low concentration of RE (La, Ce, Pr) was investigated. Magnetic and transport measurements reveal that the pinning is maximal for the concentration of heterovalent RE such that the average distance between the impurity ions in the plane of rare-earth elements close to the diameter of Abrikosov vortices in YBCO.

Pinning Enhancement by Heterovalent Substitution in Y$_{1-x}$RE$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$

To test the pinning force in high-temperature superconductors with minor doping, the Y (1− x ) Ce x Ba 2 Cu 3 O 7 system has been synthesized. Temperature dependence of magnetization has been determined. The difference of diamagnetic responses M (zfc) −  M (fc) is nonmonotonic. It has a peak at a concentration value x  = 0.0156, which corresponds to the most probable distance between cerium ions equal to eight lattice constants in the a – b plane and close to the diameter of vortices.

Enhancement of pinning in cerium doped Y(1−x)CexBa2Cu3O7 HTSC

The Y(1 − x Ce x Ba2Cu3O7 system with low cerium concentrations has been synthesized. The cerium solubility limit measured using x-ray powder diffraction analysis is about 2.4 at. %. The temperature dependences of the magnetization M(T) are measured for samples cooled in a magnetic field (FC) and in a zero field (ZFC). The difference between the magnetizations M ZFC-M FC at 77.4 K, which is proportional to the pinning potential, passes through a maximum at x = 0.0156. This concentration corresponds to the average distance (equal to eight lattice constants) between the impurity ions in the plane of the rare-earth elements, which is comparable to the diameter of Abrikosov vortices in YBa2Cu3O7.

Yu. S. Gokhfeld

Papers

Mechanism of the hysteretic behavior of the magnetoresistance of granular HTSCs: The universal nature of the width of the magnetoresistance hysteresis loop

Pinning enhancement by heterovalent substitution in Y1−xRExBa2Cu3O7−δ

Pinning Enhancement by Heterovalent Substitution in Y$_{1-x}$RE$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$

Enhancement of pinning in cerium doped Y(1−x)CexBa2Cu3O7 HTSC

Effect of heterovalent substitution of rare-earth elements on the magnetic and transport properties of YBa 2 Cu 3 O 7