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The temperature and time dependences of the magnetization of copper oxide nanoparticles 8, 13, and 18 nm in size have been investigated. Specific features of the behavior of CuO nanoparticles formed by vacuum plasma-arc synthesis as compared to other antiferromagnetic particles have been revealed. It has been shown that the bifurcation of magnetization curves upon cooling in the zero (ZFC) and nonzero (FC) magnetic fields occurs above the Neel temperature, while the usual peak of the magnetization curve is absent in the ZFC mode. The problems associated with the nonequilibrium behavior of synthesized CuO nanoparticles have been discussed.

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi1.8Pb0.3Sr1.9Ca2Cu3Ox have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.

The current-voltage characteristics of a porous superconductor Bi2Sr2Ca2Cu3Ox (Bi2223) have been measured at temperatures in the range from 10 to 90 K in magnetic fields of 0–80 kOe. The experimental dependences have been analyzed within the model allowing for pinning by clusters of a normal phase with fractal boundaries, as well as the model taking into account phase transformations of vortex matter. It has been found that the electrical resistance of the superconductor material significantly increases at temperatures of 60–70 K over the entire range of magnetic fields under consideration without changing in the sign of the curvature of the dependence R(I). It has been assumed that this behavior is associated with the specific feature of the pinning in a highly porous high-temperature superconductor, which lies in the fractality of the distribution of pinning centers in a wide range of self-similarity scales. The studied material at the aforementioned temperatures is characterized by the melting of the vortex structure.

The influence of thermomagnetic prehistory on the behavior of a resistive transition R(T) in external magnetic fields of polycrystalline YBa2Cu3O7 and Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature supercon-ductors and the Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag texture has been investigated. It has been found that, for YBa2Cu3O7, the thermomagnetic prehistory exerts a substantial influence on the dissipation in the subsystem of grain boundaries in magnetic fields up to ∼103 Oe, and this effect becomes insignificant in fields higher than ∼104 Oe. This behavior has been explained by the influence of magnetic moments of high-temperature superconductor grains on the effective magnetic field in the intergranular medium. For bismuth high-temperature superconductors, no influence of thermomagnetic prehistory on the resistive transition has been observed; however, this effect manifests itself in current-voltage characteristics at high transport current densities. There is also a radical difference in the behavior of isotherms of the magnetoresistance R(H) for the yttrium and bismuth systems. For YBa2Cu3O7, there is a clear separation between the dissipation regimes in the intergranular medium and in grains, which manifests itself even at low transport current densities as a change of sign in the curvature of the dependence R(H). For a texture based on the bismuth high-temperature superconductor, this feature has been observed only at high current densities (comparable to the critical current density at H = 0). This difference in the behavior of magnetoresistive properties of the classical high-temperature superconductor systems under investigation has been explained by relatively low irreversibility fields of the bismuth high-temperature superconductors. In these materials, simultaneous processes of dissipation can occur in an external magnetic field both in the subsystem of grain boundaries between crystallites and in the crystallites themselves.

The field dependences of the magnetoresistance of Bi1.8Pb0.3Sr1.9Ca2Cu3Ox samples with different densities, which have a foam microstructure and exhibit different diamagnetic responses, were studied at 77.4 K to identify the mechanism responsible for the formation of a negative magnetoresistance region in granular high-temperature superconductors. A region with negative magnetoresistance was found to exist in samples with magnetizations highest in absolute magnitude. This behavior finds a reasonable interpretation as due to the effect exerted by dipole moments of high-temperature superconductor grains on the effective intergranular field. The strength of this effective field has been estimated.

The hysteresis of magnetoresistance R(H) and relaxation of the remanent resistance R rem with time after magnetic field treatment of HTSC (Y-Ba-Cu-O) + CuO composites are studied. Such a composite constitutes a network of Josephson junctions wherein the nonsuperconducting component (CuO) forms Josephson barriers between HTSC grains. By comparing the experimental R rem(t) and R(H) dependences, it is shown that the relaxation of the remanent resistance is caused by the decreased magnetic field in the intergrain medium due to relaxation of magnetization. The reason is uncovered for the differences between the published values of pinning potentials determined by measuring the relaxation of magnetization or resistance and fitting them by the Anderson law.

The temperature dependences of the critical current of Y3/4Lu1/4Ba2Cu3O7 polycrystalline high-temperature superconductors subjected to annealing for different times and those of Y3/4Lu1/4Ba2Cu3O7 + BaPbO3 composites were measured. The results obtained were analyzed in terms of the Gunsenheimer-Schussler-Kummler theory considering the Andreev reflection of carriers in a “superconductor-normal metal-superconductor” (S-N-S) junction. Good agreement between the experimental and theoretical temperature dependences of the critical current over a wide temperature range for polycrystals with both natural and artificially created boundaries (composites) made it possible to estimate the effective length of grain boundaries in the high-temperature superconductors. It was revealed that the critical current density measured at 4.2 K is an exponential function of the length of grain boundaries in samples of both types.

This paper reports on the results of an investigation into the influence of magnetic fields (0–60 kOe) on the temperature dependences of the electrical resistance R(T) of the Y3/4Lu1/4Ba2Cu3O7 + CuO composites. The structure of these composites is considered to be a network of tunnel-type Josephson junctions in which a nonsuperconducting component (CuO) forms boundaries (barriers) between high-temperature superconducting crystallites. The temperature dependence R(T) of the composites has two steps characteristic of granular superconductors: (i) an abrupt change in the electrical resistance at the critical temperature of high-temperature superconducting crystallites and (ii) a smooth transition to the superconducting state under the influence of the boundaries between the crystallites. The experimental dependences R(T) are analyzed within the Ambegaokar-Halperin model of thermal fluctuations in Josephson junctions and the flux creep model. An increase in the magnetic field leads to a crossover from the Ambegaokar-Halperin mechanism to the flux creep mechanism. The temperature dependences R(T) in the range of weak magnetic fields (from 0 to 102 Oe) are adequately described by the relationship following from the Ambegaokar-Halperin model. In the range of strong magnetic fields (from 103 to 6 × 104 Oe), the dissipation obeys the Arrhenius law R ∼ exp(−U(H)/T)], which is characteristic of the flux creep model with a temperature-independent pinning energy U(H). The effective Josephson coupling energies and the pinning energies corresponding to the Ambegaokar-Halperin and flux creep mechanisms are determined.

The temperature evolution of the current-voltage (I-U) characteristic of a contact of the break-junction type with direct conduction is investigated on a polycrystalline HTSC of the Y-Ba-Cu-O system. The experimental I-U characteristics possessing a hysteresis are correctly described in the framework of the Kummel-Nicolsky theory for an S-N-S contact (S stands for a superconductor; N, for a normal metal) in which the Andreev reflection of quasiparticles from the N-S interface is considered. It is shown that the shape of the I-U curve, as well as the existence of a hysteresis, is determined by the ratio of the number of “long” and “short” intergranular boundaries in the polycrystal under investigation. The coincidence of the calculated and experimental I-U curves made it possible to estimate the effective length of “natural” intergranular boundaries in polycrystalline HTSC materials. The estimate is obtained from the experimental temperature dependence of the critical current in the sample under investigation.

Composites simulating a network of weak metallic links and consisting of a classic 1–2–3 HTSC and a BaPbO3 metal oxide with incorporated Sn, Ni, and Fe impurities have been prepared. Experimental resistivity, magnetic, and Mossbauer studies of the BaPb0.9Met0.1O3 nonsuperconducting components are presented. The transport properties of the HTSC + BaPb0.9Met0.1O3 composites have been investigated. The superconducting properties of the composites are observed to be suppressed, both when the carrier mean free path in nonsuperconducting components with tin impurities decreases, and as a result of an additional interaction of the magnetic moments of (Fe, Ni) impurities with the spins of supercurrent carriers. The experimental temperature dependences of the composite critical current are analyzed in terms of the de Gennes theory for the superconductor-normal metal-superconductor structures.

The magnetoresistance of granular La0.7Ca0.3MnO3 is studied experimentally over wide ranges of temperatures and magnetic fields. The emphasis is on anomalously large hysteresis of magnetoresistance at low temperatures (T = 4.2 K). The observed ρ(H) dependence can be qualitatively explained by spin-dependent tunneling of electrons through the dielectric boundaries of conducting granules characterized by a wide spread in the magnetic-moment magnitudes.

Measurements of the temperature dependence of the electrical resistance R(T) below the superconducting transition temperature have been performed at different values of the transport current in HTSC+CuO composites modeling a network of weak S-I-S Josephson junctions (S—superconductor, I—insulator). It has been shown experimentally that the temperature dependence R(T) at different values of the transport current is adequately described by means of the mechanism of thermally activated phase slippage developed by Ambegaokar and Halperin for tunnel structures. Within the framework of this model we have numerically calculated the temperature dependence of the critical current Jc(T) as defined by various criteria. Qualitative agreement obtains between the measured and calculated temperature dependences Jc(T).

Results are presented of an experimental study of the temperature dependence of the electrical resistivity ρ(T), critical current density Jc(T), and current-voltage characteristics of polycrystalline composites based on the high-temperature superconductor Y3/4Lu1/4Ba2Cu3O7 and copper oxide with different lithium doping levels. The experimental temperature dependence of the critical current of composites with varying volume content of the semiconductor ingredient and varying charge carrier concentration are found to be in qualitative agreement with theory which takes account of Andreev reflection of the carriers at the S-Sm and Sm-S surfaces of the S-Sm-S Josephson junction (where S is the superconductor and Sm is the semiconductor).

Composites are prepared from a high-temperature superconductor with the 1-2-3 structure and the low-temperature oxide BaPb3/4Bi1/4O3 with various volume concentrations of the high-temperature and low-temperature superconductors. The temperature dependences of the electrical resistivities and critical current densities of the composites are measured. It is shown experimentally that these composites comprise a synthetically created network of Josephson weak links (with a specified degree of coupling).

The effect of thermally activated phase slippage (TAPS) on the resistive properties of Y3/4Lu1/4Ba2Cu3O7+O7 composites has been studied. The experimental temperature dependences of the electrical resistance are described in terms of the TAPS mechanism using the temperature dependences of the tunnel-junction critical current.