K. A. Shaikhutdinov

General regularities of magnetoresistive effects in the polycrystalline yttrium and bismuth high-temperature superconductor systems

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.

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

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.

Magnetoresistance hysteresis in granular HTSCs as a manifestation of the magnetic flux trapped by superconducting grains in YBCO + CuO composites

Hysterestic behavior of the magnetoresistance of granular HTSCs and its interaction with the magnetic hysteresis are studied by measuring magnetoresistance R(H) and critical current Ic(H) of composites formed by HTSC Y0.75Lu0.25Ba2Cu3O7 and CuO. A network of Josephson junctions is formed in such composites, in which the nonsuperconducting component plays the role of barriers between HTSC grains. Hysteretic dependences R(H) of magnetoresistance are studied in a wide range of transport current density j and are analyzed in the framework of the two-level model of a granular superconductor, in which dissipation takes place in the Josephson medium and the magnetic flux can be pinned both in grains and in the Josephson medium. The interrelation between the hysteresis of critical current Ic(H) and the evolution of the hysterestic dependence R(H) of the magnetoresistance upon transport current variation is demonstrated experimentally. The effect of the magnetic past history on the hysteretic behavior of R(H) and the emergence of a segment with a negative magnetoresistance are analyzed. It is shown for the first time that the R(H) dependences are characterized by a parameter that is independent of the transport current, viz., the width of the R(H) hysteresis loop.

Mechanism of formation of a negative magnetoresistance region in granular high-temperature superconductors

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.

Current-voltage characteristics of a foamed Bi1.8Pb0.3Sr2Ca2Cu3O x high-temperature superconductor with fractal cluster structure

The influence of the structure of foamed polycrystalline bismuth-based superconductors on their critical currents and current-voltage characteristics is studied. It is found that superconducting foams have a fractal structure, and the fractal dimension of the boundary between the normal and superconducting phases is estimated. The magnetic and transport properties of superconducting foams are investigated, and the current-voltage characteristics are obtained in a wide range of currents. The effect of percolation phenomena on vortex pinning in a foamed superconductor is considered. The current-voltage characteristics of the superconducting foams at the beginning of the resistive transition are found to be in good agreement with a model in which a magnetic flux is assumed to be trapped in the fractal clusters of a normal phase.

Study of the high-coercivity material based on ε-Fe2O3 nanoparticles in the silica gel matrix

We report the results of investigations of ε-Fe2O3 magnetic nanoparticles obtained by incipient wetness impregnation of silica gel. It was established that the obtained samples with an iron content of 12‒16% mass % containing ε-Fe2O3 nanoparticles with an average size of 10 nm on the silica gel surface exhibit a room-temperature coercivity of about 10 kOe. Along with fabrication simplicity, this fact makes the prepared samples promising for application as a magnetically hard material.

Effect of an electric field on the magnetization of a SmFe3(BO3)4 single crystal

A change in the magnetization of a SmFe3(BO3)4 single crystal in response to an applied alternating electric field has been experimentally observed for the first time. The measurements have demonstrated that the magnetization oscillates not only at a frequency of the applied electric field but also at twice the frequency. The dependences of the magnetoelectric effect on the magnetic and electric fields and temperature have been measured. It has been assumed that the existence of the second harmonic of the magnetoelectric effect is due to the electrostriction.

Pinning in a porous high-temperature superconductor Bi2223

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 current-voltage characteristics of a porous superconductor Bi2Sr2Ca2Cu3Ox (Bi2223) have been measured at temperature range from 10 to 90 K. 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 R(I) dependence. The melting of the vortex structure occurs at these temperatures. 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 fractal distribution of pinning centers in a wide range of self-similarity scales.

Angular dependence of the magnetoresistance in Y3/4Lu1/4Ba2Cu3O7-CuO composites at 77 K

The angular dependence of the magnetoresistance of polycrystalline Y3/4Lu1/4Ba2Cu3O7-CuO composites has been studied. These composites represent a system of Josephson junctions and exhibit a large magnetoresistance at 77 K. In addition to the isotropic component, there is the angle-dependent component proportional to sin2ϑ, where ϑ is the angle between the directions of current and magnetic field. This behavior is unambiguous evidence for the process of flux flow in the Josephson medium realized in the composites.

Temperature evolution of the hysteresis in the current-voltage characteristic of a polycrystalline high-temperature superconductor with 1-2-3 structure

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 Kümmel-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.