M. I. Petrov

Submicron-resolved relief formation in poled glasses and glass-metal nanocomposites

The formation of a spatial relief reproducing that of the anode as a result of the thermal poling of glasses and glass-metal nanocomposites in a strong electric field has been studied by atomic force microscopy. The anode surface patterns exhibited either a square grid or a rectangular grating with a depth of 120 nm, a strip width of 0.5 μm, and a period of 1 μm manufactured using electron-beam lithography and ion etching on the surface of an n-type single crystal silicon wafer. The relief depth formed on the surface of poled samples varied within 5–15 nm, depending on the experimental conditions. The mechanism of relief formation in this system is discussed.

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.

Enhancing of magnetic flux pinning in YBa2Cu3O7−x/CuO granular composites

This paper shows that the combination of self-assembled structures in the form of “whiskers” and nanoparticles, which appear as a result of the joint sintering powders YBa2Cu3O7−x and arc nanopowders CuO, leads to a significant increase in the current density and the appearance of the peak effect in strong magnetic fields range. Very high critical current density appears from a complex vortex pinning, where the defects in the form of “whiskers” provides more energy of pinning, and nanoparticles inhibit the flux creep. Regulation of the morphology of such structures can be achieved by simple change of the concentration of nanodispersed additives. It is shown that the optimal additive is CuO equal to 20 wt. %.

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.

Specific features of the behavior of electroarc CuO nanoparticles in a magnetic field

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 Néel 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.

Composite materials on high-Tc superconductors and BaPbO3, Ag basis

The preparation, processing and characterization of the composite materials on high-T c superconductor (HTSC) basis, YBa 2 Cu 3 O 7 /Ag and Y 0.75 Lu 0.25 Ba 2 Cu 3 O 7 /BaPbO 3 , is reported. The initial components YBa 2 Cu 3 O 7 and BaPbO 3 were prepared by the standard solid state reaction technique. The microstructures of the samples were observed by scanning electron microscopy, whilst the XRD paterns of the composites HTSC+Ag and HTSC+BaPbO 3 revealed the 123 superconducting phase and the Ag or BaPbO 3 structure. The chemical compositions of the powders and the interface zone were found by employing energy dispersive spectrometry. The resistive and magnetic measurements of the composites indicated the transition temperature 93.5 K. Application of such composite materials in the construction of a superconducting fault current limiter model is reported.

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

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.

Synthesis, microstructure, and the transport and magnetic properties of Bi-containing high-temperature superconductors with a porous structure

Preliminary data on the synthesis and physical properties of polycrystalline Bi1.8Pb0.3 Sr2Ca2Cu3Ox high-temperature superconductors of low density with a foam-like microstructure are reported.

Study of magnetic flux pinning in granular YBa2Cu3O7 − y/nanoZrO2 composites

In this work, the effect of ZrO2 nanoparticles prepared in a low-pressure arc discharge plasma on magnetic flux pinning of granular YBa2Cu3O7 − y/nanoZrO2 composites has been studied. It has been shown that the ZrO2 nanoparticles do not change the superconducting transition and the microstructure of superconductors. At a temperature of 5 K, the addition of 0.5 and 1 wt % of ZrO2 nanoparticles may lead to the additional effect of magnetic flux pinning and the increase in the critical current density Jc. The Jc value for composites with 1 wt % is two times larger than that for the reference sample. The fishtail effect is observed for YBa2Cu3O7 − y/nanoZrO2 composites at the temperatures of 20 and 50 K. The problems associated with the additional effect of magnetic flux pinning of granular YBa2Cu3O7 − y/nanoZrO2 composites and the appearance of the fishtail effect have been discussed.