The effect of ferrimagnetic ordering in insulating component of composites HTSC+Yttrium Iron Garnet on its transport properties

Composites Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Fe₅O₁₂ modeling random network superconductor-ferrimagnetic-superconductor have been prepared and their transport properties (temperature dependences of resistivity ρ and critical current density j_c, current-voltage characteristics) have been studied. Below the superconducting transition temperature T_c, the ρ(T) dependences exhibit a kink at some temperature T_m. The crossover of current-voltage characteristics from ohmic-like behavior in range T_m/T_c to non-linear one below T_m is observed. Transport properties of the ‘benchmark’ composites Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Al₅O₁₂ are typical for network superconductor-insulator-superconductor Josephson junction. The behavior observed for Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Fe₅O₁₂ composites is attributed to peculiarities of supercurrent carriers tunnelling through ferrimagnetic barriers.     

Frustration mechanism of formation of a helical magnetic structure in the CuB2O4 two-subsystem antiferromagnet

A new mechanism is proposed for forming an incommensurate magnetic structure of the transverse helix type owing to the removal of frustration through intersubsystem exchange in a two-subsystem antiferromagnet. The symmetry analysis of the distribution of the Dzyaloshinskii-Moriya interaction and comparison with the experimental data on the field dependence of magnetization indicate that antisymmetric exchange and competition between symmetric exchanges cannot be responsible for forming a helical magnetic structure in CuB₂O₄.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Dominant influence of the compression effect of a magnetic flux in the intergranular medium of a granular high-temperature superconductor on dissipation processes in an external magnetic field

Experiments have been presented that demonstrate the effect of the compression of a magnetic flux in grain boundaries of a granular high-temperature superconductor in an external magnetic field on the dissipation processes. The compression of the magnetic flux is associated with the diamagnetic behavior of superconducting grains and the existence of a Josephson medium in grain boundaries. Under these conditions, grain boundaries are in an effective magnetic field that depends on the magnetic state (magnetization) of the superconducting grains. Based on the analysis of experimental data (dependences of the electrical resistance R and magnetization on the magnetic field H and temperature T, as well as current-voltage characteristics), the conclusion has been drawn that it is the temperature evolution of the effective magnetic field in the intergranular medium which primarily determines the behavior of the dependences R(T) in weak external magnetic fields of no more than ～10³ Oe. This should be taken into account in the interpretation of experiments on the magnetoresistance effect in granular high-temperature superconductors in terms of different theories. The conclusion drawn here also implies a significant correction of the previously obtained results.     

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.     

Transport properties of HTSC + Ba(Pb, Met)O3 composites as functions of the electrical and magnetic characteristics of nonsuperconducting components

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 Mössbauer studies of the BaPb_0.9Met_0.1O₃ nonsuperconducting components are presented. The transport properties of the HTSC + BaPb_0.9Met_0.1O₃ 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.     

Short-circuit current limiter utilizing a high-Tc superconductor

Experimental data are given on a prototype short-circuit current limiter utilizing a polycrystalline high-T _c superconductor of composition Y₁Ba₂Cu₃O_7−δ . The limiter comprises a seriesconnected dc circuit element immersed in liquid nitrogen. To improve the efficiency, a polycrystalline high-T _c superconductor having an S-shaped current-voltage characteristic is used as the current limiter. Zh. Tekh. Fiz. 68, 129–130 (October 1998)     

Transport properties of high-temperature superconductor + semiconductor composites with different carrier concentration

Results are presented of an experimental study of the temperature dependence of the electrical resistivity ρ(T), critical current density J _c(T), and current-voltage characteristics of polycrystalline composites based on the high-temperature superconductor Y_3/4Lu_1/4Ba₂Cu₃O₇ 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). Fiz. Tverd. Tela (St. Petersburg) 39, 829–834 (May 1997) Deceased     

Temperature of the Magnetic Ordering of the Trivalent Iron Oxide ε-Fe2O3

Physics of the Solid State - The trivalent iron oxide ε-Fe2O3 is a fairly rare polymorphic iron oxide modification, which only exists in the form of nanoparticles. This magnetically ordered...