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The experimental results of the effect of the magnetic field (up to 60 kOe) on the broadening of the resistive transition of bulk composites Y 3/4 Lu 1/4 Ba 2 Cu 3 O 7 (YBCO) + CuO are presented. These composites represent the network of the tunnel-type Josephson junctions where the copper oxide acts as a material forming barriers between YBCO crystallites. The mechanisms responsible for broadening of the resistive transition under magnetic field are discussed. The analysis of experimental R ( T ) dependences have shown that in the low field range 0–10 2 Oe, the R ( T ) dependences are described well by the Ambegaokar–Halperin (AH) model. In the range 10 3 –6 × 10 4 Oe, the dissipation follows Arrhenius law R ∼ exp(− U ( H )/ k B T ) characteristic for thermally activated flux creep model. In the range H ∼ 10 2 –10 3 , the crossover from AH to flux creep dissipation mechanisms occurs.

We study the effect of the structure on critical currents and current–voltage characteristics (CVC’s) of foamed bismuth-based polycrystalline high-temperature superconductors (HTSC). The fractal cluster structure of superconducting foams has been observed and the fractal dimension of boundaries between superconducting and normal clusters has been determined. Based on the magnetic and transport properties of the foamed polycrystalline superconductors, we have shown that the initial parts of CVC’s of the superconducting foams are described well by the model that accounts the magnetic flux trapping in fractal clusters of a normal phase.

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.

HTSC Bi 1.8 Pb 0.3 Sr 2 Ca 2 Cu 3 O x of low density has been synthesized and its structural and physical properties have been studied. Scanning electron microscopy (SEM) image shows that the material obtained has porous “foam” structure. The levitation force measured for the porous Bi 1.8 Pb 0.3 Sr 2 Ca 2 Cu 3 O x is appeared to be approximately the same as for polycrystalline YBCO of high density. These “foam” structures may be an ideal material for some practical applications of HTSCs in bulk form.

We have studied the magnetoresistive effect in bulk 1-2-3 YBCO + CuO and 1-2-3 YBCO + BaPb1−xSnxO3 composites prepared using the fast backing technique. We have found that the composites exhibit large magnetoresistance in low magnetic fields (

Y3/4Lu1/4Ba2Cu3O7 + Y3Fe5O12 composites with different volume ratios of the starting components were synthesized. The composites model an S-F-S Josephson junction network, where S stands for a superconductor and F, for a ferrimagnet. A study of the transport characteristics of the composites revealed that the temperature behavior of the electrical resistivity ρ(T) below the superconducting transition point TC is different in two regions separated by a temperature Tm. Below Tm, the current-voltage characteristics of the composites are nonlinear, while in the interval from TC to Tm the values of ρ(T) do not depend on the transport current j and magnetic field H. This behavior of ρ(T, j) and ρ(T, H) is assigned to specific features of the tunneling of superconducting carriers through the ferrimagnetic layers separating HTSC grains in the composite. Magnetic measurements showed the diamagnetic response of HTSC grains to be lower in composites with a ferrimagnet.

We have studied the low-temperature magnetoresistance of Y3/4Lu1/4Ba2Cu3O7-CuO composites obtained by fast sintering technique and established a relation between the probing to critical current density ratio j/jc and the shape of the magnetoresistance curve ρ(H). For j/jc

The results of the study of transport properties (temperature dependences of resistivity and critical current, current-voltage characteristics) of two-phase composites on the basis of Y 3/4 Lu 1/4 Ba 2 Cu 3 O 7 high-temperature superconductor (HTSC) produced by fast baking technique and modelling networks of Josephson-type weak links are reported. Non-superconducting ingredients with different magnetic structures, insulators and normal metals, function as barriers between HTSC crystallites. Besides strong suppression of superconducting properties in HTSC composites with magnetic barriers, anomalous behavior of temperature dependences of resistivity below T c of HTSC grains have been observed and investigated.

The probabilistic model to represent the behavior of an applied multi-agent system that introduces the interaction between a set of agents and Player has been developed within the framework of player-centered probabilistic computer games. The approach features are given with the aid of a game developed for testing cognitive abilities. The agent’s behavior is nondeterministic and therefore unpredictable from Player viewpoint. The system allows both coordinated and autonomous agent’s behavior that depends on availability of information about the presence and position of workable agents for each other. Agent’s behavior is determined with the aid of the algorithm that includes identification of the probabilistic model parameters using maximized objective functions representing individual and group probabilities for Player defeating. Both the model and algorithm ensure the behavior control for relevant applied multi-agent systems.

Composites representing a network of random Josephson junctions and characterized by the compositions 92.5 at. % Y3/4Lu1/4Ba2Cu3O7+7.5 at. % NiTiO3 and 92.5 at. % Y3/4Lu1/4Ba2Cu3O7+7.5 at.% MgTiO3 are synthesized, and their magnetoresistance properties are studied. The temperature dependence of the resistance R(T) measured for the composite that contains the paramagnetic NiTiO3 compound exhibits a characteristic feature below the superconducting transition temperature Tc of the high-Tc superconductor, namely, a region where R is independent of the current j and weakly depends on the magnetic field H. Below a certain temperature Tm, a strong dependence of R on j and H is observed, which is peculiar to a network of Josephson junctions. The dependences R(T, j, H) obtained for the “reference” samples with the nonmagnetic MgTiO3 compound exhibit no such features. The anomalous behavior of the HTSC + NiTiO3 composite is explained by the effect produced by the magnetic moments of Ni atoms in the insulating barriers on the transport current.

The dependence of magnetization on magnetic field M(H) was measured for textured BSCCO samples for H || c, H || ab at different temperatures. Inclusion of the pinning parameter in the Valkov-Khrustalev model [V.V. Valkov, B.P. Khrustalev, JETP 80 (1995) 680] allowed to describe successfully the asymmetric M(H) dependencies. The temperature and magnetic field dependencies of critical current and pinning parameter for H || c and H || ab were estimated., 4 pages, 1 figure, will be published in Physica C

We have studied the magneto-resistive effect in bulk Y3/4Lu1/4Ba2Cu3O7 + CuO composites prepared by the fast-sintering technique. It has been found that the composites exhibit large magnetoresistance in low magnetic fields ( jc (where jc is the critical-current density), large values of the magnetoresistance R0, up to thousands percent, are obtained in the range of weak magnetic fields (tens of oersteds) at 77 K. This effect is attractive for practical applications of these composite materials as active elements of magnetic-field sensors. The sign of the magneto-resistive effect is positive in contrast to that of manganese oxides. This may be important for some devices.

Mn2GeO4 single crystals with the olivine structure grown by the modified flux method have been investigated. Pronounced magnetic phase transitions at T1 = 47.7 K, T2 = 17 K and T3 = 5.5 K, with T2 being dependent on an applied magnetic field, have been found. Based on the data of magnetic, resonance and temperature measurements, the entire phase diagram of Mn2GeO4 has been built. Mn2GeO4 is shown to be a material with a complex magnetic structure consisting of two magnetic subsystems.

We report the results of investigations of the effect of cooling in an external magnetic field starting from the temperature over superparamagnetic blocking temperature TB on the shift of magnetic hysteresis loops in systems of ferrihydrite nanoparticles from ∼2.5 to ∼5 nm in size with different TB values. In virtue of high anisotropy fields of ferrihydrite nanoparticles and open hysteresis loops in the range of experimentally attainable magnetic fields, the shape of hysteresis loops of such objects in the field-cooling mode is influenced by the minor hysteresis loop effect. A technique is proposed for distinguishing the exchange bias effect among the effects related to the minor hysteresis loops caused by high anisotropy fields of ferrihydrite particles. The exchange bias in ferrihydrite is stably observed for particles not less than 3 nm in size or with TB over 40 K, and its characteristic value increases with the particle size.

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.

Inhomogeneous distribution of the pinning force in superconductor results in a magnetization asymmetry. A model considering the field distribution in superconductor was developed and symmetric and asymmetric magnetization loops of porous and textured Bi_{1.8}Pb_{0.3}Sr_{1.9}Ca_{2}Cu_{3}O_{x} were fitted. It is found that the thermal equilibrium magnetization realizes in crystals smaller than some size depending on temperature and magnetic field., 11 pages, 5 figures

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., Comment: 11 pages, 6 figures, will be published in SUST

The current-voltage ($I$-$V$) characteristics of break junctions of polycrystalline La$_{1.85}$Sr$_{0.15}$CuO$_4$, Y$_{0.75}$Lu$_{0.25}$Ba$_2$Cu$_3$O$_{7-\delta}$, Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$ and composite YBa$_2$Cu$_3$O$_{7-\delta}$ + Ag are investigated. The experimental $I$-$V$ curves exhibit the specific peculiarities of superconductor/normal-metal/superconductor junctions. The relation between an $I$-$V$ characteristic of network of weak links and $I$-$V$ dependencies of typical weak links is suggested to describe the experimental data. The $I$-$V$ curves of typical weak links are calculated by the K\"{u}mmel - Gunsenheimer - Nicolsky model considering the multiple Andreev reflections., Comment: 9 pages, 5 figures, 2 Tables

We investigate magnetoresistance, ρc, of single-crystal bilayer lanthanum manganites (La1−zNdz)1.4Sr1.6Mn2O7 (z = 0 and 0.1) at a transport current flowing along the crystal c axis and in external magnetic fields applied parallel to the crystal c axis or ab plane. It is demonstrated that the La1.4Sr1.6Mn2O7 manganite exhibits the positive magnetoresistance effect in the magnetic field applied in the ab sample plane at the temperatures T

The magnetization dynamics of e-Fe2O3 nanoparticles with an average size of about 9 nm is investigated. From comparison of the hysteresis loops obtained in quasi-static conditions and under pulse fields with amplitudes up to 200 kOe and pulse lengths 8–32 ms, it follows that the effective coercivity increases considerably with the variation rate of the imposed magnetic field. A theoretical explanation of this behavior is proposed. The model takes into account the superparamagnetic effects as well as the fact that magnetic anisotropy of the nanoparticles, along with the bulk term, includes a surface contribution. The latter, being of minor importance for the observed magnetic behavior of 25–100 nm particles, becomes essential when the particle size is below 10 nm. From the experimental data, a reference value of the surface anisotropy of nanodisperse e-Fe2O3 is established, and evidence is presented to the effect that below 300 K this contribution does not significantly depend on temperature.

The hysteretic behavior of critical current j C ( H ) and magneto-resistance R ( H ) of composites Y–Ba–Cu–O + CuO have been studied and presented. The composites represent the network of tunnel-type Josephson junctions where copper oxide acts as a material forming barriers between superconducting (YBCO) crystallites. The characteristic features of R ( H ) and j C ( H ) dependences are discussed in the frames of the conception of “two level superconducting system” (the Josephson media and HTSC crystallites) which is realized in the composites under study.

The current-voltage characteristic (CVC) of break junction made from polycrystalline Bi_{1.8}Pb_{0.3}Sr_{1.9}Ca_2Cu_3O_x is investigated. The experimental CVC has hysteretic feature that reflects a part of a curve with negative differential resistance. The CVC is discussed within the framework of the Kummel-Gunsenheimer-Nicolsky theory considering multiple Andreev reflections in superconductor/normal-metal/superconductor junctions., 5 pages, 1 figure

The magneto-resistive effect in bulk 123YBCO + CuO and YBCO + Normal metal BaPbO 3 composites prepared by fast backing technique has been studied. The composites exhibit broadening of resistive transition in low magnetic fields (less than 200 Oe) for a wide temperature range. Experimental dependences of resistivity versus magnetic field at various transport current densities are presented and discussed. The HTSC based composites exhibit a much higher sensitivity to weak magnetic fields at liquid nitrogen temperature, as compared to that for the “pure” HTSC ceramics. This effect is attractive for possible practical applications and may be used in magnetic field sensor devices.

The existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high-TC system Ba0.6K0.4BiO3 (TC30 K) is confirmed on Ba0.6K0.4BiO3 single crystal. At T* (T*

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.

Hysteresis and relaxation of magnetoresistance and magnetization of substituted (La0.5Eu0.5)0.7Pb0.3MnO3 lanthanum manganite in a low-temperature region (t ∼ 102–3×103 s. A comparison between time evolutions of the magnetic moment and resistance shows that magnetoresistance and magnetization are related as δR = δM n , where n = 2.5. The obtained value n is close to the characteristic value n = 2 for tunnel magnetoresistance of granular ferromagnetic metal/insulator systems.

Current–voltage characteristics of the polycrystalline substituted lanthanum manganite La0.7Ca0.3MnO3 were experimentally studied at T = 77.4 K in magnetic fields up to 13 kOe. In these characteristics, a portion of negative differential resistivity was observed above a certain threshold value of critical current density j caused, in our opinion, by nonequilibrium heating of the electron gas due to low thermal conductivity of the manganite material. Because of the nonlinearity of the current–voltage characteristics, the field dependences of resistivity ρ(H) appear extremely sensitive to the value of a transport current. In this case, the ρ(H) dependences reveal both ordinary negative and positive magnetoresistance.

The behavior of temperature dependences of electrical resistance and magnetoresistance of polycrystalline substituted lanthanum manganite (La0.5Eu0.5)0.7Pb0.3MnO3 at low temperatures was thoroughly studied. A broad hysteresis was found in the field dependences of electrical resistance in the low-temperature region. Above 40 K, no hysteresis feature was observed. The temperature T = 40 K corresponds to the temperature of minimum electrical resistance and the temperature TN to the antiferromagnet–paramagnet phase transition of the material of the intergrain boundaries. In this work we propose a model which explains the observed features of the ρ(T) and ρ(H) curves at temperatures below TN by the formation of a network of ferromagnet-antiferromagnet-ferromagnet tunnel contacts.

The effect of grain boundaries on magnetoresistance (MR) of manganites have been investigated by the comparative analysis of the properties of single-crystal and polycrystalline (La0.5Eu0.5)0.7Pb0.3MnO3. While MR of the single crystal is maximum near the Curie temperature and vanishes in the low-temperature region, the polycrystalline (La0.5Eu0.5)0.7Pb0.3MnO3 sample exhibits high MR in the low-temperature region. In order to clarify the origin of the low-temperature MR, the transport and magnetic properties of the polycrystalline (La0.5Eu0.5)0.7Pb0.3MnO3 in magnetic fields have been supplemented by study of magnetic properties and specific heat measurements. The results obtained could be attributed to spin-dependent tunneling between ferromagnetic grains through insulating antiferromagnetic grain boundaries.

In order to clarify the mechanism of hysteretic behavior of magnetoresistance of granular HTSC the magnetoresistance curves R(H) and time evolution of the resistance in constant applied magnetic fields have been studied in granular YBCO at T=77 K. It was found that on ascending branch of R(H) dependence the resistance at H=const decreased with time while on the descending branch the resistance increased with time in applied constant magnetic field. For the range of low magnetic fields (below the minimum point of the descending branch of R(H) dependence) the resistance at H=const decreased again. The behavior observed is well described by the model of granular HTSC, where the intergrain space is in effective magnetic field which is the superposition of the applied field and the field induced by superconducting grains. The time evolution of resistance reflects processes of time relaxation of magnetization of HTSC grains due to the intragrain flux creep.

A new method of preparation of bulk textured Bi2223 ceramics and Bi2223+Ag composites, based on room temperature pressing of foamed precursor (Bi2223, Bi2223+Ag) in a liquid medium, is proposed. SEM and XRD data prove a high degree of texture of the bulk samples obtained. Magnetic measurements performed in directions -axis and -planes of Bi2223 crystallites demonstrates anisotropy of magnetization confirming the texture of the ceramics. The materials obtained possess large diamagnetic response in the direction -planes of Bi2223 crystallites at both 77.4 and 4.2 K.