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 nor
mal 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.
Magnetoresistance of substituted lanthanum manganite (La0.5Eu0.5)0.7Pb0.3MnO3 in the pulse magnetic field H = 25 T was measured at different temperatures. Magnetoresistance relaxation with a characteristic time of 10 -3 s was found. It has been estab
lished that the temperature dependence of the relaxation parameter {tau}(t) at different temperatures correlates with the temperature dependence of electrical resistance R(T). The proposed relaxation mechanism is related to relaxation of conducting and dielectric phases in the sample volume under phase stratification conditions. It is shown that relaxation parameter {tau} reflects the number of boundaries in the volume and not the ratio between phase fractions.
