Magnetoresistance of superconducting grains in ceramic YBa2Cu3O7-δ high-temperature superconductors in weak magnetic fields

The current-voltage characteristics E(j)H-treat (= const) of ceramic (granular) YBa2Cu3O6.95 samples preliminarily magnetized in different transverse magnetic fields H (treat) have been measured in a zero field (H (ext) = 0) at T = 77.3 K for elucidating specific features of dissipation in superconducting grains of high-temperature superconductors (HTSCs). The current-voltage curves measured in the range 0 <= H (trap) <= H (c2J) (where H (trap) is the magnetic field trapped as a result of the pretreatment in H (treat) and H (c2J) is the upper critical field of the Josephson weak links) have been used to construct the field dependences of the magnetoresistance rho(A)(H (treat)) (j = const) of superconducting grains. It has been established that the magnetoresistance rho(A) of the superconducting grains is significantly lower than the magnetoresistance rho(J) for the Josephson medium. The dependence of rho(A) on H (treat) and on the transport current density j has been investigated. It has been shown that the dependences rho(A)(H (treat)) (j = const) exhibit a clearly pronounced tendency to saturation, rho(satur), and the value of rho(satur) increases with increasing j. It has been found that the lower critical field H (c1A) of the superconducting grains strongly depends on the transport current density.