Magnetic hysteresis and flux creep of melt-powder-melt-growth YBa2Cu3O7 superconductors

Magnetic hysteresis and flux creep characteristics in melt-powder-melt- growth YBa2Cu3O7 with Y2BaCuO5 inclusions were measured between 5 and 80 K for magnetic fields up to 5 T. The critical magnetization current densities which were calculated using the Bean model and the sample dimension show a weak dependence on magnetic field. The Jc values are of the order of 104 A/cm2 at 60 K over the investigated field range. The magnetic relaxation rate Q=-dM/d lnt first drops monotonically with increasing temperatures and then gradually saturates, which is similar to the behavior of the initial magnetization M0 versus temperature. The normalized relaxation rate S=d lnM/d lnt, plotted as a function of temperature, shows a maximum at around 30 K, and the corresponding values of S and T at the maximum are field dependent. A scaling relationship Ueff(J,H)=UiG(T)F(J/Ji)/H0.55, where G(T)=1-(T/Tx)2, F(J/Ji)(J/Ji)-n, and Ui and Ji are scaling constants, is proposed to fit the creep data from which a universal curve is attained. The characteristic temperature Tx for the scaling function G(T) is determined from the irreversibility line. The current-dependent behavior of Ueff obtained from this work agrees qualitatively with the result predicted by the theory of collective flux creep and suggests a vortex-glass state at low current density.