Magnetic hysteresis and flux creep of melt-powder-melt-growth ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ superconductors

Magnetic hysteresis and flux creep characteristics in melt-powder-melt-growth ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ with ${\mathrm{Y}}_2$ ${\mathrm{BaCuO}}_5$ 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 ${\mathit{J}}_{\mathit{c}}$ values are of the order of ${10}^4$ A/${\mathrm{cm}}^2$ 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 ${\mathit{M}}_0$ 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 ${\mathit{U}}_{\mathit{e}\mathit{f}\mathit{f}}$(J,H)=${\mathit{U}}_{\mathit{i}}$G(T)F(J/${\mathit{J}}_{\mathit{i}}$)/${\mathit{H}}^{0.55}$, where G(T)=1-(T/${\mathit{T}}_{\mathit{x}}$ ${)}^2$, F(J/${\mathit{J}}_{\mathit{i}}$)∼(J/${\mathit{J}}_{\mathit{i}}$ ${)}^{\mathrm{-}\mathit{n}}$, and ${\mathit{U}}_{\mathit{i}}$ and ${\mathit{J}}_{\mathit{i}}$ are scaling constants, is proposed to fit the creep data from which a universal curve is attained. The characteristic temperature ${\mathit{T}}_{\mathit{x}}$ for the scaling function G(T) is determined from the irreversibility line. The current-dependent behavior of ${\mathit{U}}_{\mathit{e}\mathit{f}\mathit{f}}$ 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.