Magnetic hysteresis and flux creep of YBa2Cu3O(x) grown by the melt-powder-melt-growth (MPMG) process

摘要

Magnetic hysteresis and flux creep of melt-powder-melt-growth (MPMG) YBa(sub 2)Cu(sub 3)O(sub x) containing nominal 0, 25 and 40 mole% Y(sub 2)BaCuO(sub 5) (211) were measured in the temperature range of 5 to 80 K and in magnetic fields up to 5 T. With the introduced fine dispersion of second phase 211 particles, the critical magnetization current density J(sub c) shows a weak field dependence over a wide range of temperature, and the effective pinning energy U(sub eff) is much enhanced. From these results, a functional expression U(sub eff)(J,T) = (minus) U(sub o) G(T) (J (vert bar)J(sub i))(sup n) is obtained, where G(T) = (1 (minus) (T(vert bar)T(sub x))(sup 2))(sup 2) with Tx = 82.5 K near the irreversibility temperature. The observed power-law relationship of U(sub eff)(J, T) clearly demonstrates two of three regimes as predicted by the theory of collective flux creep, namely n = 3/2 and 7/9 for J < J(sub c) and J (much lt) J(sub c), respectively. In addition, the divergence of U(sub eff) at low current densities also suggests the existence of a vortex-glass state.