Magnetic hysteresis and flux creep of melt-powder-melt-growth (MPMG) YBa2Cu3Ox containing nominal 0, 25 and 40 mole% Y2BaCuOs (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 Jc shows a weak field dependence over a wide range of temperature, and the effective pinning energy Ueff is much enhanced. From these results, a functional expression UeffCA T) =-U0 G(J) (J/Ji)n is obtained, where G(T) = [1 - (T/TX)2]2 with Tx = 82.5 K near the irreversibility temperature. The observed power-law relationship of Ueff(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 < JC and J Jc, respectively. In addition, the divergence of Ueff at low current densities also suggests the existence of a vortex-glass state.
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