FLUX-CREEP AND FISHTAIL IN MELT-TEXTURED AND SINGLE-CRYSTALLINE YBA2CU3O7

Time-dependent decay of the magnetic moment and magnetization measurem ents were used to study pinning and flux creep in two samples with and without a j(B) maximum in nonzero field (fishtail effect). From both measurements the E(j) relation was reconstructed for B \\ c and the ch aracteristic current exponent mu was obtained. At high j and low B mu values are between 1 and 4; with increasing B mu passes a maximum and approaches negative values. At low j mu is below 0.5 and not dependent on B or j. This behavior, which is qualitatively the same in both sam ples, is compared with proposed phase diagrams of the vortex lattice. Large mu values are correlated with the plateau of the normalized cree p rate S almost-equal-to 0.025; both observations indicate low relaxat ion and are found in that low B region for which j(B) has its minimum. This observation rejects a dominating influence of relaxation on the fishtail effect.