Local magnetic measurements in Nd1.85Ce0.15CuO4-δ crystals show a 'fishtail' anomaly in the magnetization curves together with anomalous relaxation behavior similar to that measured in YBa2Cu3O7-x crystals, suggesting a universal flux dynamics in the field range of the fishtail peak.
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circles in Figure 1 describe the field dependence of the exponent p obtained by fitting Eq. (1) to the experimental data. At low fields p changes from about 0.2 to the highest value of more than 1 in agreement with the collective creep theory - these values correspond to the crossover from the single vortex creep regime to the bundle regime 151. However, at higher fields p decreases down to values less than 0.2 and it would imply an inconceivable crossover to a single vortex regime (p = l/7) which is expected only for low fields and high values of j. Thus, the p values at high fields are inconsistent with the collective creep theory. As argued by Abulafia et al. in their measurements of YBCO [21i,t is possible to explain this behavior as indicating an elastic-to-plastic creep crossover. As compared to YBCO, NCCO exhibits smaller T,, larger anisotropy 161,a nd lower field range for the fishtail anomaly. Moreover, the origin of the fishtail in NCCO and YBCO may be different . Yet, in both cases, similar anomaly in the relaxation is observed in the field range of the fishtail. The connection between these two phenomena requires further investigation. This work was supported in part by the Israel Science Foundation and by the Heinrich Hertz Minerva Center for High Temperature Superconductivity. Y.Y. and E.Z. acknowledge support of the U.S.A.-Israel Binational Science Foundation.