We report on a new type of correlated nanometer-scale pinning structure observed in a melt-processed (Nd0.33Eu0.38Gd0.28)Ba(2)Cu(3)O(y) (NEG-123). It consists of NEG/Ba-rich clusters in the stoichiometric NEG-123 matrix forming a lamellar array with a period of a few nanometers. These lamellas appear within regular twins, thus representing their fine substructure-sometimes straight, sometimes wavy. This new material structure correlates well with the significant enhancement of pinning at high fields, represented by irreversibility field above 14 T at 77 K (B parallel c). We believe that the new pinning medium enables one to significantly broaden the limits for high-field applications.

The scaling and relaxation behavior around the fishtail minimum is studied in detail in a wide temperature range ͑3-70 K͒ on DyBa 2 Cu 3 O 7Ϫ␦ single crystals exhibiting a pronounced fishtail effect. Magnetic hysteresis loops ͑MHL's͒ normalized with respect to the height and position of the fishtail maximum fall on a universal curve which form can be derived from the phenomenological model of a thermally activated flux creep proposed by Perkins et al. ͓Phys. Rev. B 51, 8513 ͑1995͔͒. This universal curve tends at low fields towards zero. At low temperatures, the drop of j s at low fields is usually masked by a wide central peak. By subtracting the universal curve from the experimental j s (B) data we separate the contribution of the central peak. It has a simple, exponentially decaying field dependence. This implies that the fishtail minimum at low fields might be understood as a result of an overlapping of two contributions originating from separate pinning mechanisms: one active mainly at high fields and dying away with B going to zero and another one ͑responsible for the central peak of the MHL͒ vanishing rapidly with increasing field. This concept is also supported by relaxation experiments. These experiments confirm that the shape of MHL's is given by a dynamic equilibrium between the induction, pinning, and relaxation processes. ͓S0163-1829͑97͒05305-8͔

No abstract