FISHTAIL SHAPE IN THE MAGNETIC HYSTERESIS LOOP FOR SUPERCONDUCTORS - INTERPLAY BETWEEN DIFFERENT PINNING MECHANISMS

The scaling and relaxation behavior around the fishtail minimum is stu died in detail in a wide temperature range (3-70 K) on DyBa2Cu3O7-delt a single crystals exhibiting a pronounced fishtail effect. Magnetic hy steresis loops (MHL's) normalized with respect to the height and posit ion of the fishtail maximum fall on a universal curve which form can b e derived from the phenomenological model of a thermally activated flu x creep proposed by Perkins et al. [Phys. Rev. B 51, 8513 (1995)]. Thi s universal curve tends at low fields towards zero. At low temperature s, 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 simpl e, exponentially decaying field dependence. This implies that the fish tail minimum at low fields might be understood as a result of an overl apping of two contributions originating from separate pinning mechanis ms: one active mainly at high fields and dying away with B going to ze ro and another one (responsible for the central peak of the MHL) vanis hing rapidly with increasing field. This concept is also supported by relaxation experiments. These experiments confirm that the shape of MH L's is given by a dynamic equilibrium between the induction, pinning, and relaxation processes.