Magnetic hysteresis loops between 5 and 50K were measured in crystals which had been grown via the spiral or layer-by-layer growth mechanisms, using a KCl flux or a self-flux and a high temperature gradient. Spiral-grown crystals, with a high density of spiral steps, exhibited a strong fishtail effect in samples with both low and high critical temperatures. In the case of layer-by-layer grown crystals, with an extremely smooth surface, which were annealed in O-N, a weak fishtail effect was observed between 20 and 40K. The fishtail effect disappeared when the spirals were removed from the crystal surface. The fishtail effect for layer-by-layer grown crystals was controlled mainly by the O content. The peak effect was fully reversible, in the layer-by-layer grown crystals, by suitable annealing in O and N. On the basis of this comparison, it was concluded that the peak effect was caused by surface pinning or by O vacancies in spiral-grown and layer-by-layer grown crystals. Transmission electron microscopy showed that dislocation networks were not responsible for the fishtail effect. In the case of layer-by-layer grown crystals, the presence of O vacancies was a necessary but not a sufficient condition for the fishtail peak effect; but the networking of these vacancies could play a dominant role.

Comparative Studies of the Fishtail Effect Associated with Surface Pinning and Oxygen Vacancy Network in Spiral and Layer-by-Layer Grown Bi2Sr2CaCu2Oy Single Crystals. X.L.Wang, M.Ionescu, J.Horvat, X.Z.Liao, H.K.Liu, S.X.Dou: Superconductor Science and Technology, 1998, 11[10], 1041-4