COMPARATIVE-STUDIES OF THE FISHTAIL EFFECT ASSOCIATED WITH SURFACE PINNING AND OXYGEN VACANCY NETWORK IN SPIRAL AND LAYER-BY-LAYER GROWN BI2SR2CACU2OY SINGLE-CRYSTALS

Magnetic hysteresis loops (M-H) between 5 and 50 K were measured on Bi (2)Sr(2)CaCu(2)Oy (Bi-2212) crystals grown by the spiral or layer-by-l ayer growth mechanism using KCI flux and self-flux with a large temper ature gradient growth technique. The spiral-grown crystals with a larg e density of spiral steps showed a strong fishtail effect with H-peak at 1000-2000 Oe between 20 and 50 K, for both high-T-c (86 K) and low- T-c (76 K, oxygen underdoping) samples. For the layer-by-layer-grown c rystals with an extremely smooth surface and annealed in oxygen-nitrog en, a weak fishtail effect with H-peak at 300 Oe was observed between 20 and 40 K. The fishtail effect disappeared when the spirals were rem oved from the crystal surface, whereas the fishtail effect for the lay er-by-layer-grown crystals was mainly controlled by oxygen content. Th e peak effect is fully reversible in the layer-by-layer-grown crystals by a proper annealing in oxygen and in nitrogen. From this comparison we conclude that the peak effect in Bi-2212 is caused by either surfa ce pinning or oxygen vacancies for spiral- and layer-by-layer-grown cr ystals. Furthermore, the TEM study helps to show that the dislocation networks are not responsible for the fishtail effect. In the layer-by- layer-grown crystals, the presence of oxygen vacancies is a necessary but not a sufficient condition for the fishtail peak effect, but the n etworking of these vacancies may play a dominant role.