Comparative studies of the fishtail effect associated with surface pinning and oxygen vacancy network in spiral and layer-by-layer grown single crystals

Abstract

Magnetic hysteresis loops (M-H) between 5 and 50 K were measured on (Bi-2212) crystals grown by the spiral or layer-by-layer growth mechanism using KCl flux and self-flux with a large temperature gradient growth technique. The spiral-grown crystals with a large density of spiral steps showed a strong fishtail effect with at 1000-2000 Oe between 20 and 50 K, for both high- (86 K) and low- (76 K, oxygen underdoping) samples. For the layer-by-layer-grown crystals with an extremely smooth surface and annealed in oxygen-nitrogen, a weak fishtail effect with at 300 Oe was observed between 20 and 40 K. The fishtail effect disappeared when the spirals were removed from the crystal surface, whereas the fishtail effect for the layer-by-layer-grown crystals was mainly controlled by oxygen content. The 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 surface pinning or oxygen vacancies for spiral- and layer-by-layer-grown crystals. 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 networking of these vacancies may play a dominant role.