GROWTH MECHANISM AND ANOMALOUS MAGNETIZATION PEAK EFFECT IN SPIRAL-GROWN Bi_2Sr_2CaCu_2O_y CRYSTALS

摘要

Crystals of Bi_2Sr_2CaCu_2O_y (Bi2212) have been grown using KC1 as flux. It was found that the Bi2212 single crystals growth was mainly controlled by a spiral growth mechanism if the crystals were grown at the flux surface. Single, double and multi-spirals with square and polygonal shapes were observed by atomic force microscope and optical microscope. Square shape spirals were observed to be due the (110) growth twins. Magnetic hysteresis loops (M-H) were measured on the spiral-grown Bi-2212 crystals. An anomalous peak effect at magnetic field of 1000-2000 Oe was observed both in high T_c (86 K) and oxygen under-doped (T_c = 76 K) spiral-grown crystals between 20 and 40 K. Surface pinning associated with the growth spirals are suggested to be responsible for the strong peak effect for the spiral-grown Bi-2212 crystals and not oxygen vacancies nor screw dislocations.