THE PENETRATION LENGTH-LAMBDA, THE BULK CRITICAL-FIELD H-C1 AND THE GEOMETRICAL BARRIER IN TL2BA2CACU2O8 SINGLE-CRYSTALS

Magnetization curves of Tl2Ba2CaCu2O8 crystal were measured in applied fields up to 8 Tesla. At temperatures above 25 K we observe reversibl e magnetization at high magnetic fields. Equilibrium magnetization val ue was derived from these data. The penetration length lambda and the bulk critical field H-c1 Were determined by fitting the theoretical cu rves to the reversible magnetization data. The magnetization correspon ding to the first maximum is much higher than the calculated value bas ed on the measured equilibrium magnetization at high fields. We conclu de that geometrical barrier effects change the magnetization curves dr astically.