The penetration length λ, 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 reversible magnetization at high magnetic fields. Equilibrium magnetization value was derived from these data. The penetration length λ and the bulk critical field H_cl were determined by fitting the theoretical curves to the reversible magnetization data. The magnetization corresponding to the first maximum is much higher than the calculated value based on the measured equilibrium magnetization at high fields. We conclude that geometrical barrier effects change the magnetization curves drastically.