EFFECT OF THE REVERSIBILITY REGION ON THE LOW-TEMPERATURE VORTEX STRUCTURE IMAGED BY BITTER MAGNETIC DECORATION

We have combined magnetic measurements and imaging of the vortex latti ce at low fields in Tl2Ba2CuO6-delta single crystals with different ox ygen content. Those with high critical temperature T-c > 80 K have a r eversible region while those with low T-c < 30 K have an irreversible magnetization in the whole range of temperatures, up to T-c. The prese nce of the reversible region allows the formation of ordered vortex st ructures, while amorphous structures are obtained when no reversible r egion is present below T-c. The results are discussed in terms of vort ex nucleation processes in the individual pinning regime.