We have combined magnetic measurements and imaging of the vortex lattice at low fields in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{6\mathrm{-}\mathrm{\delta }}$ single crystals with different oxygen content. Those with high critical temperature ${\mathrm{T}}_{\mathrm{c}}$>80 K have a reversible region while those with low ${\mathrm{T}}_{\mathrm{c}}$<30 K have an irreversible magnetization in the whole range of temperatures, up to ${\mathrm{T}}_{\mathrm{c}}$. The presence of the reversible region allows the formation of ordered vortex structures, while amorphous structures are obtained when no reversible region is present below ${\mathrm{T}}_{\mathrm{c}}$. The results are discussed in terms of vortex nucleation processes in the individual pinning regime.

DOI:https://doi.org/10.1103/PhysRevB.55.14610