On the equilibrium magnetization of high-Tc superconductors below the irreversibility line

By scaling isothermal magnetization data measured at different temperatures in the mixed state of high-Tc superconductors, we show that in some cases the sample magnetization, measured in increasing magnetic field below the irreversibility line, is identical with the equilibrium magnetization even in magnetic fields well within the irreversible regime. This surprising behavior can hardly be explained in terms of traditional models of vortex pinning in the bulk of the sample.