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We show that the usually observed logarithmic U(J) dependence at large magnetic fields is related to the increasing number of free flowing vortices distributed in the weak pinning centers, with respect to the vortices in the strong pinning centers in the flux creep regime. Increasing the flux density (i.e. Increasing the external magnetic field) the flux distribution on the pinning centers has a larger dispersion (/spl sigma//sub u//spl Gt/k/sub B/T) and this leads to a logarithmic U(J) behavior. The comparison with the experimental data seems to prove this idea.