  We report experimental evidence of the effect of an applied magnetic field on
the non-thermal magnetic relaxation in a disk-shaped type-I lead
superconductor. The time evolution of the irreversible magnetization proves to
be logarithmic for a wide range of temperatures and magnetic field values along
the descending branch of the hysteresis cycle. When the intensity of the
magnetic field increases, the crossover temperature separating the thermal and
non-thermal regimes of magnetic relaxation is found to decrease, whereas the
rate at which such relaxation occurs is observed to increase. These results are
discussed in the framework of a recent model for quantum tunneling of
normal-superconductor interfaces through the distribution of pinning energy
barriers generated by structural defects in the sample, considering that the
strength of the barriers decreases with the magnetic field. A phase diagram
describing the dynamics of interfaces during flux expulsion in the intermediate
state as a function of temperature and magnetic field is constructed.
