LAUSR

Using small-angle neutron scattering, we investigated the behavior of a metastable vortex lattice state in MgB2 as it is driven towards equilibrium by an ac magnetic field. This shows an activated behavior, where the ac field amplitude and cycle count are equivalent to, respectively, an effective “temperature” and “time.” The activation barrier increases as the metastable state is suppressed, corresponding to an aging of the vortex lattice. Furthermore, we find a crossover from a partial to a complete suppression of metastable domains depending on the ac field amplitude, which may empirically be described by a single free parameter. This represents an unconventional kind of collective vortex behavior, not governed by pinning, most likely due to the nucleation and growth of equilibrium vortex lattice domains.