  The vortex lattice in MgB2 is characterized by the presence of long-lived
metastable states, which arise from cooling or heating across the equilibrium
phase boundaries. A return to the equilibrium configuration can be achieved by
inducing vortex motion. Here we report on small-angle neutron scattering
studies of MgB2, focusing on the structural properties of the vortex lattice as
it is gradually driven from metastable to equilibrium states by an AC magnetic
field. Measurements were performed using initial metastable states obtained
either by cooling or heating across the equilibrium phase transition. In all
cases, the longitudinal correlation length remains constant and comparable to
the sample thickness. Correspondingly, the vortex lattice may be considered as
a system of straight rods, where the formation and growth of equilibrium state
domains only occurs in the two-dimensional plane perpendicular to the applied
field direction. Spatially resolved raster scans of the sample were performed
with apertures as small as 80 microns, corresponding to only 1.2*10^6 vortices
for an applied field of 0.5 T. These revealed spatial variations in the
metastable and equilibrium vortex lattice populations, but individual domains
were not directly resolved. A statistical analysis of the data indicates an
upper limit on the average domain size of approximately 50 microns.
