  We have studied non-equilibrium phase transitions in the vortex lattice in
superconducting MgB2, where metastable states are observed in connection with
an intrinsically continuous rotation transition. Using small-angle neutron
scattering and a stop-motion technique, we investigated the manner in which the
metastable vortex lattice returns to the equilibrium state under the influence
of an ac magnetic field. This shows a qualitative difference between the
supercooled case which undergoes a discontinuous transition, and the
superheated case where the transition to the equilibrium state is continuous.
In both cases the transition may be described by an an activated process, with
an activation barrier that increases as the metastable state is suppressed, as
previously reported for the supercooled vortex lattice [E. R. Louden et al.,
Phys. Rev. B 99, 060502(R) (2019)]. Separate preparations of superheated
metastable vortex lattices with different domain populations showed an
identical transition towards the equilibrium state. This provides further
evidence that the vortex lattice metastability, and the kinetics associated
with the transition to the equilibrium state, is governed by nucleation and
growth of domains and the associated domain boundaries.
