  We study the dynamical response of a two-dimensional Ising model subject to a
square-wave oscillating external field. In contrast to earlier studies, the
system evolves under a so-called soft Glauber dynamic [P.A. Rikvold and M.
Kolesik, J. Phys. A: Math. Gen. 35, L117 (2002)], for which both nucleation and
interface propagation are slower and the interfaces smoother than for the
standard Glauber dynamic. We choose the temperature and magnitude of the
external field such that the metastable decay of the system following field
reversal occurs through nucleation and growth of many droplets of the stable
phase, i.e., the multidroplet regime. Using kinetic Monte Carlo simulations, we
find that the system undergoes a nonequilibrium phase transition, in which the
symmetry-broken dynamic phase corresponds to an asymmetric stationary limit
cycle for the time-dependent magnetization. The critical point is located where
the half-period of the external field is approximately equal to the metastable
lifetime of the system. We employ finite-size scaling analysis to investigate
the characteristics of this dynamical phase transition. The critical exponents
and the fixed-point value of the fourth-order cumulant are found to be
consistent with the universality class of the two-dimensional equilibrium Ising
model. As this universality class has previously been established for the same
nonequilibrium model evolving under the standard Glauber dynamic, our results
indicate that this far-from-equilibrium phase transition is universal with
respect to the choice of the stochastic dynamics.
