  Free energy as a function of polarization is calculated for the
square-lattice $J_1$-$J_2$ Ising model for $J_2 < |J_1|/2$ using the random
local field approximation (RLFA) and Monte Carlo (MC) simulations. Within RLFA,
it reveals a metastable state with zero polarization in the ordered phase. In
addition, the Landau free energy calculated within RLFA indicates a geometric
slab-droplet phase transition at low temperature, which cannot be predicted by
the mean field approximation. In turn, restricted free energy calculations for
finite-size samples, exact and using MC simulations, reveal metastable states
with a wide range of polarization values, but with only two domains. Taking
into account the dependence of the restricted free energy on the
nearest-neighbor correlations allows us to identify several more metastable
states. The calculations also reveal additional slab-droplet transitions at
$J_2 > |J_1|/4$. These findings enrich our knowledge of the $J_1$-$J_2$ Ising
model and the RLFA as a useful theoretical tool to study phase transitions in
spin systems.
