  We investigate nonthermal phase transitions that may occur after
post-inflationary preheating in a simple model of a two-component scalar field
with the effective potential $\lambda (\phi_i^2 - {\rm v}^2)^2/4$, where
$\phi_1$ is identified with the inflaton field. We use three-dimensional
lattice simulations to investigate the full nonlinear dynamics of the model.
Fluctuations of the fields generated during and after preheating temporarily
make the effective potential convex in the $\phi_1$ direction. The subsequent
nonthermal phase transition with symmetry breaking leads to formation of cosmic
strings even for ${\rm v} \gg 10^{16}$ GeV. This mechanism of string formation,
in a modulated (by the oscillating field $\phi_{1}$) phase transition, is
different from the usual Kibble mechanism.
