  Using a Ginzburg-Landau model, we study the phase transition behavior of
compressible Ising systems at constant volume by varying the temperature $T$
and the applied magnetic field $h$. We show that two phases can coexist
macroscopically in equilibrium within a closed region in the $T$-$h$ plane. It
occurence is favored near tricriticality. We find a field-induced critical
point, where the correlation length diverges, the difference of the coexisting
two phases and the surface tension vanish, but the isothermal magnetic
susceptibility does not diverge in the mean field theory. We also investigate
phase ordering numerically.
