  The Landau theory was applied to treat the phase diagrams for a multiferroic
with two second order phase transitions taking into account the coupling of the
primary order parameters with strain. Two order parameters are coupled
biquadratically which corresponds to the magnetoelectric materials. The
coupling with strain is assumed to be linear in strain and quadratic in order
parameters. Three ordered phases are discussed. Analytic relationships were
obtained for the phase transition temperatures and for elastic modulus changes
through the phase transitions. Strong influence of the coupling with strain on
the phase diagrams was shown.
