  At room temperature, GdMnO3 is a paraelectric and paramagnetic with a
distorted perovskite structure of orthorhombic symmetry (space group Pnma). On
cooling, it undergoes a phase transition sequence to a magnetic incommensurate
phase (k=delta a*; Tc1=42K) and a A-type antiferromagnetic phase (Tc2=27K). At
low temperatures (T<12K), a magnetic field applied along the a-axis
destabilizes the antiferromagnetic phase and induces a first order transition
to a magnetic commensurate modulated phase (delta=1/4) that is also
ferroelectric (P//c). This work analyses this field induced phase transition
from the point of view of the symmetry and Landau theory.
