On the magnetically driven ferroelectric phase in GdMnO3

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.