Change in lattice modulation upon gigantic magnetoelectric transition in   GdMnO3 and TbMnO3
  Single-crystal synchrotron x-ray diffraction measurements in strong magnetic fields have been performed for magnetoelectric compounds GdMnO3 and TbMnO3. It has been found that the P || a ferroelectric phase induced by the application of a magnetic field at low temperatures is characterized by commensurate lattice modulation along the orthorhombic b axis with q = 1/2 and q = 1/4. The lattice modulation is ascribed to antiferromagnetic spin alignment with a modulation vector of (0 1/4 1). The change of the spin structure is directly correlated with the magnetic-field-induced electric phase transition, because any commensurate spin modulation with (0 1/4 1) should break glide planes normal to the a axis of the distorted perovskite with the Pbnm space group. 
T.Arima, T.Goto, Y.Yamasaki, S.Miyasaka, K.Ishii, M.Tsubota, T.Inami,   Y.Murakami, and Y.Tokura