Sinusoidal electromagnon in RMnO3: Indication of anomalous   magnetoelectric coupling
  The optical spectra in the family of multiferroic manganites RMnO3 is a great puzzle. Current models can not explain the fact that two strong electromagnons are present in the non-collinear spin cycloidal phase, with only one electromagnon surviving the transition into the collinear spin sinusoidal phase. We show that this is a signature of the presence of anomalous magnetoelectric coupling that breaks rotational invariance in spin space and generates oscillatory polarization in the ground state. 
Markku P. V. Stenberg and Rog\'erio de Sousa
cond-mat.str-el cond-mat.mtrl-sci