Field induced linear magneto-elastic coupling in multiferroic TbMnO3
  We have used in-field neutron and X-ray single crystal diffraction to measure the incommensurability &#948; of the crystal and magnetic structure of multiferroic TbMnO3 . We show that the flop in the electric polarization at the critical field HC, for field H along the a&#8722; and b&#8722;axis coincides with a 1st order transition to a commensurate phase with propagation vector &#954; = (0, 1/4, 0). In-field X-ray diffraction measurements show that the quadratic magneto-elastic coupling breaks down with applied field as shown by the observation of the 1st harmonic lattice reflections above and below HC . This indicates that magnetic field induces a linear magneto-elastic coupling. We argue that the commensurate phase can be described by an ordering of Mn-O-Mn bond angles. 
N. Aliouane, D. N. Argyriou, J. Strempfer, I. Zegkinoglou, S.   Landsgesell, and M. v. Zimmermann