We have used in-field neutron and X-ray single crystal diffraction to measure the incommensurability δ 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− and b−axis coincides with a 1st order transition to a commensurate phase with propagation vector κ = (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.
@article{arxiv.cond-mat/0508634,
title = {Field induced linear magneto-elastic coupling in multiferroic TbMnO3},
author = {N. Aliouane and D. N. Argyriou and J. Strempfer and I. Zegkinoglou and S. Landsgesell and M. v. Zimmermann},
journal= {arXiv preprint arXiv:cond-mat/0508634},
year = {2009}
}