Field-Induced Linear Magnetoelastic Coupling in Multiferroic TbMnO3

We have used in-field neutron and x-ray single-crystal diffraction to measure the incommensurability {delta} of the crystal and magnetic structures of multiferroic TbMnO{sub 3}. We show that the flop in the electric polarization at the critical field HC, for field H along the a and b axes, coincides with a first-order transition to a commensurate phase with propagation vector {kappa}=(0,1/4,0). In-field x-ray diffraction measurements show that the quadratic magnetoelastic coupling breaks down with applied field as shown by the observation of the first harmonic lattice reflections above and below HC. This indicates that magnetic field induces a linear magnetoelastic coupling.