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Abstract:
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 TbMnO3. We show that the flop in the
electric polarization at the critical field H-C, 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 H-C. This
indicates that magnetic field induces a linear magnetoelastic coupling.
