Strong coupling between the spin polarization of Mn and Tb in multiferroic TbMnO₃ determined by x-ray resonance exchange scattering

We report on an x-ray resonance exchange scattering (XRES) study of multiferroic TbMnO₃. Magnetic scattering is observed close to the Mn K edge and the Tb L_III and L_II edges. Surprisingly, Tb shows XRES also in the paraelectric phase, where neutron diffraction results suggested the 4f moments to be disordered. The temperature dependence of the XRES intensities shows a distinct kink close to the ferroelectric transition temperature T_C for all absorption edges. We were able to model the temperature dependence for all edges, assuming a strong coupling of Mn and Tb magnetism via the spin polarized 5d conduction band. We argue that the hybridization between Mn and Tb states is a prerequisite for the multiferroic effect in this material.