Weak exchange striction between the 4f and 3d ions in the multiferroic GdMn2 O5

We used single-crystal neutron diffraction to investigate the magnetic structures of the multiferroic GdMn2O5. The system undergoes a first-order incommensurate to commensurate magnetic transition below 33 K, accompanied by the appearance of electric polarization P. Upon cooling, P increases smoothly while the magnetic order shows an abrupt enhancement in intensity below 20 K owing to the large increase of the rare-earth Gd moment. The contrasting temperature evolution of the magnetic order and polarization indicates the polarization is mainly driven by the exchange striction between the magnetic Mn ions. The incommensurate phase in the intermediate temperature range has a cycloidal modulation along the c axis, with spin configuration projected in the ab plane similar to the commensurate phase. The lack of observable bulk polarization suggests the exchange striction is not the dominant mechanism for the polarization in the incommensurate phase.