Origin of the large ferroelectric polarization enhancement under high pressure for multiferroic DyMnO3 studied by polarized and unpolarized neutron diffraction

The multiferroic perovskite rare earth manganites RMnO3 (R=Dy, Tb, Gd) are known as multiferroics exhibiting pressure-induced gigantic ferroelectric polarization. In this study, we have investigated the magnetic orderings in the pressure-induced phases for DyMnO3, by neutron diffraction and spherical neutron polarimetry (SNP) experiments up to 8.0 GPa. The magnetic ordering for Mn spins changes from the incommensurate bc-cycloid to the commensurate collinear E-type structure with kMn=0,12,0 above 4.0 GPa, which is concomitant with the appearance of a giant ferroelectric polarization. The magnetic ordering for the Dy spins has been determined to be a noncollinear spin structure with a and b spin components and kDy=(0,12,0) for the low- and high-pressure phases. The magnetic field along the a axis, Ha, affects the Dy ordering, which is seen in the changes in the k vector from kDy=(0,12,0) in Ha≤3T to kDy=(0,0,0) in Ha≥3T. Considering the lattice distortion generated by the determined magnetic orderings through the exchange striction mechanism, we conclude that the exchange striction for rare earth and Mn bonds, which is added to the uniform polarization generated by the E-type Mn ordering, is strongly related to the significant magnetic field enhancement of ferroelectric polarization in the high-pressure phase of the rare earth manganites.