Magnetic order and ferroelectricity inRMnO3 multiferroic manganites: coupling between R- and Mn-spins

Abstract

Combining polarized and unpolarized neutron scattering techniques with x-ray resonant magneticscattering we have studied the coupling between the Mn- and R-spin-ordering in the multiferroicRMnO3,R = Tb and Dy. Polarized neutron diffraction reveals the moment orientation associatedwith the various modes describing the complex magnetic ordering observed inTbMnO3, while neutron diffraction in high magnetic fields allows the identification of theorigin (Mn versus Tb) of the various modes. In this way we identify significantCx and Fz contributions from Tb arising from the coupling of Tb moments to the Mn cycloidal ordering.The x-ray studies give further insight into this coupling. In the ferroelectric phase, bothTbMnO3 and DyMnO3 show an induced ordering of the R-ion with a propagation vector clamped to the Mn ordering. Whilein TbMnO3 this clamping leads to a ground state in which the two propagation vectorsτTb andτMn obey therelation 3τTb−τMn = 1, in DyMnO3 the ferroelectric polarization is effectively enhanced. The theoreticalanalysis of these effects not only explains the observed behavior forR = Tb and Dy but can also be applied to understand the Mn–R interaction in the related compounds withR = Gd and Ho. Finally we show both experimentally and theoretically how the Mn–Rcoupling can enhance the ferroelectric polarization in this manganite’s multiferroics.