Structure study on multiferroic materials: Hexagonal manganites and PbFe0.5Nb0.5O3

Abstract

Among multiferroic materials that have both ferroelectricity and magnetism, the properties of hexagonal manganite (RMnO3) and perovskite type PbFe0.5Nb0.5O3 were studied by means of a structure analysis. For h-RMnO3 having a unique two dimensional triangular lattice, the structural transition path from the paraelectric P63=mmc structure to the ferroelectric P63cm structure was studied. Not using normal Rietveld refinement but instead with a mode analysis using the peak intensity, we found that the structural transition is a single transition and that the R-OP displacement mode resembles a first-order transition, while the MnO5 tilting mode and the polarization mode behave as second order transitions. Ferroelectric transitions and magnetic property changes were also studied in YMnO3 after doping non-magnetic Al and Ga ion into Mn sites. The R-OP displacement mode, which shows first-order transition behavior, gradually changes to a smooth second-order transition as the Al doping ratio gets increased. Because Al ions are smaller than Mn ions, the local strain induced by the size difference is the likely cause of this outcome. We also note that there are the decrease in magnetic transition temperature, low temperature ferromagnetism and low energy excitation with the doping ratio of Al and Ga. The structure of perovskite type PbFe0.5Nb0.5O3 was studied by both high-resolution neutron diffraction and total scattering methods. Our diffraction study, the structure confirmed Cm structure and by a local structure study, we also found that the local structure showed Pb displacement and oxygen disorder even above the structural transition temperature. In this systematic study of multiferroic materials, I suggest a new methods of structural analysis that will be helpful to those undertaking structural studies of other multiferroic materials.