Abstract
How magnetoelectric coupling actually occurs on a microscopic level in multiferroic BiFeO3 is not well known. By using high-resolution single crystal neutron diffraction techniques, we have determined the electric polarization of each individual element of BiFeO3, and concluded that magnetostrictive coupling suppresses the electric polarization at the Fe site below T-N. This negative magnetoelectric coupling appears to outweigh the spin current contributions arising from the cycloid spin structure, which should produce positive magnetoelectric coupling.