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