  Muon spin rotation and resonant soft X-ray scattering experiments on
prototype multiferroics RMn2O5 (R = Y, Sm) are used to demonstrate that the
local electric displacements are driven by the spin-current (SC) mechanism.
Small local electric displacements were evaluated by observing spin
polarization at ligand O ions, for which implanted muons served as an extremely
sensitive probe. Our results for YMn2O5 provide evidence that the spin
polarization of O ions forming a spin cycloid chain with Mn spins increases in
proportion to the vector spin chirality (Si x Sj ) of the Mn ions. This
relationship strongly indicates that the charge transfer between O and Mn ions
is driven by the SC mechanism, which leads to the ferroelectricity accompanying
O spin polarization.
