To clarify the microscopic origin of the gigantic magnetoelectric effect in multiferroics, we have investigated the variation of an electric polarization ($P$) vector under a rotating magnetic field ($H$) for a cycloidal helimagnet ${\mathrm{Eu}}_{0.55}{\mathrm{Y}}_{0.45}{\mathrm{MnO}}_3$ as a canonical example. $P$ rotates smoothly by rotating $H$ around the magnetic propagation wave vector $k$, which can be well understood by the rotation of the conical spin structure around $k$. We also show that the rotation process of the conical spin structure under $H$ is crucial for the retention or reversal of the spin helicity or equivalently of the direction of $P$.

• Received 8 August 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.197207