Rotation of an electric polarization vector by rotating magnetic field in cycloidal magnet Eu_0.55Y_0.45MnO₃

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 Eu_0.55Y_0.45MnO₃ 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.