Molecular-spin dynamics study of electromagnons in multiferroic RMn2O5

We investigate the electromagnon in magnetoferroelectrics RMn2O5 using combined molecular-spin dynamics simulations. We confirm that the origin of the electromagnon modes observed in the optical spectra is due to the exchange-striction interaction between the magnons and the phonons, and the dielectric step at the magnetic phase transition is due to the appearance of the electromagnon in the low-temperature phase in these materials. The magnetic anisotropy breaks the rotational symmetry of the magnetic structures and, as a result, the electromagnon splits into three modes in RMn2O5. We find that the electromagnon frequencies are very sensitive to the magnetic wavevector along the a direction qx. Therefore, the electromagnon frequencies of TmMn2O5 (qx ∼ 0.467) are expected to be much higher than those of other materials of the family, such as R= Tb, Y, Ho, etc (qx ∼ 0.48). We further calculate the electromagnons in the magnetic field, and find a new mode appearing in the magnetic field. Although the modes' frequencies change significantly under magnetic field, the total static dielectric constant contributed from the electromagnons does not change much in the magnetic field, suggesting that the colossal magnetodielectric effects in these materials may not be caused by the electromagnons.