Magnetic excitation mode in multiferroics Ba₂CoGe₂O₇

Magnetic excitation mode in the multiferroic Ba₂CoGe₂O₇ are clarified by the unpolarized and polarized neutron scattering measurements. The unpolarized neutron spectrum reveals the existences of two dispersive excitations below 2.5 meV and one dispersionless excitation at 4 meV. By analyzing the polarized neutron spectra, the mode of the observed three excitations are identified. Three fluctuations, the transverse fluctuation in the a-b plane, that in the c-direction, and the longitudinal fluctuation of the ordered moment, are involved in the magnetic excitation of Ba₂CoGe₂O₇. The determined modes are consistent with the calculation by the extended spin-wave theory. The longitudinal fluctuation takes on a main role in the electromagnon mode of Ba₂CoGe₂O₇. (author)