Raman scattering from one and two magnons in magnetoelectric LiNiPO4

Polarized Raman scattering in magnetoelectric ${\mathrm{LiNiPO}}_{4}$ shows a sharp resonant peak at 37 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the magnetically ordered phase, originating from a one-magnon excitation of ${\mathrm{Ni}}^{2+}\phantom{\rule{4pt}{0ex}}S=1$ localized moments at the zone center. Also, a broad component with maximum intensity at $\ensuremath{\sim}65\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ is observed and successfully modeled in terms of light scattering from two-magnon excitations within the framework of the Fleury-Loudon theory using five relevant exchange parameters, providing an independent experimental confirmation of their values previously obtained with inelastic neutron scattering data in this material. An additional peak at 58 ${\mathrm{cm}}^{\ensuremath{-}1}$, already reported in previous works, shows no detectable Zeeman splitting for magnetic fields up to 6 T along the crystallographic a and c directions, excluding one-magnon scattering as a possible assignment for this peak. The possible nature of this excitation is discussed.