Raman scattering from one and two magnons in magnetoelectric LiNiPO4

Polarized Raman scattering in magnetoelectric LiNiPO4 shows a sharp resonant peak at 37 cm−1 in the magnetically ordered phase, originating from a one-magnon excitation of Ni2+S=1 localized moments at the zone center. Also, a broad component with maximum intensity at ∼65cm−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 cm−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