Raman scattering from one and two magnons in magnetoelectric LiNiPO4

Polarized Raman scattering in magnetoelectric LiNiPO4 shows a sharp resonant peak at 37cm−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 58cm−1, already reported in previous works, shows no detectable Zeeman splitting for magnetic fields up to 6T 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.