  Detailed spin-wave spectra of magneto-electric LiNiPO4 have been measured by
neutron scattering at low temperatures in the commensurate (C)
antiferromagnetic (AF) phase with ordering temperature 20.8 K. An anomalous
low-energy mode is observed at the modulation vector of the incommensurate (IC)
AF phase appearing above the 20.8 K. A linear spin-wave model based on
Heisenberg exchange couplings and single ion anisotropies accounts for all the
observed spin-wave dispersions and intensities. Along the b axis an unusually
strong next-nearest-neighbor AF coupling competes with the dominant
nearest-neighbor AF exchange interaction and causes the IC structure.
