Light Scattering by Phonons and Magnons in NaNiF3

The weak four-sublattice ferromagnet NaNi${\mathrm{F}}_3$ shows one- and two-magnon Raman scattering. The two-magnon scattering is perfectly described by a two-sublattice isotropic cubic antiferromagnet model with one exchange integral between magnetic neighbors ($J=41.8\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$). The two one-magnon low-frequency lines, centered at 6.5 and ${11.5}_5$ ${\mathrm{cm}}^{-1\mathrm{}}$ have been studied as a function of the direction and the amplitude of an applied magnetic field and versus the temperature. A two-sublattice model, taking into account the isotropic exchange, the antisymmetric and symmetric anisotropic exchanges, and the single-ion anisotropy, provides a satisfactory agreement with the experiment. The most important term after the isotropic exchange is the Dzyaloshinski—Moriya antisymmetric exchange, but other coefficients are necessary to give an account of the observed properties: Seven of them were derived from our experimental data, which enables us to explain all the known magnetic properties of NaNi${\mathrm{F}}_3$. Besides this we discuss the one-phonon Raman scattering of NaNi${\mathrm{F}}_3$.