Light Scattering by Phonons and Magnons in NaNiF3

Abstract

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 ($J=41.8$ ${\mathrm{cm}}^{\ensuremath{-}1}$). The two one-magnon low-frequency lines, centered at 6.5 and ${11.5}_{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ 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${\mathrm{F}}_{3}$.