Light Scattering from Phonons and Magnons in RbNiF3

The spectrum of inelastically scattered laser light from RbNi${\mathrm{F}}_3$ is investigated as a function of temperature between 10 and 300°K and of magnetic field between 0 and 50 kOe. RbNi${\mathrm{F}}_3$ crystallizes in a structure isomorphic to the hexagonal modification of BaTi${\mathrm{O}}_3$, space group ${D_{6h}}^{14}$. Since there are six formula units per unit cell, there are 90 phonon modes, of which 33 are predicted to be Raman-active. Of these we have observed and identified 27 Raman-active phonons. Below 139°K, RbNi${\mathrm{F}}_3$ becomes ferrimagnetic. We have observed light scattering from a two-magnon excitation of net spin zero. The identification of this feature is based on the temperature and magnetic field dependence of its frequency and width, which are presented in detail. The behavior of the magnetic scattering, which persists above the transition temperature, is discussed in relation to earlier magnon sideband experiments.