Magnetic Effects on the Optical Absorption Spectrum of RbNiF3

The polarized optical-absorption spectrum of the hexagonal ferrimagnet RbNi${\mathrm{F}}_3$ is considered for the transitions from the ground state ${}_{}{}^3{}_{}{}^{}A_2^{}{}_{}{}^{}$ to excited states ${}_{}{}^3{}_{}{}^{}T_2^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}{T}_2_{}^a{}_{}{}^{}$. For the ${}_{}{}^3{}_{}{}^{}T_2^{}{}_{}{}^{}$ manifold, an effective Hamiltonian is constructed and its parameters are adjusted to match the levels of the observed spectrum. Estimates are made for the excited-state exchange field; however, because the exchange interaction appears somewhat anisotropic, the preciseness of these estimates is limited by the adequacy of the anisotropic exchange Hamiltonian to describe the effects produced by reorientation of the exchange field direction. Oscillator strengths, calculated from the resulting wave functions are in reasonable agreement with the measured values for this manifold. The reorientation of the exchange field from the basal plane to the $c$-axis direction is seen to manifest itself in a very direct way through the behavior of the oscillator strengths of some of the features in the ${}_{}{}^1{}_{}{}^{}{T}_2_{}^a{}_{}{}^{}$ manifold.