Light Scattering from Magnons and Excitons in RbCoF3

We have determined the energies and linewidths of excitons and of one- and two-magnon states in the antiferromagnetic perovskite RbCo${\mathrm{F}}_3$, by laser-light scattering experiments. Because ${\mathrm{Co}}^{2+}$ ions at octahedral sites have orbital angular momenta which are unquenched by the crystal field, the magnetic excitons in RbCo${\mathrm{F}}_3$ are not pure spin waves, but posses an orbital character. We have measured the energies and widths of such excitons as functions of temperature from 4 °K to above the Néel temperature $T_N=(101\mathrm{}\pm 2)$ °K. The results are compared with pure-spin systems, such as Fe${\mathrm{F}}_2$ and Mn${\mathrm{F}}_2$, and with the known temperature dependence of the RbCo${\mathrm{F}}_3$ sublattice magnetization. For the magnon-pair scattering, a moment analysis of the line shape has been performed from 4 °K to $1.3T_N$; the results are in qualitative agreement with those of Brya et al. on Mn${\mathrm{F}}_2$. Measurement of the energies and selection rules for high-energy ($\sim 1000$-${\mathrm{cm}}^{-1\mathrm{}}$) excitons has allowed us to conclude that the spin-orbit parameter ($\lambda \approx -146\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$) is about equal to that in KCo${\mathrm{F}}_3$. By comparison, the magnon-pair energy (336 ${\mathrm{cm}}^{-1\mathrm{}}$ at 4 °K) indicates that the exchange constant ($J\approx -9\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$) is about 15% less than in KCo${\mathrm{F}}_3$, in good agreement with the ratio of Néel temperatures for the two isomorphs. The magnon energy is observed to renormalize very rapidly with increasing temperature (31 ${\mathrm{cm}}^{-1\mathrm{}}$ at 4 °K to 17 ${\mathrm{cm}}^{-1\mathrm{}}$ at 70 °K), more rapidly by far than the sublattice magnetization. The tetragonal lattice distortion at $T_N$ is inferred to be unrelated to "soft"-phonon modes, but to arise instead from a Jahn-Teller distortion; Kanamori's theory of Jahn-Teller effects in crystals such as CoO and RbCo${\mathrm{F}}_3$ with unquenched orbital angular momentum gives the right sign and magnitude for the $\frac{\mathrm{}(a-c)\mathrm{}}{a}$ distortion.