Excitations of isolated clusters of Co2+ magnetic ions in KZn0.92Co0.08F3

Abstract

Inelastic neutron scattering from clusters of ${\mathrm{Co}}^{2+}$ ions in cubic KZn${\mathrm{F}}_3$ is reported. From measurements of the lowest pair mode at (1.676±0.008) THz, we find that the exchange between ${\mathrm{Co}}^{2+}$ ions is surprisingly isotropic, with any bond anisotropy being less than 2.4%. An accurate value of the isotropic exchange parameter has been obtained [$J=(0.330\mathrm{}\pm 0.005)$ THz] free of the many-body effects that complicate the interpretation of exchange coupling in concentrated crystals. Neutron scattering was also observed from triplet and quartet states at (2.47±0.02) and (1.09±0.05) THz, respectively, and a higher-frequency pair state at 8.8 THz. The results are interpreted in terms of an elementary theory of cluster spectra for ions with nonzero angular momentum.