The magnetism in ${\mathrm{RbCoCl}}_3$⋅${2\mathrm{H}}_2$O ($T_N$=2.97 K) is studied by high-temperature dynamic susceptibility and low-temperature electron-spin resonance. In order to describe the experiments a Hamiltonian is derived that includes spin-orbit coupling, tetragonal and rhombic distortions, exchange in the mean-field approximation, and an external magnetic field. A simultaneous fit to three mutual perpendicular susceptibilities yields consistent values for the crystal-field parameters and an intrachain exchange constant of J/$k_B$=-5.0 K. The final Hamiltonian in the effective-spin s=(1/2) formalism is more or less of the planar type. The zero-field electron-spin-resonance spectra are described in terms of magnon bound states, i.e., the elementary excitations of the proposed Hamiltonian. The frequency independence and the angular dependence of both microwave and external fields are explained by the model. The temperature variations of the absorption intensity and linewidth are qualitatively discussed using simplified models.

• Received 26 September 1985

DOI:https://doi.org/10.1103/PhysRevB.33.4854