Electron spin resonance is studied for 2.6 K <T<6.4 K at 9GHz. The absorption peak lies at H..0 and has maximum intensity just above TN. From a realistic spin Hamiltonian we infer that magnon-bound state resonance was observed. The linewidth follows a T4 law for T>TN.
Nijhof, E. J., van der Vlist, H., & Gerritsma, G. J. (1986). Magnon-bound state resonance in the quasi-1d antiferromagnet RbCoCl3 ·2H2O. Journal of magnetism and magnetic materials, 54-57(Part 3), 1197-1198. https://doi.org/10.1016/0304-8853(86)90781-X