The spin-lattice relaxation rate (T1-1) of the hydrogen nuclei in (C6H11NH3)CuBr3 (CHAB) has been measured for 1.21 is completely dominated by the two-magnon (Raman) process, whereas for vB /T0.8 exhibit systematic deviations from linear spin-wave theory, which can fully be explained by the effect of sine-Gordon solitons. The apparent discrepancies between the observed magnetization and various classical model predictions seem to originate primarily from the poor description of the linear excitations by these models.
Kopinga, K., Tinus, A. M. C., Jonge, de, W. J. M., & de Vries, G. C. (1987). Nuclear-spin-lattice relaxation and magnetization of the ferromagnetic S=1/2 chain system (C6H11NH3)CuBr3 : contribution of magnons and solitons. Physical Review B: Condensed Matter, 36(10), 5398-5410. https://doi.org/10.1103/PhysRevB.36.5398