Nuclear-spin-lattice relaxation and magnetization of the ferromagnetic S=(1/2 chain system (C6H11NH3)CuBr3: Contribution of magnons and solitons

The spin-lattice relaxation rate ($T_1^{\mathrm{-}1}$) of the hydrogen nuclei in (${\mathrm{C}}_6$ ${\mathrm{H}}_{11}$ ${\mathrm{NH}}_3$)${\mathrm{CuBr}}_3$ (CHAB) has been measured for 1.2<T<6.5 K and applied fields 0<B<70 kG along the crystallographic a, b, and c directions. For B∥c, in which case CHAB can be mapped to a sine-Gordon system, the contribution of linear and nonlinear excitations to $T_1^{\mathrm{-}1}$ has been investigated. The relaxation rate for √B /T>1 is completely dominated by the two-magnon (Raman) process, whereas for √B /T<1, a three-magnon term has to be included to obtain a fair description of the data. The quantitative effect of solitonlike excitations on $T_1^{\mathrm{-}1}$ is too small to be unambiguously identified. The magnetization was measured for 1.4<T<10 K and fields 0<B<50 kG along c. The data for T/ √B >0.8 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.