Experimental study of high-temperature spin dynamics in one-dimensional Heisenberg paramagnets

From room-temperature measurements of proton spin-lattice relaxation rate in (CH₃)₄NMnCl₃ (TMMC) and CsMnCl₃ ·{} 2H₂O (CMC) as a function of magnetic field →H the shape of the low-frequency spectral density of the spin fluctuations is directly obtained. For →H parallel to the chain axis the spectral density contains a diffusive term which diverges as ω^-1/2. The experimental results for →H∥→c are in good agreement with a theoretical calculation in TMMC in which the two-spin correlation function is assumed to decay at long times as t^-1/2. The lack of frequency dependence of →H perpendicular to the chain axis indicates a difference in the "cutoff" time of the two-spin correlation function for the spin components parallel and perpendicular with respect to the external magnetic field.