Proton nuclear magnetic resonance investigation of the spin dynamics in cobalt based one-dimensional magnetic molecular chains

H-1 nuclear magnetic resonance was used to investigate the spin dynamics of the molecular chain Co(hfac)(2)NITPhOMe. This helical chain is composed of alternating Co(II) ions and radical moieties with dominant nearest-neighbor antiferromagnetic interaction (Jsimilar to220 K). Below 50 K, the chain behaves as a 1/2-1/2 Ising chain, with the electronic spins freezing before a transition to three-dimensional order. Previous results stated that the relaxation time tau(M) of the magnetization M follows a thermally activated mechanism tau(M)=tau(0) exp(Delta/T) with tau(0)approximate to3(+/-1)x10(-11) s and Deltaapproximate to152(+/-2) K. Measurements of the H-1 NMR spin-lattice relaxation rate T-1(-1) were performed as a function of the temperature for two different external magnetic fields (0.35 and 1.7 T). Below 50 K, where the 1/2-1/2 spin chain approximation is valid, explanation of the experimental data was found by assuming the presence of two thermally activated relaxation mechanisms. One of these corresponds to the correlation time detected by M while a second mechanism, undetected in the magnetization measurements, becomes effective in the low temperature range. The possibility of the two nuclear relaxation mechanisms being related to different spin dynamics of the Co(II) and the radical spins, respectively, is discussed.