Critical slowing down of the spin dynamics in antiferromagnetic rings

We report 1H nuclear magnetic resonance spin-lattice relaxation rates T-11 vs. T at different magnetic field values, for molecular antiferromagnetic (AFM) rings with STotal = 0 singlet ground state. New results in {CsFe8}, {Cr8} and {Fe6} are compared with previous results in {Fe6} and {Fe10}. In all cases a strong enhancement of T 1-1 was observed with a maximum occurring at some temperature T0 which is found to be field dependent. It is shown that the critical enhancement appears to be a robust universal feature in these AFM rings for which a general theory is still lacking