NUCLEAR-SPIN RELAXATION IN S = 1 HEISENBERG ANTIFERROMAGNETIC CHAINS IN HIGH-FIELD

The proton spin-lattice relaxation rate 1/T1 has been measured in the two compounds Ni(C2H8N2)2NO2 (ClO4) (NENP) and [Ni(NO2)3N(CH3)4] (TMNIN), which provide good examples of S = 1 antiferromagnetic chains. Both the field H and the temperature dependencies are well analyzed in the framework of the Haldane conjecture. The prediction of a mode whose energy decreases with H is confirmed. The energy damping of this mode is observed to be also reduced by H. For both compounds, a possible energy renormalization is suggested as H approaches the critical field, where the spin system is expected to recover a magnetic behavior.