SPIN FLUCTUATION AND STATIC PROPERTIES OF THE LOCAL MOMENTS IN THE HALDANE-GAP SYSTEM NI(C2H8N2)2NO2(CLO4) STUDIED BY H-1-NMR

The nuclear spin-lattice relaxation time T1 of H-1 nuclei in the S = 1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C2H8N2)2NO2(ClO4) (NENP) was measured at temperatures above 1.4 K in the field range be low 12 T. The field dependence of the relaxation rate T1(-1) exhibited a maximum at 9.5 T, which corresponds to the critical field, for the field parallel to the chain at low temperatures below 4 K. The experim ental results were analyzed by treating the magnetic excitation as fre e fermions. The H-1 NMR spectra were also measured for various fields and temperatures. The results showed the appearance of static local mo ments even at fields as low as 3 T. Their temperature and field depend ences were explained by considering the mixing of the ground state and the first excited state in the Haldane-gap system.