Spin fluctuation and static properties of the local moments in the Haldane-gap system Ni(C2H8N2)2NO2(ClO4) studied byH1NMR

The nuclear spin-lattice relaxation time ${\mathit{T}}_1$ of ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ nuclei in the S=1 quasi-one-dimensional Heisenberg antiferromagnet Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$(${\mathrm{ClO}}_4$) (NENP) was measured at temperatures above 1.4 K in the field range below 12 T. The field dependence of the relaxation rate ${\mathit{T}}_1^{\mathrm{-}1}$ 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 experimental results were analyzed by treating the magnetic excitation as free fermions. The ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR spectra were also measured for various fields and temperatures. The results showed the appearance of static local moments even at fields as low as 3 T. Their temperature and field dependences were explained by considering the mixing of the ground state and the first excited state in the Haldane-gap system.