Proton Spin-Lattice Relaxation in the Quasi-One-DimensionalS=1 Heisenberg Antiferromagnet Ni(C2H8N2)2NO2(ClO4)

Proton spin-lattice relaxation time T 1 in the Haldane-gap system with S =1, Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ), was measured at temperatures between 1.4 K and 77 K. The relaxation rate T 1 -1 decreased over more than three orders of magnitude with decreasing temperature, thus suggesting scarce development of the antiferromagnetic short-range order. Monotonous decrease of T 1 -1 above 6 K was analyzed on the basis of the theory for insulating paramagnets using static susceptibility data. To explain the behavior of T 1 -1 below 6 K, which accompanies a rounded peak, we consider the effect of local-field fluctuation of the order of the nuclear Larmor period, which is associated with magnetic excitation from the nonmagnetic ground state.