Magnetic properties of the Haldane-gap material NENB

Results of magnetization and high-field ESR studies of the new spin- 1 Haldane-chain material Ni(C₂H₈N₂)₂NO₂ are reported. A definite signature of the Haldane state in NENB was obtained. From the analysis of the frequency-field dependence of magnetic excitations in NENB, the spin-Hamiltonian parameters were cal-
culated, yielding Δ/k_B = 17.4 K, gk = 2.14, D/k_B = 7.5 K, and |E/k_B| = 0.7 K for the Haldane gap, g factor. and the crystal-field anisotropy, respectively. The presence of fractional S = 1/2 chain-end states, revealed by ESR and magnetization measurements, is found to be responsible for spin-glass-freezing effects. In addition, extra states in the excitation spectrum of NENB have been observed in the vicinity of the Haldane gap, whose origin is discussed.