Excitation spectrum and magnetic properties of the Haldane-gap material NENB

Results of high-field ESR and magnetization 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 calculated, yielding \Delta/k_{B} = 17.4~K, g_{\parallel}=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 constants, 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, which origin is discussed.