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

Results of high-field ESR and magnetization studies of the new spin-1 Haldane-chain material [Ni(C$_2$H$_8$N$_2$)$_2$NO$_2$](BF$_4$) (NENB) 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.