Magnetic properties of the Haldane-gap material [Ni(C₂H₈N₂)₂NO₂](BF₄)

Results of magnetization and high-field electron spin resonance (ESR) studies of the new spin-1 Haldane-chain material [Ni(C2H8N2)2NO2] (BF4) (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 1/kB = 17.4 K, g‖ = 2.14, D/kB = 7.5 K and |E/kB| = 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; their origin is discussed. 6 Author to whom any correspondence should be addressed. New Journal of Physics 10 (2008) 033008 1367-2630/08/033008+09$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft