HALDANE-GAP OBSERVATION IN TMNIN BY ELECTRON-SPIN-RESONANCE

Electron spin resonance (ESR) measurements on powder samples of the qu asi-one-dimensional S = 1 Heisenberg antiferromagnet (CH3)(4)NNi(NO2)( 3) (TMNIN) were performed for the first time in the frequency range 55 -400 GHz in magnetic fields up to 14 T at temperatures down to 1.3 K. Resonance lines can be associated with transitions between excited sta tes in the framework of a field theoretical model. The held dependence of the resonance modes points to a zero-field splitting of the triple t states at about 2.9 K due to a planar anisotropy. The temperature de pendence of the line intensity confirms the existence of a Haldane gap in TMNIN with Delta/k(B) = 3.4 K.