Magnon spin resonance in the Haldane spin chains of Ni(C2H8N2)NO2ClO4

Abstract
We present a high-frequency electron spin resonance study of the magnetic transitions occurring between excited states in a Haldane spin system. We show that these transitions are induced between the low-energy magnon states, which develop at the boundary (q≊π) of the Brillouin zone. Our study provides therefore, the first evidence that, in the Haldane phase, the excitations associated with the quantum gaps correspond actually to a s=1 magnetic state.