Multi-frequency ESR studies on a Haldane magnet in a field-induced phase at ultra-low temperatures

We report the results of multi-frequency electron spin resonance (ESR) measurements on single crystals of Ni(C5H14N2)2N3(PF6) which is regarded as the one-dimensional Heisenberg antiferromagnet with spin one, namely the Haldane magnet, at very low temperatures down to about 100 mK. We observed the lowest resonance branch below about 500 mK for the field along the chain direction (H||c), which was observed previously only in an inelastic neutron scattering experiment at 30 mK. We compare the resonance branch with that calculated by a phenomenological field theory, and discuss the field dependence and the temperature sensitivity of this ESR branch.