  Spin resonance absorption of the triplet excitations is studied
experimentally in the Haldane magnet PbNi2V2O8. The spectrum has features of
spin S=1 resonance in a crystal field, with all three components, corresponding
to transitions between spin sublevels, being observable. The resonance field is
temperature dependent, indicating the renormalization of excitation spectrum in
interaction between the triplets. Magnetic resonance frequencies and critical
fields of the magnetization curve are consistent with a boson version of the
macroscopic field theory [Affleck 1992, Farutin & Marchenko 2007], implying the
field induced ordering at the critical field, while contradict the previously
used approach of noninteracting spin chains.
