High-field multifrequency electron-spin-resonance study of the Haldane magnet Ni(C₅H₁₄N₂)₂N ₃(PF₆)

In order to understand physical properties of the field-induced phase of a spin-gap system, we performed high-field and multifrequency electron-spin- resonance (ESR) measurements on single crystals of the S=1 quasi-one-dimensional Heisenberg antiferromagnet, namely, the Haldane magnet, Ni (C5 H14 N2) 2 N3 (PF6), abbreviated as NDMAP. This compound has an energy gap (Haldane gap) at zero field and one of the excited triplet branches goes down on applying magnetic fields, resulting in the gap closing at a critical field Hc around 5 T that is slightly different depending on the field direction. First, we studied the angular dependence of spin excitations below 14 T. Two sets of resonance modes caused by two types of Ni2+ chains in NDMAP are observed. These data are analyzed by comparing with a phenomenological field theory (PFT). The experimental results between Hc and about 12 T are well fitted with the calculated ones by the PFT, but the fitting above 12 T is not satisfactory. Therefore, we studied spin excitations at much higher magnetic fields up to about 55 T. Several ESR signals are observed above Hc for each crystallographic axis, and one or two of them survive in the high-field region above about 15 T. One mode approaches a paramagnetic resonance line at high fields and the other mode broadly changes with magnetic fields. These modes fit well with the conventional antiferromagnetic resonance modes with biaxial anisotropy. This result suggests that the quantum fluctuations are suppressed by strong magnetic field and the spin excitations change from a quantum nature to a classical one in high magnetic fields.