Neutron scattering studies in the linear-chain haldane materials, NDMAZ and NDMAP

Neutron scattering experiments were carried out in order to investigate the field-induced antiferromagnetic long-range order and spin dynamics for the new Haldane compounds Ni(C₅H₁₄N₂)₂N₃(ClO ₄) (NDMAZ) and Ni(C₅H₁₄N₂)₂N₃(PF ₆) (NDMAP). We observed the energy gaps corresponding to the excited triplet, Δ_x = 1.61 meV, Δ_y = 1.76 meV, Δ_z = 3.2 meV for NDMAZ and Δ_x = 0.48 meV, Δ_y = 0.64 meV for NDMAP. The dispersion in NDMAZ was well explained by the theoretical curve with the intra-chain interaction J = 50 K. The field dependence of the gap energy below critical field (H_c), was observed for NDMAZ and NDMAP as applying the field perpendicular to the chain direction. The field dependence of the excitation levels in NDMAZ was well explained by the fermionic field-theory treatment of the one-dimensional Heisenberg antiferromagnetic Hamiltonian with these gap energies and g-value, 2.3. In NDMAP, we found that the excitation peaks at Q = (0 0 0.5) became unobservably weak above H_c. This result would indicate a level crossing between the ground and the excited level at H_c and the change of the ground state above H_c. No trace of the magnetic Bragg peak due to the field induced ordering could be observed in the present study with our non-deuterated single crystal sample.