Multi-frequency ESR in the haldane magnet NDMAP below 1 K

The S=1 quasi-one dimensional Heisenberg antiferromagnet Ni(C ₅H₁₄N₂₎₂N₃(PF₆), abbreviated as NDMAP, has been studied by electron spin resonance (ESR) in a magnetic field exceeding the critical field (H_c) above which this compound shows a long range order (LRO) at sufficiently low temperatures. Our recently developed ESR apparatus with a dilution refrigerator is utilized to study the spin excitations of NDMAP above H_c for Hc, because this compound exhibits the LRO below about 1 K. The spin excitation modes above H_c observed below 1 K do not show any difference from those at 1.5 K and the lowest excitation mode observed at 30 mK in the inelastic neutron experiments above H_c was not observed at about 200 mK in this study. Accordingly, the result suggests that the lowest excitation mode is very sensitive to temperature.