A near-ideal molecule-based Haldane spin chain*

We report on a new molecule-based magnet NiI2(3,5-lutidine)4 characterized using x-ray diffraction, electron-spin resonance, magnetometry, inelastic neutron scattering and muon-spin relaxation. The system is a near-ideal realization of the S = 1 Haldane chain with intrachain exchange J = 17.5 K, energy gaps of 5.3 K and 7.7 K split by easy-axis anisotropy D=−1.2 K, and with no sign of magnetic order down to 20 mK. The value D/J=−0.07 implies one of the most isotropic Haldane systems yet discovered, while the ratio Δ/J = 0.40(1) (where Δ is the average gap size) is close to the ideal theoretical value, suggesting a very high degree of magnetic isolation of the chains. The Haldane gap is closed by orientation-dependent critical fields 5.3 T and 4.3 T, which are readily accessible experimentally. The fully polarized state occurs above 46.0 T. The results are explicable within the so-called fermion model, which is in contrast to other reported easy-axis Haldane systems. The flexible nature of this molecule-based material readily permits tuning via applied pressure and introduction of disorder along the exchange pathway.