Near-ideal molecule-based Haldane spin chain.

The molecular coordination complex NiI2 (3, 5-lut)4 [where (3,5-lut) = (3,5-lutidine) = (C7H9N)] has been synthesized and characterized by several techniques including synchrotron x-ray diffraction, electron-spin resonance, superconducting quantum interference device magnetometry, pulsed-field magnetization, inelastic neutron scattering, and muon spin relaxation. Templated by the configuration of 3,5-lut ligands the molecules pack in-registry with the Ni–I ··· I–Ni chains aligned along the c axis. This arrangement leads to an uncommon through-space I ··· I magnetic coupling which is directly measured in this work. The net result is a near-ideal realization of the S = 1 Haldane chain with J = 17.5 K and energy gaps of = 5.3 K ⊥ = 7.7 K, split by the easy-axis single-ion anisotropy D = −1.2 K. The ratio D/J = −0.07 affords one of the most isotropic Haldane systems yet discovered, while the ratio 0/J = 0.40(1) (where 0 is the average gap size) is close to its ideal theoretical value, suggesting a very high degree of magnetic isolation of the spin chains in this material. The Haldane gap is closed by orientation-dependent critical fields μ0H c = 5.3 T and μ0H⊥ c = 4.3 T, which are readily accessible experimentally and permit investigations across the entirety of the Haldane phase, with the fully polarized state occurring at μ0H s = 46.0 T and μ0H⊥ s = 50.7 T. The results are explicable within the so-called fermion model, in contrast to other reported easy-axis Haldane systems. Zero-field magnetic order is absent down to 20 mK and emergent end-chain effects are observed in the gapped state, as evidenced by detailed low-temperature measurements.