Near-ideal molecule-based Haldane spin chain

Abstract

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=17.5K and energy gaps of Δ∥=5.3K Δ⊥=7.7K, split by the easy-axis single-ion anisotropy D=-1.2K. The ratio D/J=-0.07 affords one of the most isotropic Haldane systems yet discovered, while the ratio Δ0/J=0.40(1) (where Δ0 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 μ0Hc∥=5.3T and μ0Hc⊥=4.3T, which are readily accessible experimentally and permit investigations across the entirety of the Haldane phase, with the fully polarized state occurring at μ0Hs∥=46.0T and μ0Hs⊥=50.7T. 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 20mK and emergent end-chain effects are observed in the gapped state, as evidenced by detailed low-temperature measurements.