Near-ideal molecule-based Haldane spin chain

Abstract

The molecular coordination complex NiI_{2}(3,5-lut)_{4} [where (3,5-lut) = (3,5-lutidine) =(C_{7}H_{9}N)] 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 Δ_{0}/J=0.40(1) (where Δ_{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 μ_{0}H_{c}^{∥}=5.3T and μ_{0}H_{c}^{⊥}=4.3T, which are readily accessible experimentally and permit investigations across the entirety of the Haldane phase, with the fully polarized state occurring at μ_{0}H_{s}^{∥}=46.0T and μ_{0}H_{s}^{⊥}=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.