Magnetic ground-state of the one-dimensional ferromagnetic chain compounds M(NCS)2(thiourea)2; M = Ni, Co
Abstract
The magnetic properties of the two isostructural molecule-based magnets, Ni(NCS)2(thiourea)2, S = 1, [thiourea = SC(NH2)2] and Co(NCS)2(thiourea)2, S = 3/2, are characterised using several techniques in order to rationalise their relationship with structural parameters and ascertain magnetic changes caused by substitution of the spin. Zero-field heat capacity and muon-spin relaxation measurements reveal low-temperature long-range ordering in both compounds, in addition to Ising-like (D < 0) single-ion anisotropy (DCo -100 K, DNi -10 K). Crystal and electronic structure, combined with DC-field magnetometry, affirm highly quasi-one-dimensional behaviour, with ferromagnetic intrachain exchange interactions JCo≈+4 K and JNi+100 K and weak antiferromagnetic interchain exchange, on the order of J' -0.1 K. Electron charge and spin-density mapping reveals through-space exchange as a mechanism to explain the large discrepancy in J-values despite, from a structural perspective, the highly similar exchange pathways in both materials. Both species can be compared to the similar compounds MCl2(thiourea)4, M = Ni(II) (DTN) and Co(II) (DTC), where DTN is know to harbour two magnetic field-induced quantum critical points. Direct comparison of DTN and DTC with the compounds studied here shows that substituting the halide Cl- ion, for the NCS- ion, results in a dramatic change in both the structural and magnetic properties.