Magnetic ground state of the one-dimensional ferromagnetic chain compounds M(NCS)2(thiourea)2 (M = Ni, Co)

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 characterized using several techniques in order to rationalize their relationship with structural parameters and to 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 (D-Co similar to -100 K, D-Ni similar to -10 K). Crystal and electronic structure, combined with dc-field magnetometry, affirm highly quasi-one-dimensional behavior, with ferromagnetic intrachain exchange interactions J(Co) approximate to +4 K and J(Ni) similar to +100 K and weak antiferromagnetic interchain exchange, on the order of J' similar to -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 known to harbor 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.