The magnetic properties of the two isostructural molecule-based magnets—$\mathrm{Ni}{\left(\mathrm{NCS}\right)}_2{\left(\mathrm{thiourea}\right)}_2, S=1 [\mathrm{thiourea}=\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2]$ and $\mathrm{Co}{\left(\mathrm{NCS}\right)}_2{\left(\mathrm{thiourea}\right)}_2, S=3/2$—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 ($D_{\mathrm{Co}}\sim -100$ K, $D_{\mathrm{Ni}}\sim -10$ K). Crystal and electronic structure, combined with dc-field magnetometry, affirm highly quasi-one-dimensional behavior, with ferromagnetic intrachain exchange interactions $J_{\mathrm{Co}}\approx +4$ K and $J_{\mathrm{Ni}}\sim +100$ K and weak antiferromagnetic interchain exchange, on the order of $J^{\prime }\sim -0.1$ 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 $M{\mathrm{Cl}}_2{\left(\mathrm{thiourea}\right)}_4, M$ = 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 ${\mathrm{Cl}}^{-}$ ion for the ${\mathrm{NCS}}^{-}$ ion results in a dramatic change in both the structural and magnetic properties.

• Received 26 November 2020
• Accepted 3 February 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.034401