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---$\mathrm{Ni}{(\mathrm{NCS})}_{2}{(\mathrm{thiourea})}_{2}, S=1 [\mathrm{thiourea}=\mathrm{SC}{({\mathrm{NH}}_{2})}_{2}]$ and $\mathrm{Co}{(\mathrm{NCS})}_{2}{(\mathrm{thiourea})}_{2}, S=3/2$---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 ($Dl0$) single-ion anisotropy (${D}_{\mathrm{Co}}\ensuremath{\sim}\ensuremath{-}100$ K, ${D}_{\mathrm{Ni}}\ensuremath{\sim}\ensuremath{-}10$ K). Crystal and electronic structure, combined with dc-field magnetometry, affirm highly quasi-one-dimensional behavior, with ferromagnetic intrachain exchange interactions ${J}_{\mathrm{Co}}\ensuremath{\approx}+4$ K and ${J}_{\mathrm{Ni}}\ensuremath{\sim}+100$ K and weak antiferromagnetic interchain exchange, on the order of ${J}^{\ensuremath{'}}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\ensuremath{-}0.1$ 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 $M{\mathrm{Cl}}_{2}{(\mathrm{thiourea})}_{4}, M$ = 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 ${\mathrm{Cl}}^{\ensuremath{-}}$ ion for the ${\mathrm{NCS}}^{\ensuremath{-}}$ ion results in a dramatic change in both the structural and magnetic properties.