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(NH${}_2$)${}_2$] 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 ($D_{\mathrm{Co}}\sim$ -100 K, $D_{\mathrm{Ni}}\sim$ -10 K). Crystal and electronic structure, combined with DC-field magnetometry, affirm highly quasi-one-dimensional behaviour, 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 $M$Cl${}_2$(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.

arXiv commentMain text: 13 pages - 11 figures, 2 tables. Supplemental information: 6 pages - 2 figures, 6 tables

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• Zigzag magnetic order in a novel tellurate compound Na${}_{4-\delta }$NiTeO${}_6$ with $S$ = 1 chains
• Semiclassical approach to spin dynamics of a ferromagnetic S=1 chain