3 d Transition Metal Contribution to the Magnetic Behavior of Chiral Y6 M x Si2S14 and Tb6CoSi2S14

Abstract A series of quaternary rare-earth sulfide materials Y6MxSi2S14 (M = Ti, V, Cr, Mn, Co, Ni) and Tb6CoSi2S14 were synthesized by solid-state methods to investigate the effects of the 3d transition metal on the respective magnetic properties. The elemental composition was confirmed by single-crystal X-ray diffraction (SCXRD), Rietveld refinements of powder X-ray diffraction (PXRD) data, and energy-dispersive X-ray spectroscopy (EDXS). An abrupt change in unit cell parameters and corresponding M-M distances is observed when x, the concentration of transition metal M, changes from 0.67 to 1. Among all Y-containing compounds, Y6NiSi2S14 is the only compound that showed antiferromagnetic (AFM) ordering at 2.5 K and a 2-step metamagnetic transition. According to the Curie–Weiss fits, most Y6MxSi2S14 compounds have negative values for the Weiss constant, θCW. Only Y6Cr0.67Si2S14 produced a positive θCW of +19 K, suggesting ferromagnetic coupling between nearest-neighbor Cr atoms. Isothermal magnetization at 2 K showed that the Cr- and Co-containing compounds reach magnetic saturation at 2 T, whereas those containing Ti, V, and Mn remain far from saturation even at 7 T. To clarify whether the Co sublattice is ordered in the presence of a magnetic Tb cation, magnetic structure determination was performed on Tb6CoSi2S14 (TN = 9 K) using single-crystal neutron diffraction. The experimental data were best described by a spin-density-wave model in which the Tb moments are arranged noncollinearly at 120° angles in the ab plane, with their amplitudes varying sinusoidally along the c-axis in accordance with the propagation vector k = (0, 0, 0.445). The Co moments in Tb6CoSi2S14 remain largely disordered, unlike the collinear Fe sublattice in the previously reported Tb6FeSi2S14 counterpart, which had the Fe moments aligned along the c-axis.