Charge density wave-templated spin cycloid in topological semimetal NdSbxTe2−x−δ

Magnetic topological semimetals present open questions regarding the interplay of crystal symmetry, magnetism, band topology, and electron correlations. Ln${\mathrm{Sb}}_{x}{\mathrm{Te}}_{2\text{\ensuremath{-}}x\text{\ensuremath{-}}\ensuremath{\delta}}$ (Ln denotes Lanthanide) is a family of square-net-derived topological semimetals that allows compositional control of band filling, and access to different topological states via an evolving charge density wave (CDW) distortion. Previously studied Gd and Ce members containing a CDW have shown complex magnetic phase diagrams, which implied that spins localized on Ln interact with the CDW, but to this date no magnetic structures have been solved within the CDW regime of this family of compounds. Here, we report on the interplay of the CDW with magnetism in ${\mathrm{NdSb}}_{x}{\mathrm{Te}}_{2\text{\ensuremath{-}}x\text{\ensuremath{-}}\ensuremath{\delta}}$ by comparing the undistorted square net member ${\mathrm{NdSb}}_{0.94}{\mathrm{Te}}_{0.92}$ with the CDW-distorted phase ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$, via single-crystal x-ray diffraction, magnetometry, heat capacity, and neutron powder diffraction. ${\mathrm{NdSb}}_{0.94}{\mathrm{Te}}_{0.92}$ is a collinear antiferromagnet with ${T}_{N}\ensuremath{\sim}2.7\phantom{\rule{4pt}{0ex}}\mathrm{K}$, where spins align antiparallel to each other, but parallel to the square net of the nuclear structure. ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$ exhibits a nearly fivefold-modulated CDW (${q}_{\text{CDW}}=0.18$), isostructural to other Ln${\mathrm{Sb}}_{x}{\mathrm{Te}}_{2\text{\ensuremath{-}}x\text{\ensuremath{-}}\ensuremath{\delta}}$ at similar $x$. ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$ displays more complex magnetism with ${T}_{N}=2.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, additional metamagnetic transitions, and an elliptical cycloid magnetic structure with ${q}_{\text{mag}}=\ensuremath{-}0.41{\mathrm{b}}^{*}$. The magnitudes of ${q}_{\text{CDW}}$ and ${q}_{\text{mag}}$ exhibit an integer relationship, $1+2{q}_{\text{mag}}={q}_{\text{CDW}}$, implying a coupling between the CDW and magnetic structure. Given that the CDW is localized within the nonmagnetic distorted square net, we propose that conduction electrons ``template'' the spin modulation via the Ruderman-Kittel-Kasuya-Yosida interaction.