Complex antiferromagnetic order in the metallic triangular lattice compound SmAuAl4Ge2

The compounds $\mathit{Ln}{\mathrm{AuAl}}_{4}{\mathrm{Ge}}_{2}$ ($\mathit{Ln}=$ lanthanide) form in a structure that features two-dimensional triangular lattices of $\mathit{Ln}$ ions that are stacked along the crystalline $c$ axis. Together with crystal electric field effects, magnetic anisotropy, and electron-mediated spin exchange interactions, this sets the stage for the emergence of strongly correlated spin and electron phenomena. Here we investigate ${\mathrm{SmAuAl}}_{4}{\mathrm{Ge}}_{2}$, which exhibits weak paramagnetism that strongly deviates from conventional Curie-Weiss behavior. Complex antiferromagnetic ordering emerges at ${T}_{\mathrm{N}1}=13.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=7.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where heat capacity measurements show that these transitions are first and second order, respectively. These measurements also reveal that the Sommerfeld coefficient is not enhanced compared to the nonmagnetic analog ${\mathrm{YAuAl}}_{4}{\mathrm{Ge}}_{2}$, consistent with the charge carrier quasiparticles exhibiting typical Fermi liquid behavior. The temperature-dependent electrical resistivity follows standard metallic behavior, but linear magnetoresistance unexpectedly appears within the ordered state. We compare these results to other $\mathit{Ln}{\mathrm{AuAl}}_{4}{\mathrm{Ge}}_{2}$ materials, which have already been established as localized $f$-electron magnets that are hosts for interesting magnetic and electronic phases. From this, ${\mathrm{SmAuAl}}_{4}{\mathrm{Ge}}_{2}$ emerges as a complex quantum spin metal, inviting further investigations into its properties and the broader family of related materials.