Complex Antiferromagnetic Order in the Metallic Triangular Lattice Compound SmAuAl$_4$Ge$_2$

The compounds $Ln$AuAl$_4$Ge$_2$ ($Ln$ $=$ lanthanide) form in a structure that features two-dimensional triangular lattices of $Ln$ 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 SmAuAl$_4$Ge$_2$, which exhibits weak paramagnetism that strongly deviates from conventional Curie-Weiss behavior. Complex antiferromagnetic ordering emerges at $T_{\rm{N1}}$ $=$ 13.2 K and $T_{\rm{N2}}$ $=$ 7.4 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 YAuAl$_4$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 $Ln$AuAl$_4$Ge$_2$ materials, which have already been established as localized $f$-electron magnets that are hosts for interesting magnetic and electronic phases. From this, SmAuAl$_4$Ge$_2$ emerges as a complex quantum spin metal, inviting further investigations into its properties and the broader family of related materials.