  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.
