Yb magnetic instability in YbMn6Ge6−xSnx

Yb exhibits unusual physical behavior in ${\mathrm{YbMn}}_{6}{\mathrm{Ge}}_{6\ensuremath{-}x}{\mathrm{Sn}}_{x}$ with coexisting intermediate valent state and magnetism. We complete the $(x,T)$ magnetic phase diagram of this system by using dc magnetization and powder neutron-diffraction experiments in a composition range ($4.60\ensuremath{\le}x\ensuremath{\le}5.30$) where the alloys were previously reported to two-phase separate. In these compounds, Mn ferromagnetically orders near room temperature while the intermediate valent Yb sublattice ($\ensuremath{\upsilon}\ensuremath{\approx}2.8$) obeys a Doniach-like behavior. Upon increasing the $4f$ conduction electron hybridization strength through Sn for Ge substitution, the Yb magnetic moment continuously reduces as a result of the enhancement in the Kondo screening. Meanwhile the Yb magnetic ordering temperature reaches its maximum ${T}_{\mathrm{Yb}}\ensuremath{\sim}125$ K in the alloy with $x=4.65$, before dropping upon further Sn doping. The Yb magnetic instability is located near $x\ensuremath{\sim}5.23$, in the vicinity of which the nature of the quantum phase transition remains to be elucidated.