Magnetic phase diagram ofCeCu2(Si1−xGex)2measured with low-temperature thermal expansion

Abstract

We investigated the evolution of the magnetism in the alloy $\mathrm{Ce}{\mathrm{Cu}}_{2}{({\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x})}_{2}$ by means of low-temperature thermal expansion measurements on large single crystals with $0.01\ensuremath{\leqslant}x\ensuremath{\leqslant}0.45$. The results evidence a new magnetic phase diagram more complex than that obtained in previous studies on polycrystals. The two main features are a second order transition from a paramagnetic to an antiferromagnetic state with a transition temperature ${T}_{N}(x)$ continuously increasing with Ge content and a first order transition corresponding to some change in the magnetically ordered structure at ${T}_{1}(x)\ensuremath{\leqslant}{T}_{N}(x)$. ${T}_{1}(x)$ and ${T}_{N}(x)$ seem to merge at $x\ensuremath{\approx}0.25$ leading to a tetracritical point at this concentration. An analysis of the Gr\"uneisen parameter suggests that at this critical concentration a transition from rather localized f electrons for $x>0.25$ to composite heavy fermions for $x<0.25$ occurs. This strongly supports the itinerant scenario for the quantum-critical point observed in pure $\mathrm{Ce}{\mathrm{Cu}}_{2}{\mathrm{Si}}_{2}$.