Magnetic phase diagram of CeCu2(Si1-xGex)2 measured with low-temperature thermal expansion -: art. no. 094409

We investigated the evolution of the magnetism in the alloy CeCu2(S1-xGex)(2) by means of low-temperature thermal expansion measurements on large single crystals with 0.01 <= x <= 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) <= T-N(x). T-1(x) and T-N(x) seem to merge at x approximate to 0.25 leading to a tetracritical point at this concentration. An analysis of the Gruneised 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 CeCu2Si2.