We have determined the magnetic phase diagram of CeCu${}_2$(Si${}_{1-x}$Ge${}_x)$${}_2$ using specific heat, susceptibility, and resistivity measurements and additional thermal expansion and muon spin resonance experiments. The system evolves continuously from the heavy-fermion state at $x=0\mathrm{}$ to an antiferromagnetically (AF) ordered state at $x=1\mathrm{}$, though the magnetic structure undergoes significant modifications. The results strongly suggest that the AF ordering emerges from the $A$ phase of pure CeCu${}_2$Si${}_2$. The phase diagram can be divided into three regions: Low Ge content $(x<~0.2)$ leads to an enhancement of the $A$ phase of pure CeCu${}_2$Si${}_2$ and to a moderate decrease of the Kondo temperature. Despite this increase of the magnetic character, we could observe superconductivity up to $x=0.1\mathrm{}$. At large Ge content $\mathrm{}(x>\mathrm{}0.5\mathrm{})\mathrm{}$ the behavior resembles that of pure CeCu${}_2$Ge${}_2$. The intermediate region is characterized by the appearance of a second magnetic transition below $T_N,$ which seems to be of first-order type. The appearance of this transition leads to a well-defined critical point at $x=0.3\mathrm{}$ and suggests the possibility of another critical point at $x\simeq 0.5$.

• Received 5 November 1996

DOI:https://doi.org/10.1103/PhysRevB.56.678