Onset of magnetic order inYbCu5−xAlx

Spectroscopic, magnetic, thermal, and transport measurements under pressure are presented which were performed on ${\mathrm{YbCu}}_{5-x}{\mathrm{Al}}_x$ intermetallics. The substitution of Cu by Al in ${\mathrm{YbCu}}_5$ drives the system from an almost divalent behavior of the Yb ion in ${\mathrm{YbCu}}_5$ to a trivalent state in ${\mathrm{YbCu}}_3{\mathrm{Al}}_2$. A compensation between the normal thermal expansion and a valence-driven contraction was found for ${\mathrm{YbCu}}_4\mathrm{Al}$. Antiferromagnetic order sets in for $x>\mathrm{}1.5\mathrm{}$, where an increasing Al content causes both a growing transition temperature and rising saturation moments reaching 2 K and about 2.1${\mu }_{\mathrm{B}},$ respectively, for $x=2\mathrm{}$. At the critical concentration $(x\approx 1.5)$ the possibility of a non-Fermi-liquid state is analyzed together with its tuning by pressure.