Onset of magnetic order in YbCu 5 − x Al x

Spectroscopic, magnetic, thermal, and transport measurements under pressure are presented which were performed on ${\mathrm{YbCu}}_{5\ensuremath{-}x}{\mathrm{Al}}_{x}$ 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 $xg1.5$, where an increasing Al content causes both a growing transition temperature and rising saturation moments reaching 2 K and about 2.1${\ensuremath{\mu}}_{\mathrm{B}},$ respectively, for $x=2$. At the critical concentration $(x\ensuremath{\approx}1.5)$ the possibility of a non-Fermi-liquid state is analyzed together with its tuning by pressure.