Structure, crystal fields, magnetic interactions, and heavy-fermion behavior in (Ce1−xLax)3Al

Abstract

We report measurements of the resistivity \ensuremath{\rho}, susceptibility \ensuremath{\chi}, and specific heat C of the alloys (${\mathrm{Ce}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{La}}_{\mathrm{x}}$ ${)}_{3}$ Al. At room temperature these form in the hexagonal ${\mathrm{Ni}}_{3}$ Sn structure (\ensuremath{\alpha}-${\mathrm{Ce}}_{3}$ Al); at low temperatures a structural transition to a monoclinic phase occurs for 0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.3 (\ensuremath{\gamma}-${\mathrm{Ce}}_{3}$ Al); and a transition with a similar feature in the resistivity occurs for 0.75\ensuremath{\leqslant}1 (\ensuremath{\gamma}-${\mathrm{La}}_{3}$ Al). Crystal fields have strong effects on these measurements: analysis of the specific heat suggests that for ${\mathrm{Ce}}_{3}$ Al two excited doublets occur at temperatures ${\mathrm{T}}_{1\mathrm{c}\mathrm{f}}$ \ensuremath{\approx}75 K and ${\mathrm{T}}_{2\mathrm{c}\mathrm{f}}$ \ensuremath{\approx}130 K above the ground-state doublet, and that these splittings decrease significantly on alloying; this causes a similar decrease in the Curie-Weiss temperature ${\mathrm{\ensuremath{\theta}}}_{\mathrm{h}}$ obtained from the high-temperature susceptibility \ensuremath{\chi}=C/(T+${\mathrm{\ensuremath{\theta}}}_{\mathrm{h}}$ ). The derivative d\ensuremath{\rho}/dT of the low-temperature resistivity is negative over a range of temperature for all x (0\ensuremath{\leqslant}x1), which is a characteristic sign of heavy-fermion (Kondo) behavior; various measures of the Kondo temperature ${\mathrm{T}}_{\mathrm{K}}$ , taken from the analysis of \ensuremath{\rho}, \ensuremath{\chi}, and C, consistently suggest that ${\mathrm{T}}_{\mathrm{K}}$ decreases by an order of magnitude on alloying, from \ensuremath{\approx}10 K for small x to \ensuremath{\approx}1 K for large x. Fits to the low-temperature specific heat which include a lattice contribution, a crystal-field contribution, and an S=1/2 Kondo contribution describe the data well for x=0.95; but for 0.3\ensuremath{\leqslant}x\ensuremath{\leqslant}0.82 the specific heat peak is larger and narrower than predicted by Kondo theory and a peak occurs in the resistivity, suggesting that coherence due to magnetic correlations plays a role for these concentrations. For the monoclinic phase, peaks in \ensuremath{\rho}, \ensuremath{\chi}, and C indicate antiferromagnetic order, where the N\'eel temperature decreases with x from its value ${\mathrm{T}}_{\mathrm{N}}$ =2.5 K for x=0. The specific heat is linear at the lowest temperatures, even in the antiferromagnetic phase, which suggests that the magnetic order coexists with Kondo behavior.