Effects of Y- and La-doping on the magnetic ordering, Kondo effect, and spin dynamics in Ce_1-MRu₂Al₁₀

The influence of Y- and La-substitution for Ce on the competing Kondo effect and magnetic ordering, as well as on spin dynamics in the Kondo semiconductor CeRu₂Al₁₀have been investigated by means of thermal, electronic, and magnetic properties. The parent compound CeRu₂Al₁₀is known to be a controversial antiferromagnet with high magnetic ordering temperatureT₀= 27 K. A small negative chemical pressure caused by La-doping results rapid suppression ofT₀and spin gap energy Δ, compared to a small positive pressure caused by Y-doping. Upon Y- and La-doping, the electrical resistivityρ(T) illustrates the evolution from dense Kondo semiconductor to incoherent single-ion Kondo behaviour, and hence weakens thec-fhybridization and thus lowers the Kondo temperature. The 5% Y- and La-doped compounds show enormous enhancement in the thermoelectric power and complex behaviour in the Hall resistivity belowT₀due to an abrupt change in charge carrier mobility with temperature. The magnetic contribution to electrical resistivityρ_mag(T) (≳50% La) and specific heatC_P(T)/T(≳70% La) evidence non-Fermi-liquid behaviour at low temperature in the La-doped system, due to interplay of atomic disorder with spin-fluctuation. Application of magnetic field suppresses the spin-fluctuation inC_P(T)/Tand eventually emerges to Fermi-liquid state in the 95% La-doped compound in 9 T. The magnetic phase diagram illustrates that the strength of the Kondo interaction in the doped systems are primarily controlled by the effect of volume change as described by the compressible Kondo lattice model. We ascribe the fascinating observation ofT_K≃ 4T₀to anisotropy in the single-ion crystal electric field in the presence of strong anisotropicc-fhybridization.