Magnetic ordering and superconductivity in the R₂Ir₃Ge₅(R=Y, La, Ce-Nd, Gd-Tm, Lu) system

Abstract

We report crystal structure, electrical resistivity, magnetic susceptibility, isothermal magnetization, and heat-capacity studies on polycrystalline samples of the intermetallic series R₂Ir₃Ge₅ (R=Y, La, Ce-Nd, Gd-Tm, Lu) from 1.5 to 300 K. We find that the compounds for R=γ, La-Dy, crystallize in the tetragonal Ibam (U₂Co₃Si₅ type) structure whereas the compounds for R=Er-Lu, crystallize in a different orthorhombic structure with a space group Pmmn. Samples of Ho₂Ir₃Ge₅ were always found to be multiphase. The compounds for R=Y-Dy which adopt the Ibam type structure show a metallic resistivity with a tendency of saturation at high temperatures whereas the compounds with R=Er, Tm, and Lu show an anomalous behavior in the resistivity with a semiconducting increase in ρ as we go down in temperature from 300 K. Interestingly we had earlier found a positive temperature coefficient of resistivity for the Yb sample in the same temperature range. We will compare this behavior with similar observations in the compounds R₃Ru₄Ge₁₃ and RBiPt. La₂Ir₃Ge₅ and Y₂Ir₃Ge₅ show bulk superconductivity below 1.8 K and 2.5 K, respectively. Our results confirm that Ce₂Ir₃Ge₅ shows a Kondo-lattice behavior and undergoes antiferromagnetic ordering below 8.5 K. Most of the other compounds containing magnetic rare-earth elements undergo a single antiferromagnetic transition at low temperatures (T<~12K) while Gd₂Ir₃Ge₅, Dy₂Ir₃Ge₅, and Nd₂Ir₃Ge₅ show multiple transitions. The T_N's for most of the compounds roughly scale with the de Gennes factor, which suggests that the primary mechanism of interaction leading to the magnetic ordering of the magnetic moments may be the Ruderman-Kittel-Kasuya-Yosida interaction. The ordering temperature of 8.5 K for Ce₂Ir₃Ge₅ is anomalously large compared to the T_N for Gd₂Ir₃Ge₅ which is about 12 K. There are signs of strong CEF influence on the measured properties for the series.