Low-temperature studies of the magnetic and superconducting properties of the R₂Ir₃Si₅ (R=Y,La,Ce-Nd,Gd-Tm) system

Abstract

Polycrystalline samples of the ternary rare-earth silicide compounds R₂Ir₃Si₅ (R=Y,La,Ce-Nd,Gd-Tm) have been prepared and characterized using room temperature powder x-ray, magnetic susceptibility, electrical resistivity, and low-temperature (1.6-30 K) zero-field heat capacity measurements. All the compounds crystallize in the tetragonal U₂Co₃Si₅ type structure (space group Ibam). The effective moments estimated from Curie-Weiss fits to the high-temperature inverse susceptibility data suggest that Ir may carry an induced moment in these compounds. The systematics of the lattice parameters across the series suggest that Ce is trivalent in Ce₂Ir₃Si₅, whereas the magnetic, transport, and heat capacity measurements reveal a nonmagnetic behavior for this compound, suggesting that this is a strongly hybridized compound similar to Ce₂Rh₃Si₅. Pr₂Ir₃Si₅ also does not order down to 1.8 K. Large and prominent peaks at low temperatures in the susceptibility and heat capacity of the other magnetic rare-earth-containing compounds indicate bulk magnetic ordering of trivalent rare-earth moments in these compounds. The heat capacity data for Gd₂Ir₃Si₅ show three anomalies, one above and one below the main λ type antiferromagnetic transition peak at 11.8 K, while the data for Tb₂Ir₃Si₅ also suggest two transitions. Most of the compounds have transition temperatures which follow the de Gennes scaling. However, Nd₂Ir₃Si₅ and Tb₂Ir₃Si₅ have anomalously large transition temperatures compared to that expected by the de Gennes scaling. Large crystalline electric field effects are indicated by the reduced value of the magnetic entropy at 30 K, and this could be a possible reason for the deviation of the ordering temperature T_N from the de Gennes factor. From the temperature dependence of the magnetic entropy we conclude that Nd₂Ir₃Si₅, Tb₂Ir₃Si₅, Dy₂Ir₃Si₅, and Ho₂Ir₃Si₅ are in a doublet ground state. The anomalies in the susceptibility and heat capacity near 2 K for La₂Ir₃Si₅ and 2.9 K for Y₂Ir₃Si₅ suggest that these compounds undergo a transition into the superconducting state at low temperatures. A reduced jump ΔC/γT_C at T_C and a large linear term in the temperature dependence of the heat capacity in the superconducting state are observed, and we compare this with similar behavior seen in the nonmagnetic superconducting members of the structurally related series R₂Fe₃Si₅, where it has been suggested that a fraction of the Fermi surface remains normal below T_C.