C S Garde

Resistivity and thermopower studies on La3X (XAl, Sn, In, Ru, Ir, Co, Ni, Ge, Ga) systems

Abstract We report resistivity (ρ) and thermopower (S) measurements on the La3X (XAl, Sn, Ru, Ir, Co, Ni, Ge, Ga) and La3−xGdxIn (x = 0, 0.02, 0.06, 0.14, 0.20) systems between 1.7 and 300 K. All the systems except La3Ir and La3−xGdxIn (x = 0.14, 0.20) become superconducting with Tc below 10 K. The x = 0.14 and 0.20 systems order magnetically at 4.7 and 7.1 K respectively. The ρ curve of all the compounds shows a strong negative curvature at intermediate temperatures followed by its saturation at higher temperatures. Further, some of the compounds exhibit a T2 dependence at low temperatures. The low temperature T2 dependence and the high temperature saturation effect in the ρ behaviour of these compounds have been suggested to arise from the scattering of conduction electrons from the spin fluctuations in the d-band. The low temperature extremum features in the S data could also arise from spin fluctuation effects. A sudden change in the slope of the ρ curve for some alloys, e.g. La3Al and La3In, at Ts≈27 K has been observed, which may be related to lattice transformation effects.

Antiferromagnetism in CeAu2Si2- and CeAg2Si2-based Kondo-lattice systems

We report the electrical resistivity rho , thermopower S and thermal conductivity lambda of Ce1-xLaxAu2Si2, Ce1-yYyAu2Si2, Ce1-xLaxAg2Si2 and Ce1-yYyAg2Si2 (0<or=x, y<or=1) systems. The rho curves show a sudden change in slope at the antiferromagnetic ordering temperature TNrho . In the CeAu2Si2-based alloys, the TN depression, due to the dilution of Ce atoms by La or Y, is accounted for solely by the weakening of the inter-site interaction temperature scale TRKKY. On the other hand, in the CeAg2Si2-based alloys, apart from TRKKY, the single-ion Kondo temperature TK also plays an important role in determining the nature of the TN variation. The rho and lambda behaviours, at both low and high temperatures, show clear evidence for the Kondo effect in the presence of crystal fields.

Resistivity and thermopower studies of Ce3-xLaxAl alloys

The authors present resistivity ( rho ) and thermopower (S) measurements at temperatures between 3 K and 300 K on Ce3-xLaxAl alloys (x=0,0.15,0.30,0.45,0.6,0.9,1.2). All the alloys showed evidence of Kondo-like scattering (occurrence of rho min and Smin around 30 K). The low-temperature Smin crossed over to positive values at x>or=0.45, giving rise to a positive thermopower over the entire temperature range. The low-temperature positive peak (around 5-10 K) in S and rho measurements gave evidence of a coherence effect. This peak was found to vary with La concentration in the thermopower studies. At higher temperatures (50-100 K), alloys with 0<or=x<or=0.6 showed thermal hysteresis in both types of measurements, indicating the presence of lattice transformation effects.

Magnetic, transport and lattice instability behaviour in Co2NbSn-based systems

Co2(Nb1-xVx)Sn, Co2NbSn1-yGay and Co2NbSn1-zAlz alloy systems exhibit itinerant ferromagnetic behaviour as observed from magnetization and susceptibility studies. Spin fluctuation effects are observed from magnetization, resistivity and thermopower data. The thermal expansion data gave evidence of structural transformation effects due to the formation of a pseudogap in the density of states near the Fermi level.

Magnetic and superconducting behaviour of the (R = La, Pr, Gd, Tb, Ho and Er) systems

Magnetic and transport behaviours of (R = La, Pr, Gd, Tb, Ho and Er) systems have been studied between 2 and 300 K. At low temperatures, all the systems except for R = La exhibit complex magnetic ordering effects. The R = La system exhibits superconductivity. At high temperatures, the transport behaviour has been explained in the framework of the s - d model arising from the interaction between the 3d and the 5d electrons of Ni and R, respectively.

Electrical resistivity, thermopower and thermal conductivity studies of (Sm1-xYx)Cu2 and RCu2 (R identical to Gd, Pr or Tb) systems

The authors report the electrical resistivity rho , thermopower S and thermal conductivity lambda studies of (Sm1-xYx)Cu2 and RCu2 (R identical to Gd, Pr or Tb) systems between 4.2 and 300 K. The value of the antiferromagnetic ordering temperature TN for the ((Sm1-xYx)Cu2 systems is found to become depressed, almost linearly, with Y substitution. The value of TN extrapolates to zero as x to 1. Such a behaviour is attributed solely to the dilution effects of Sm by Y atoms. Chemical pressure effects on the value of TN due to addition of Y atoms are found to be negligible. The lambda curves of SmCu2, GdCu2 and TbCu2 exhibit a peak for T or= 0.10. The S curves exhibit several extremum features, at T <or= TN, which is ascribed to the variation in the density of states at the Fermi level.

Thermoelectric power and resistivity studies in the Kondo-lattice system CeGa2 with Sn or Al substitutions and RGa2 (R identical to Ho,Dy,Tb) alloys

Resistivity rho and thermopower S studies between 1.7 and 300 K have been carried out to examine Kondo and magnetic ordering effects in Ce(Ga1-xSnx)2, Ce(Ga0.9Al0.1)2 and RGa2 (R identical to Ho,Dy,Tb) alloys. Evidence for the presence of the Kondo effect in CeGa2 was deduced from the high-temperature logarithmic variation observed in the rho data, supporting the conclusions of the earlier neutron studies. In the Sn-doped alloys (x>0.1), the Kondo effect is also found to be present as observed through the evolution of rho min. Such a rho min behaviour is not seen in the case of pure CeGa2 alloy. Further, for the Sn-doped alloys, the magnetic ordering temperature is depressed compared with the pure CeGa2 alloy. The development of the low-temperature magnetic order was found in RGa2 as well as in the Al-doped alloy, whereas both Kondo and magnetic ordering effects were observed in the Sn-doped samples.

Thermal irreversibility effects close to the antiferromagnetic transition temperature in Pr(Cu1-xNix)2Si2 systems

The electrical resistivity of Pr(Cu1-xNix)2Si2 systems has been measured between 1.7 and 300 K. We observe, for the first time in Pr-based ternary systems, thermal hysteresis effects, at temperatures below their antiferromagnetic ordering and temperatures, in a narrow concentration range of 0.075<or=x<or=0.15. This hysteresis effect is also corroborated by the magnetization results. It could be related either to spin reorientation effects or to the quadrupolar strains distorting the lattice.

Structural and magnetic ordering effects in (Tb1-xYx)Cu systems

Structural transformation and magnetic ordering effects have been observed at temperatures TS and TN, respectively, in polycrystalline as well as in single-crystalline samples of (Tb1-xYx)Cu. For the TbCu (x=0)system, the values of TS and TN are found to be the same for the polycrystalline sample, whereas they differ for the single-crystalline one. Furthermore, for all values of x, the value of TN is the same for both the above classes of samples. However, the value of TS for the single-crystalline samples is found to be larger by about 10-20 K compared to that for the polycrystalline ones.

Electrical resistivity, thermopower and thermal conductivity studies in RNiGe (RGd, Tb, Dy, Er, Ho, Y) systems

Abstract We report here transport studies on the rare earth equiatomic germanide RNiGe systems between 4.2 and 300 K. Some of the systems (R  Gd, Tb, Dy) show evidence of magnetic transition from our electrical resistivity ϱ studies. For R  Tb, two clear transitions at temperatures of 18 K and 9 K have been observed in contrast to a single transition, at 7 K, from the earlier magnetic study. In the case of R  Er, no magnetic transition has been observed down to 4.2 K from our ϱ studies, whereas the previous magnetic study revealed the presence of a magnetic transition at around 6 K. In addition, both the thermopower S and the thermal conductivity λ data exhibit complex behaviour at low temperatures.