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Magnetoresistance (MR) studies on polycrystalline R2Ni3Si5, (R=Y, rare earth) which order antiferromagnetically at low temperatures, are reported here. MR of the Nd, Sm, and Tb members of the series exhibit positive giant magnetoresistance, largest among polycrystalline materials (85%, 75%, and 58% for Tb2Ni3Si5, Sm2Ni3Si5, and Nd2Ni3Si5, respectively, at 4.4 K in a field of 45 kG). These materials have, to the best of our knowledge, the largest positive GMR reported ever for any bulk polycrystalline compounds. The magnitude of MR does not correlate with the rare earth magnetic moments. We believe that the structure of these materials, which can be considered as a naturally occurring multilayer of wavy planes of rare earth atoms separated by Ni–Si network, plays a role. The isothermal MR of other members of this series (R=Pr,Dy,Ho) exhibits a maximum and a minimum, below their respective TN’s. We interpret these in terms of a metamagnetic transition and short-range ferromagnetic correlations. The short-range ferromagnetic correlations seem to be dominant in the temperature region just above TN.

The formation of two new materials, Sm2Ni3Si5 and Gd2Ni3Si5, of the rare‐earth series R2M3Si5 (R denotes rare earth and Y) and their magnetic and transport properties are reported here. These materials crystallize in the orthorhombic U2Co3Si5‐type structure (space group Ibam). The magnetic susceptibility of Gd2Ni3Si5 follows a Curie–Weiss behavior with an effective magnetic moment 8.1μB/Gd ion. The material orders antiferromagnetically at ≊15 K. The magnetic susceptibility of Sm2Ni3Si5 exhibits a deviation from the Curie–Weiss behavior which is attributed to the low‐lying excited state of Sm3+ ions and also to crystal‐field effects. This material also orders antiferromagnetically, but at ≊11 K. This value of the ordering temperature does not seem to follow the de Gennes scaling with respect to that of Gd2Ni3Si5.

We have investigated the thermal and electrical properties of filled skutterudites belonging to the series Ce1−zYbzFe4Sb12 and Cey∕2Yby∕2Fe4−x(Co∕Ni)xSb12, as well as their potential for thermoelectric power generation. In the first series, increasing the Yb fraction decreases the resistivity and thermopower. These effects are related to the variations of valence of Yb with the Yb fraction (from 2.2 in Yb0.92Fe4Sb12 to 2.7 in Ce0.85Yb0.05Fe4Sb12). The power factor is increased in Ce0.40Yb0.53Fe4Sb12 while the thermal conductivity is reduced and this leads to an improved figure of merit in this compound when compared to Ce0.85Fe4Sb12. Co or Ni substitution was used to tune the carrier concentration and improve the power factor: we measure a ZT=0.5 at 500K in Ce0.44Yb0.32Fe3.02Co0.98Sb12 and an extrapolation leads to ZT=0.95 at 800K, which is close to state-of-the-art Ce0.28Fe1.52Co2.48Sb12 although the composition is far from being optimized. Double filling happens to be a promising path to provide good th...

SmRuSn3 is a unique compound among the known Sm‐based valence fluctuation (VF) systems. Its crystallographic structure has two inequivalent Sm sites and Sm ions at only one of them are in VF state while the Sm ions in the other site orders magnetically. Our 119Mossbauer studies show a quadrupolar splitting at the Sn site, consistent with the noncubic symmetry of the Sn site. A broadening of the Mossbauer spectrum is seen due to magnetic ordering of the material. The transferred hyperfine field at Sn site at 4.2 K is small.

Neutron diffraction has been used to investigate the magnetic order of RNi2B2C. For R=Er the system orders antiferromagnetically at T N =6.8 K, and this long range order coexists with superconductivity (T c =11 K). The magnetic structure is an incommensurate, transversely polarized spin‐density‐wave state, with the modulation wave vector δ along the a axis and the moments along b. δ has a temperature‐independent value of 0.5526 (2π/a), with the structure squaring up at low temperatures. For R=Ho the moments also prefer to reside in the a–b plane, but initially an incommensurate c‐axis spiral state forms upon cooling, with T N ≊T C ≊8 K. This c‐axis spiral consists of ferromagnetic sheets of holmium moments in the a–b plane, but with each sheet rotated by ∼163° as one proceeds along the c axis. Small a‐axis peaks are also observed above the reentrant superconducting transition over a narrow temperature range, but the c‐axis peaks dominate. Just below the reentrant transition at ∼5 K the magnetic system locks‐in to a simple commensurate antiferromagnetic structure, which permits superconductivity to be restored. The c‐axis spiral, the a‐axis component, the commensurate antiferromagnetic structure, and the superconducting phase are all in a delicate balance energetically, and this balance may be easily shifted by subtle changes in composition, magnetic field, and pressure. DyNi2B2C orders antiferromagnetically at T N =11 K, with the same commensurate antiferromagnetic structure as found for the holmium material at low temperature. The existence of superconductivity in some samples of DyNi2B2C is consistent with the antiferromagnetic structure observed.

From our x‐ray (LIII‐edge) absorption (XAS) investigations of Ce2Ni3Si5, we show that Ce‐valence is temperature dependent; it is 3.07 and 3.11 at 280 and 8 K, respectively. We also report on our resistivity measurements of two related materials Ce2−xRxNi3Si5 (R=Y, Gd and x=0.1). Absence of any qualitative difference in the resistivities of these two samples suggests that the enhancement of resistivity at low temperature on introduction of impurity atoms is due to Kondo hole scattering implying that Ce2Ni3Si5 is a concentrated Kondo system.