E. Felder

Unusual magnetism of hexaborides

Various recent experimental investigations have revealed unusual magnetic properties of hexaborides with divalent cations M2‘. EuB 6 is ferromagnetic below 16 K and its low-temperature properties show remarkable similarities to those of manganese oxides, exhibiting the phenomenon of colossal magnetoresistance. Close to the phase transition as well as far below the ordering temperature, EuB 6 exhibits anomalous features, that are brie#y discussed. Alkaline-earth hexaborides are close to a metal}insulator transition and it has been found that, in a narrow range of electron doping, an itinerant-type of ferromagnetic order, stable up to temperatures of the order of 600}900 K, is established. This remarkable phenomenon is suspected to be due to the peculiar electronic band structure of these materials. ( 2000 Elsevier Science B.V. All rights reserved.

Impurity effects in UBe13

Abstract The effect of a few per cent of thorium replacing uranium in the heavy-fermion superconductor UBe13 is dramatic. The low-temperature properties are drastically altered and a second transition of a controversial nature occurs below the onset of superconductivity. Other non-magnetic impurities are seen from various preliminary measurements to yield promise of different results. Normally such impurity studies show only gradual changes in properties, which are useful for extracting superconducting parameters, but the large effects seen in this heavy-fermoin superconductor highlight its exotic nature.

Physics of YbBiPt

YbBiPt has a low temperature linear specific heat coefficient of 8J/mole-Yb K{sup 2} and a small specific-heat anomaly at 0.4K. We discuss new experiments on specific-heat of diluted YbBiPt, and magnetic field dependent effects and electrical resistivity in pure YbBiPt. We argue that in this material the Kondo and crystal-field energy scales are small and of comparable magnitude, placing YbBiPt in the same class as many Uranium heavy-electron compounds.

Magnetic phase transition at 88 K in Na 0.5 Co O 2 revealed by Na 23 NMR investigations

Na0.5CoO2 exhibits a metal-insulator transition at 53 K upon cooling. The nature of another transition at 88 K has not been fully clarified yet. We report the results of measurements of the electrical conductivity, the magnetic susceptibility and 23Na NMR on a powder sample of Na0.5CoO2, including the mapping of NMR spectra, as well as probing the spin-lattice relaxation rate and the spin-spin relaxation rate, in the temperature range between 30 K and 305 K. The NMR data reflect the transition at T_X very well but provide less evidence for the metal-insulator transition at T_MI. The temperature evolution of the shape of the spectra implies the formation of a staggered internal field below T_X, not accompained by a rearrangement of the electric charge distribution. Our results thus indicate that in Na0.5CoO2, an unusual type of magnetic ordering in the metallic phase precedes the onset of charge ordering, which finally induces an insulating ground state.

Electrical transport, magnetic, and thermal properties of FeSi

We report measurements of the electrical conductivity, magnetoresistance, Hall effect, magnetic susceptibility, and specific heat, measured in different temperature ranges between 0.05 and 300 K, on a high-quality FeSi single crystal grown by vapour transport. The semiconductor-like temperature dependence of the resitivity of FeSi arises essentially from thermal activation of charge carriers across two different energy gaps. Several features suggest thats-d hybridization effects are important. At low temperatures some evidence of electron-electron interactions is found.

Comparative study of the third-order magnetic susceptibility and the specific heat of antiferromagnetic MnBr2·4H2O

We report a comparative study of the third-order magnetic susceptibility χ3(T) and of the specific heatCp(T) of an antiferromagnetically ordering MnBr2·4H2O single crystal withTN=2.13 K. The dominant feature in χ3(T) is a positive singularity nearTN. The critical exponents of χ3 are negative both above and belowTN. On the contrary, the critical behaviour of the magnetic contributionCm(T) to the specific heatCp(T) is logarithmic belowTN and may be described by a large and positive critical exponent definitely exceeding the value of 0.3 aboveTN. These results suggest that the critical behaviour of χ3(T) of a moderately anisotropic antiferromagnet is qualitatively different from that ofCm(T).

Low-temperature magnetic and thermodynamic properties of Fe1 − xCoxSi

Abstract We report measurements of the low-field AC magnetic susceptibility χ′(T,ƒ) and of the specific heat Cp(T) for polycrystalline samples of Fe1−xCoxSi with 0 ⩽ x ⩽ 0.03 at low temperatures. For x ⩾ 0.01, the χ′(T) curves show maxima indicative of a spin-glass-type freezing of magnetic moments below 1 K. The small values of Tf/x and the Tf(f) variations both suggest a weak magnetic interaction. The specific heat Cp(T) contains a term γT which raises with increasing x from 2.1 T mJ mol−1 K−1 for FeSi to 7.6 T mJ mol−1 K−1 for Fe0.97Co0.03Si.

Low-temperature electrical transport in icosahedral Al−Re−Pd

Measurements of the electrical conductivity σ(T) and magnetoconductivity Δσ(H) of icosahedral Al70Re10Pd20 and Al70Re8.6Pd21.4 quasicrystals at low temperatures and in high magnetic fieldsH are reported. Below 0.2 K σ of Al70Re10Pd20 varies as σ(T)=σ0+αT1/2 with σ0=30Ω-1cm-1 and a<0. In agreement with expectations presuming a substantial Coulomb interaction, Δσ(H) is negative. For Al70Re8.6Pd21.4 σ(T) decreases with decreasingT and saturates at the level σ0=1.7Ω-1cm-1 below 0.1 K. In all measured magnetic fields up to 40 kOe, Δσ(H) is negative below 1 K and remains temperature-independent at the lowest temperatures. The saturation of σ(T) implies that in this material Coulomb interactions among itinerant electrons are negligible.

Low-temperature thermal properties of icosahedral Al−Re−Pd

We report measurements of the thermal conductivity λ(T) and the specific heat,Cp(T) of icosahedral Al70Re8.6Pd21.4 at low temperatures. Below 1 K, the behaviour of the quasilattice thermal conductivity λph(T) is compatible with the scattering of vibrational excitations involving tunneling states and grain boundaries. The specific heatCp(T) contains a small contribution ∼γT. Our λph(T) results indicate that a substantial part ofγT must be attributed to the excitations of tunneling states. Below 0.4 K,Cp(T) increases with decreasingT and, at the lowest temperatures, varies asT−2 suggestive of an interaction of the quadrupolar moments of185Re and187Re nuclei with the gradient of electric fields induced by their neighbours.

Low-temperature thermal properties of decagonal Al65Cu20Co15

We report measurements of the specific heatCp(T) and of the thermal conductivity λ(T) along both periodic and quasiperiodic directions on single-grained samples of decagonal Al65Cu20Co15 quasicrystals at low temperatures. The electronic contribution to λ(T) in the periodic direction is compatible with the prediction of the Wiedemann-Franz law in the limits of low and high temperatures. The phonon thermal conductivity λph in the periodic direction shows a maximum at 25 K. The dominant feature in λph (T) measured along a direction in the quasiperiodic plane is an extended plateau between 30 and 60 K, in agreement with the concept of generalized Umklapp processes in quasicrystals. The coefficient γ=0.512 mJg-atom−1K−2 of the linear term toCp(T) indicates a low density of electronic states atEF.