T F Smith

Elastic properties of (Hf,Zr)V2 superconducting compounds

Youngs modulus and internal friction have been measured between 6K and room temperature for several (Hf1-xZrx)V2 alloys (x=0, 0.25, 0.5, 0.75 and 1.0). A V shaped minimum is found in Youngs modulus for each of the alloys. For HfV2 and ZrV2 the minimum is centred about a temperature at which, as has been confirmed by X-ray diffraction, there is a structural distortion. A peak in internal friction near this temperature is observed and discussed on the basis of current understanding of the structural transformation.

Thermodynamic study of the magnetic ordering of Fe3?x Mn x Si alloys

The compounds Fe3−xMnxSi exhibit a complex magnetic behavior for 0.6⩽x⩽1.75 involving ferromagnetic ordering followed by a reordering at lower temperature. Measurements of the thermal expansion α and the specific heat Cp show only a weak anomaly at the Curie temperature, TC, whereas a large, relatively sharp peak occurs at the magnetic reordering temperature, TR. Thermal expansion measurements for the Fe2.2Mn0.8Si compound made about TR are indicative of long-range ordering that can be suppressed in an applied field of ∼0.4T. Magnetization measurements show that low fields, 1–2 mT, strongly influence the bulk magnetization. Irreversible magnetization behavior, similar to that in spin-glass systems, is displayed.

Anisotropic thermal expansion of polycrystalline V25.75at%Si

Abstract Thermal expansion measurements along three mutually orthogonal directions in polycrystalline V 3 Si display anisotropy up to about 90K, indicating that the onset of the cubic-tetragonal structural transformation in this compound can take place at temperatures well above those suggested by x-ray diffraction studies. In addition a small volume change accompanies the transformation. The results are discussed in relation to other anomalous properties of this compound.

Internal friction in A15 and C15 superconductors in the temperature range 4-150K

Measurements of the internal friction for the propagation of longitudinal ultrasonic waves in the kHz range have been made for samples of Nb3Sn, V 25.25 at.% Si, HfV2 and ZrV2 all of which undergo lattice transformations above their superconducting transition temperatures. Large peaks in the internal friction are observed just below the lattice transformation temperatures of HfV2 and ZrV2, but not for Nb3Sn and V 27.25 at.% Si, though the latter does exhibit a much smaller peak at a temperature above the transformation.

Gruneisen parameters, electron-phonon enhancement and superconductivity for Pd-H alloys

Measurements of the low-temperature (1.5-30K) and room temperature thermal expansion have been made for palladium-hydrogen alloys with H/Pd<or approximately=0.7. The low-temperature data have been resolved into the individual electronic and lattice components and values for the corresponding Gruneisen parameters in the zero-temperature limit have been calculated. The influence of the optic modes on the volume dependence of the electron-phonon interaction parameter and its relationship to the contribution to gamma e from the electron-phonon enhancement and the magnitude of the pressure dependence of the superconducting transition temperature of PdH is discussed.

Anomaly in the low temperature thermal expansion of alloys near the critical composition for magnetism

Abstract The thermal expansion of Ni-Cr alloys, near the critical concentration xM at which ferromagnetic order appears, shows a small characteristic anomaly below about 10 K. This anomaly increases in size as xM is approached, then abruptly changes sign going through xM. We deduce from the expansion data that the ground-state magnetovolume in the ferromagnetic Ni-Cr alloys is positive, and suggest a qualitative picture that accounts for the sign change of the anomaly. Our explanation provides support for the notion that spin fluctuations significantly affect volume. Similar behaviour appears to be seen in other alloys.

The low-temperature thermal expansion of polycrystalline V3Ge

Abstract The expansion coefficient for polycrystalline V3Ga has been measured in three mutually perpendicular directions through the sample using capacitance dilatometry. Anisotropy is found at temperatures below 40 K. This behaviour is discussed in terms of the related physical properties for V3Ge and can be explained in terms of a coupling between the elastic compliance constants and an internal stress in the sample.

Magnetic Gruneisen parameters for some spin glasses

Values for the magnetic Gruneisen parameter gamma M for CuMn, (Cu-10 at.% Al)Mn and AgMn alloys determined form low-temperature thermal expansion and specific heat capacity measurements are reported. mu M is found to be both temperature- and concentration-dependent in contrast to the simple RKKY model for the spin-glass ordering. The concentration dependence of mu M differs significantly from that reported earlier based upon a combination of thermal expansion and specific heat capacity measurements made independently on different samples. Dipolar anisotropy is discussed as a possible cause of the dependence of mu M on temperature and magnetic impurity concentration.

Hall coefficient of TaS2-xSex layer compounds

The Hall coefficients of several TaS2-xSex layer compounds have been measured as a function of temperature between 4K and 200K. Anomalous changes in RH, which are associated with the occurrence of charge density wave periodic lattice distortion formation, have been observed and the variation of the onset temperature T0 with composition has been obtained. Values of Tc and dTc/dP have been obtained for single-crystal TaSe2 which are higher than those reported for polycrystal material. A strong inverse correlation is found between T0 and the superconducting transition temperature Tc. A comparison between T0 and dTc/dP indicates that the pressure-induced increase of Tc for these compounds is not entirely due to the suppression of the charge density wave, as previously suggested.

Low-temperature thermophysical properties of Fe3Si

Abstract Measurements of the thermal expansion of Fe3Si in the temperature range 2-300 K are reported. The low-temperature linear thermal expansion of Fe3Si is accurately described by the expression α = (-3·5±0·5)x 10−9 T + (47middot;07plusmn;2) × 10−12 T 3 K−1, being the sum of a linear electronic contribution and a cubic lattice contribution. The thermal expansion is negative below 24 K. The failure to find evidence for low-temperature T 3/2 spin-wave contributions to the low-temperature thermophysical properties of Fe3Si is shown to be consistent with the very low values for the spin-wave thermophysical coefficients predicted on the basis of neutron-scattering experiments. Values for the electronic and lattice Gruneisen parameters in the low-temperature limit are calculated. The negative electronic Gruneisen parameter for Fe3Si is shown to be consistent with band-structure calculations which all predict a large peak in the electronic density of states at the Fermi surface.