Search

The effects of uniaxial stresses on c axis as well as a axis oriented thin films of YBa 2 Cu 3 O 7− x have been investigated. We show that the effects depend strongly on the microstructure. Thus the a axis oriented films exhibit non monotonous uniaxial pressure effects. On the contrary, c axis oriented films show reversible effects. Our results are compared with those previously published.

The electrical resistivity and the lattice parameters have been studied as a function of pressure on the quasi-twodimensional purple bronze KMo 6 O 17 which shows a Peierls transition towards a commensurate charge density wave state. The Peierls temperature is found to be first slightly decreased for pressures smaller than 6 kbar, then strongly increased above. This increase is associated to an anomalous contraction of the lattice parameters in the plane of the layers. The corresponding large increase of the compressibility above 16 kbar at 300 K is associated to the pretransitional regime of the Peierls transition as a function of pressure. These results are attributed mainly to an improved nesting of the Fermi surface under pressure.

We present a detailed study of the superconducting properties of crystals of Pb 2 Sr 2 Y 1− x Ca x Cu 3 O 8 (x between 0.36 and 0.39), in particular the irreversibility line obtained from magnetoresistance measurements up to 20 teslas, and the variation of T c with hydrostatic pressure (H.P.) up to 16.6. kbars and quasi-hydrostatic pressure (Q.H.P.) up to 100 kbars. This variation of T c with pressure strongly depends on the nature of the applied pressure, which is attributed to a strong sensitivity of T c to defects introduced in QHP experiments. The anisotropy of the irreversibility field H ∗ is similar to that of YBa 2 Cu 3 O 7 although the distance between the double copper-oxygen planes is much larger for Pb 2 Sr 2 Y 1− x Ca x Cu 3 O 8 .

We have measured the resistivity of electron-doped superconductors Ln 1.85 Ce 0.15 CuO 4− y (Ln = Nd, Sm, Eu) under quasihydrostatic pressure up to 100 kbar. All the samples remain superconducting for this pressure. The ratio d log T c / d log V was found to be of the same order of magnitude as the one measured under hydrostatic pressure up to 20 kbar, weak and positive, in contrast with the hole doped high T c superconductors. The effect of applied pressure on T c does not compare with that of chemical pressure.

We have synthesized a new perovskite of the K2NiF4 structure Sr2VO4. The formal valence of vanadium is 4+. We have performed X-ray and neutron experiments which show that the octahedra surrounding the vanadium is elongated. The compound is insulating and antiferromagnetic. These properties are reminiscent of those of La2CuO4 which becomes a high Tc superconductor when doped, the copper d9 ion being replaced by the vanadium d1 ion.

We have studied the electronic specific heat of various nickel-platinum alloys near the critical concentration for the appearance of ferromagnetism. The electronic specific heat coefficient has a maximum for an atomic concentration of 42.5 ± 0.5% nickel. This maximum coincides with that of the high field susceptibility. The results obtained are discussed qualitatively.

We report magnetization, resistivity and small angle neutron scattering experiments on cubic B20 FexCo1−x SI alloys (0.3 ⩽ x ⩽ 0.9) and CoxMn1−xSi allpys (x = 0.02, 0.04 and 0.06). The magnetically ordered FexCo1−xSi alloys and the CoxMn1−xSi alloys with x = 0.02 and 0.04 are found to be helimagnetic with very long period. The low temperature resistivity of the magnetically ordered FexCo1−xSi alloys exhibits an anomalous positive magnetic contribution. The occurence of the Dzyalosyinsky-Moriya spin-orbit type interaction in these non-centrosymmetric alloys is given as a possible explanation of the helimagnetic order.

Magnetization, NMR and coercive force measurements were performed on amorphous Ni1−cPc alloys for concentration ranging between 0.15 and 0.25. The experimental results show that the magnetism is not of the same nature below and above some critical concentration c0 which determines the concentration ranges: the first one (0.15 < c < 0.18) presents the characters of a weak homogeneous ferromagnetism, while inhomogeneities dominate the magnetic properties of the second one (0.18 < c < 0.25).

We have measured the thermal variation of the magnetization of weakly ferromagnetic Ni0.47Pt0.53, Ni0.52Pt0.48, Ni0.04Pd0.96 and Ni0.05Pd0.95 alloys in fields up to 150 kOe. The values of the stiffness coefficients D for the magnon dispersion curves are deduced. Small angle neutron scattering measurements with and without a magnetic field were carried out on Ni0.45Pt0.55, Ni0.50Pt0.50 and Ni0.04Pd0.96 alloys. An independent measurement of D has been obtained from the cut off angle of the intensity of the scattered neutrons. The results are compared.

The (FeTi)x(CoTi)1−x pseudobinary alloys are weekly ferromagnetic in the range 0.4

The electrical resistivity of CePd3 has been measured between 1.5 and 300 K under hydrostatic pressures up to 20 kbar. The temperature TM for the maximum of resistivity increases with pressure at the rate dTMdp = 0.65 K kbar−1. The value, −5.2, found for ∂ lnM∂ lnV is in good agreement with previous data based on elasticity measurements and thermodynamical properties.

Magnetization measurements of amorphous FeGe alloys containing 20 and 30 at.% iron have been made under high field and high pressure. These alloys possess a weakly itinerant ferromagnetic character although large inhomogeneous effects are superposed.

The cubic Laves phase of YCo2 has a strongly enhanced susceptibility χy. The variation of χy with pressure at 40 K has been measured and the initial slope is d log χy/d log V = 14 ± 2. This result is correlated with the pressure dependence of the Curie temperatures of A Co2 alloys.