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We report a comparative study of the electrical resistivity under hydrostatic pressure up to 18?kbar of the quasi-two-dimensional conductors (PO2)4(WO3)2m (m = 4, 5, 6) and ?-Mo4O11, which show Peierls transitions towards a charge density wave state. The pressure dependences of the transition temperatures are discussed in relation with the hidden nesting properties of these oxides. The results for ?-Mo4O11 are compared with previous works.

The influence of hydrostatic pressure on the transport properties of (La1-xRx)0.67Ca0.33MnO3 ( ,Tb) ferromagnetic manganites is investigated. The enhancement of the Curie temperature TC under pressure agrees with previous data. In the paramagnetic range, the resistivity can be represented by a Mott localisation law, with a characteristic temperature T0 decreasing with pressure. The variation of TC with pressure is compared to the effect induced by replacing La by a magnetic rare earth in (La1-xRx)0.67Sr0.33MnO3 manganites ( , ..., Tm). The main effect is not related to the decrease of the mean radius of the cation, but to an additional scattering by the magnetic moment of the rare earth.

CoVSb is a weak itinerant ferromagnet with one electron in the conduction band. Its physical properties (magnetization, resistivity, Hall effect, thermopower, pressure dependence of resistivity, ESR) and its electronic structure calculated by the KKR method are compared to those of some other semi-Heusler phases with semiconducting or metallic behaviour. It appears that the largest magnetic moment is borne by vanadium atoms. The Curie temperature is mainly related to Stoner-like excitations, even if spin fluctuations play an important role as revealed by the resistivity and ESR data. The problem of the atomic disorder between transition metals is discussed.

Des forts courants pulses d'une duree comprise entre 1,25 et 20 ms ont ete appliques a T = 77 K a des conducteurs Bi:2223 textures par forgeage a chaud ou frittes. Pour des pulses de courant , deux comportements sont observes : quand la temperature moyenne de l'echantillon est inferieur a , le materiau retourne dans l'etat supraconducteur avant la fin du pulse de courant. Au dessus de , l'echantillon transite dans l'etat normal apres le pulse de courant a cause de l'elevation de temperature. Ceci induit une puissance instantanee importante appliquee au conducteur de 182 GW/m 3 . A I = 15 I c , des champs electriques de 400 a 1000 V/m sont obtenus sur des elements frittes et textures.

In this study, the transition of Bi 1.8 Pb 0.4 Sr 2 Ca 2.2 O 10.3+ x ceramics in the form of both textured and sintered bars is investigated at 77 K using high pulsed currents with a duration varying from 1.25 to 20 ms. Two types of behaviour are observed: (i) When the average temperature T of the sample is lower than T c = 110 K,a critical current is recovered at the end of the pulse, probably because of good thermal exchange between the sample surface and the bath and because the critical current density exists up to 100 K; (ii) above this temperature, the sample remains normal after the pulse because the energy dissipated is too high. No deterioration in the sample is observed and experiments can be reproducibly repeated. The maximum instantaneous power applied to the material is 140 GW/m 3 and the maximum energy per unit volume transmitted to the sample after a 20 ms pulse is 925 MJ/m 3 . V ( I ) curves for sintered and textured samples are compared. At 15 times I c , the measured electric fields are 1000 V/m and 400 V/m for textured and sintered samples, respectively. Using specific heat data and assuming adiabatic heating, the average temperature reached after pulse is calculated. Resistivity curves are obtained for sintered and textured materials which are in good agreement with classical measurements on small samples.

Bulk textured BiPbSrCaCuO (2223) ceramics have been prepared by solidification in a magnetic field. We have determined the optimal temperature required in the melt in order to maximize the effect of the magnetic field. X-ray pole figures and scanning electron microscopy have been used to characterize the nature and degree of texture. Magnetic and electrical transport measurements in the superconducting state are also reported. Transport critical current densities as high as 1450 A/cm 2 at 77 K and 0 tesla have been reached. This value is significantly greater than that, 900 A/cm 2 , obtained following the same heat treatment in the absence of an applied magnetic field.

Complex a.c. susceptibility measurements in a broad field range () have been made on Bi - Pb - Sr - Ca - Cu - O (Bi-2223 phase) bulk samples obtained by combined magnetic melt texturing and hot pressing. Highly anisotropic properties are observed. Improved intergranular coupling is obtained in the direction perpendicular to the applied stress and the magnetic field direction. An intragranular peak is also observed for the same direction. This is the first time that an intragranular loss peak has been clearly seen in a bismuth-based compound, which indicates that the intragranular contribution is strongly increased.

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.

We have developed a new versatile apparatus combining the effect of hot pressing and the effect of magnetic field to texture HTSC (high-temperature superconductor) ceramics. The experimental set-up allows application of uniaxial stress up to 1100 degrees C under a maximum pressure of 60 MPa, in a controlled atmosphere and a magnetic field of 8 T. We have optimized the cycle for stress application along with the thermal cycle to produce highly-textured ceramics. In this paper, we describe the stress and thermal cycles used to texture Bi-Pb-Sr-Ca-Cu-O (2223 phase) ceramics. We present the physical and chemical characterizations of the samples which show a strong degree of crystallite orientation.

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.

The transport properties of the electron-doped cuprates ${\mathit{M}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (M=Pr, Nd, and Sm) single crystals are investigated under hydrostatic pressure up to 2 GPa. The effects of pressure on both the c-axis (${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$) and the in-plane (${\mathrm{\ensuremath{\rho}}}_{\mathrm{ab}}$) resistivity are measured from room temperature down to the superconducting state. It is found that at 300 K, ln${\mathrm{\ensuremath{\rho}}}_{\mathrm{ab}}$/dP\ensuremath{\approxeq}d ln${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$/dP\ensuremath{\approxeq}-3% ${\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1}$. The pressure does not change the temperature dependence of the normal-state resistivity, which at high temperature in both directions follows a \ensuremath{\rho}\ensuremath{\sim}A(P)${\mathit{T}}^{2}$ law, but the coefficient A(P) lowers as pressure is increased. Moreover, we have found that for any lanthanide host (Pr, Sm, and Nd), the onset of superconductivity does not change with pressure (P2 GPa). These experimental results raise questions on the significance of the already proposed general behavior of ln${\mathit{T}}_{\mathit{c}}$/dP on ${\mathit{T}}_{\mathit{c}}$ and on the claimed ``antisymmetry'' of the pressure dependence of ${\mathit{T}}_{\mathit{c}}$ in electron-doped and hole-doped high-temperature superconductors.

The transport properties of the electron-doped cuprates Ln 2- x Ce x CuO 4 (Ln z.bbnd; Pr, Nd and Sm) single crystals are investigated under hydrostatic pressure up to 2 GPa. The effects of pressure on both the c -axis (ϱ c and the in-plane (ϱ ab resistivity are measured from room temperature down to the superconducting state. It is found that at 300 K, dln ϱ ab /d P ≈dln ϱ c /d P ≈ −3% GPa −1 . The pressure does not change the temperature dependence of the normal state resistivity, which at high temperature in both directions follows a ϱ ∼ A ( P ) T 2 law, but the coefficient A ( P ) decreases as pressure is increased. Moreover, we have found that for any lanthanide host (Pr, Sm and Nd) the onset of superconductivity does not change with pressure ( P

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 report the effect of hydrostatic pressure on the resistivity anomaly in KMo6O17 resulting from charge-density-wave (CDW) formation. The rate ofdecay of the Peierls temperature with pressure is δT p/δp = −2Kkbar−1 for p>6kbar and δT p/δp = +l0K kbar−1 for p>6kbar. The observed behaviour is ascribed to two competing mechanisms. At low pressures, interlayer couplings and a stiffening of the lattice are involved. At higher pressures, intralayer couplings and a strong enhancement of the density of states at the Fermi level are responsible for the large increase of T p observed.

Measurements of the in-plane and c -axis normal-state resistivity in a superconducting Nd 1.85 Ce 0.15 CuO 4−δ single crystal are reported. The resistivity anisotropy of this n-type material is ≤250, much smaller than BiSCCO and comparable to YBaCuO. Both, ϱ ab ( T ) and ϱ c ( T ) displays a metallic-like positive temperature coefficient of resistivity with a basic T 2 dependence. We discuss some possible origins of this peculiar temperature dependence.

Single crystals of SrxMo6S8 with different strontium concentrations (x 10 K under 20 kbar). The structural transition (Ts) and superconducting critical (Tc) temperatures under pressure for stoichiometric and substoichiometric samples can be described by the same curve, if chemical pressure is taken into account. Comparison is made with EuMo6S8 and related compounds where the cations are magnetic.

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 measured the pressure effects on the superconducting transition temperature (Tc) of Sm2−xCexCuO4−y for various x and y which show an asymmetrical behavior contrasting with what is observed in La2−xSrxCuO4 compounds. Previous data under hydrostatic pressure up to 20 kbar on the Ln1.85M0.15CuO4−y compounds are surveyed and the results of new resistivity measurements under quasihydrostatic pressure up to 100 kbar are given. Unlike the hole-doped high Tc superconductors, the quantity dLogT c dLogV was found to be weakly positive in the whole range of pressure. A simple chemical pressure analysis based on the dependence of Tc on the cell volume (derived from X-ray data) in the series of rare-earths, is not consistent with the pressure effects. The data are discussed in terms of interaction between magnetism and superconductivity.

We study the superconducting properties of the Electron.-doped compounds Ln2-xMxCuO4-y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th). Three characteristic temperatures of the superconducting transition are defined, corresponding to: the macroscopic resistive transition midpoint (Tc mid), the diamagnetic signal onset (Tcχ), and the intragrain transition temperature (Tcg). Tcχ and Tcg are found to vary weakly in the series of the studied compounds, in contrast with Tc mid, which is strongly reduced in Eu1.85Ce0.15CuO4-y and Sm1.85Th0.15CuO4-y samples.

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.

Measurements of the superconducting transition temperature {ital T}{sub {ital c}} in the electron-doped copper-oxide compounds {ital L}{sub 2{minus}{ital x}}M{sub x}CuO{sub 4{minus}{ital y}} under pressure {ital P} to {similar to}20 kbar reveal that, in general, {ital T}{sub {ital c}} decreases with {ital P}, in contrast to the behavior of hole-doped copper-oxide compounds. The quantity {ital d} ln{ital T}{sub {ital c}}/{ital d} ln{ital V} exhibits a dependence on {ital T}{sub {ital c}} similar to that of hole-doped compounds, but opposite in sign, and can be as large as {similar to}24. Correlation of structural and transport properties indicates competition between interlayer and intralayer effects in these materials.

International audience; We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba8@Si–46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I8@Si–46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp3 silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C60.

We have investigated a sintered Gd 1.85 Th 0.15 CuO 4 sample in a SQUID magnetometer under pressure up to 10 kbar. From high-field magnetic measurements, we have deduced a large decrease under pressure of the internal field associated to the weak ferromagnetism of the Cu sublattice. We have studied by magnetic susceptibility measurements in low field the pressure effect on the Neel temperature, on the magnetic ordering temperature of the rate earth and on the intermediate spin reorientation temperature. Possible implications for electron-doped superconductivity of related compounds are discussed.

Sintered Nd1.85Ce0.15CuO4-δ samples were characterized. The absence of pressure effects on the critical temperature was confirmed. The magnetoresistance measurements can be understood from the anisotropy of critical fields of single crystals.

Texturing of high-temperature superconducting ceramics leads to an improved intergranular coupling. This is demonstrated on the basis of ac susceptibility measurements obtained when texturing Bi-2223 polycrystals through combined magnetic melt texturing and hot pressing (MMTHP) and Y123 bulk samples obtained through combined magnetic melt texturing and zone melting (MMTZM).

We discuss the magnetic properties of lead doped Bi-2223 bulk samples obtained through combined magnetic melt texturing and hot pressing (MMTHP). The ac complex susceptibility measurements are achieved over a broad ac field range (1 Oe{lt}h{sub ac}{lt}100 Oe) and show highly anisotropic properties. The intergranular coupling is improved in the direction perpendicular to the applied stress and magnetic field direction, and an intragranular loss peak is observed for the first time. A comparison is made with other bismuth-based compounds and it is shown that the MMTHP process shifts the ac irreversibility line (ac IL) toward higher fields. It is also shown that all the ac IL{close_quote}s for quasi 2D bismuth-based compounds show a nearly quadratic temperature dependence and deviate therefore strongly from the linear behavior observed in quasi 3D compounds and expected from a critical state model.{copyright} {ital 1997 Materials Research Society.}

We describe a new texturing process for superconducting ceramics which combines the effects of magnetic melt texturing (MMT) and hot pressing (HP), referred to in the following as the MMHPT process. This process essentially involves three steps: (i) a hot pressing at a temperature Tp, (ii) a partial melting in a magnetic field at a slightly higher temperature TM, (iii) a second hot pressing at the same temperature Tp. Optimization of the thermomechanical treatment in a magnetic field applied to the (2223) Bi-Pb-Sr-Ca-Cu-O cuprates led to highly textured and dense samples carrying transport current density Jc of 3800 A cm-2 at 77 K in zero field.

Magnetic melt texturation has been applied to ceramics of Bi-Pb-Sr-Ca-Cu-O (2223), including critical current density, Jc of 1090 A/cm 2 at 77 K. Hot pressing of such ceramics increases Jc up to 1667 A/cm 2 . The best Jc of 2500 A/cm 2 is obtained for ceramics hot pressed in the solid state, just below the melting temperature. The effects of the different processes are discussed.

We have optimized a texturing process of Bi-Pb-Sr-Ca-Cu-O (2223) superconducting ceramics by hot pressing. The Hoechst precursor powder has been first pressurized at 1 GPa and then heated under uniaxial stress up to 35 MPa, just below the melting temperature between 850°C and 857°C. We have obtained large textured samples having critical transport current densities J c as large as 2500 A/cm 2 . In this work, we have optimized the stress value as well as the temperature treatment on the basis of characterizations such as magnetization and electrical-transport measurements.

We propose an {ital H}-{ital T} phase diagram of granular superconductors based on the separate analyses of the intragranular critical fields and that of an assembly of Josephson junctions. The model is examined experimentally and its predictions are applied to several problems of high-temperature superconductivity like the {ital H}{sub {ital c}2}({ital T}) enhancement with a temperature reduction, the width of the {ital R}({ital T}) transition under an applied magnetic field, and a double-step {ital J}{sub {ital c}}({ital H}) variation.

Nonmonotonic forms of resistive superconducting transition as a function of temperature and magnetic field have been observed in granular high-${\mathit{T}}_{\mathit{c}}$ ceramics of the electron-type family ${\mathit{L}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$. The behavior is compared with that of low-temperature granular superconductors and is treated by the conventional terms of the energy gap and Josephson and quasiparticle tunnelings. The values of the intragranular upper critical field ${\mathit{H}}_{\mathit{c}2}$ perpendicular to the a-b plane of Sm-Ce-Cu-O are extracted. We find evidence for the appearance of superconductivity in individual grains at temperatures significantly higher than the resistance onset temperature.

An unusual form of superconducting transition has been observed in some of the superconducting compounds {ital L}{sub 2{minus}{ital x}}{ital M}{sub {ital x}}CuO{sub 4{minus}{ital y}} ({ital L}=Pr,Nd,Sm,Eu; {ital M}=Ce,Th) under low magnetic fields. When the temperature is reduced below the critical one, the sample's resistance first increases, then drops to a nonzero value, increases again, and finally decreases at low temperature, creating a characteristic double-peak transition. The behavior is explained in terms of intra- and intergranular superconducting transitions.

Rare-earth-based Chevrel phases constitute model systems to study both superconductivity and magnetic order but results may be hindered by the quality of the sample. Materials elaboration is thus essential to separate out extrinsic effects (secondary phases) from those due to the Mo 6 S 8 clusters and the rare-earth contribution. Crystal growth, transport and magnetic properties of the quaternary systems (RE, Eu)Mo 6 S 8 (RE-Sm, Yb) are presented. Different RE/Eu nominal ratios allowed to cover a large range of concentrations for single crystals. Superconducting transitions are narrow and as high as 9 K for ytterbium-rich samples, while the structural transition of the ternary EuMo 6 S 8 ( T s = 110 K) is largely diminished by the partial substitution of Eu by RE.

We report resistivity and AC susceptibility measurements under external magnetic field in NdCeCuO single crystals. From resistivity data we have estimated the upper critical fields, coherence lengths and anisotropy factor; AC susceptibility has allowed the extraction of the activation energy values, which turn out to be higher than the p-type HTSC ones.

Magnetization measurements in Pr2NiO4 single crystals were made up to 70kOe, for different orientations of the applied magnetic field with respect to the crystallographic axes, in the range from 1.5 to 300K. The Ni sublattice becomes antiferromagnetically ordered at TN=325K, as reported from previous neutron diffraction experiments1. There is also a structural transition at TC1=117K1, which allows a ferromagnetic component in the low temperature phase. Below TC1 different magnetic anomalies are observed, which have been interpreted as spin reorientations. Finally at low temperature the Pr sublattice is also ordered. Four different transitions induced by the external applied magnetic field parallel to the c-axis are visible in the range from 0 to 70 kOe, for T

We studied the superconducting properties of the electron-doped compounds Ln2−xMxCuO4−y (Ln = Pr, Nd, Sm, Eu; M = Ce and Th). The granular structure of the samples influences strongly their superconducting behavior, causing an unusual “double - peak” resistive transition in low magnetic fields. The intragrain transition temperature was found to vary weakly in the series of the studied compounds in contrast with the macroscopic resistive transition midpoint. The ceramics remain superconducting under quasihydrostatic pressure up to 100 kbar. We found that the chemical pressure alone can't be responsible for the absence of superconductivity in the Gd - based compound.

We study the physical properties of Bi 2 Sr 2 CaCu 2 O 8+ x platelet-like single crystals with a zero resistance superconducting transition T c of ≅ 80 K. Normal state electrical resistivity and upper critical fields are more anisotropic than in the YBa 2 Cu 3 O 7 single crystals. The normal state resistivity along the a-b plane linearly increases with the temperature as in the YBa 2 Cu 3 O 7 compounds. Along the c -axis, the resistivity is very high and exhibits a metallic behaviour which can be induced by defects instead to be intrinsic. The superconducting transition temperature, T c , increases under pressure at a rate of ≅ 0.2 K/kbar. Superconducting needle-like crystals are also studied. They are not single crystals but are composed of two intergrowing phases. One of them is insulating, the other one superconducting. This last phase could be different from the “2212” one. Similarities between the BiSrCaCuO compounds and La 2 CuO 4 compounds are underlined.

The electrical resistance R of SmB6 as a function of temperature T and pressure P has been measured in the range 1 K

From low-field magnetisation measurements the authors give evidence for the existence of helimagnetism in magnetically ordered FexCo1-xSi alloys. They report small-angle neutron scattering experiments on two samples which are confirmed to be helimagnetic with a helix long period of 430 AA for Fe0.9Co0.1Si and 295 AA for Fe0.8Co0.2Si.

The pressure dependence of the magnetic transition temperature for CoMnGe 1- x Si x was determined using the ac susceptibility method. For the compositions: x =0 and x =0.1 a discontinuous magnetostructural phase transition has been observed and a triple point TR was found. By confronting the changes of the specific volume at Curie temperature T c with d T c /d P measured at pressures up to 1.5 GPa, the character of the magnetic moments of the Mn and Co atoms has been determined. It has been found additionally that for composition: 0.75 x T AF-F /d P depends on the silicon content.

High pressures and high magnetic field behaviour give indications on the nature of magnetism encountered in transition metal alloys. We present experiments performed under pressure up to 7 kbar, in fields up to 150 kOe, between 1.4 K and 300 K. The results are used to discuss the occurence of ferromagnetism in Fe x Co 10 x Si and Fe x Co 1- x Ti systems. These systems exhibit two critical concentrations for the onset of ferromagnetism. Their magnetic properties are very sensitive to magnetic fields and high pressures and this allows us to characterize them as weak itinerant ferromagnets. A more localized behaviour is evidenced for the iron rich alloys and the appearance to ferromagnetism is different for cobalt rich side and for iron rich one.

The occurence of ferromagnetism is described in the nickel-platinum Ni x Pt 1-x disordered alloys and in the rare earth-cobalt, TCo 2 compounds. The low temperature moments depend on chemical content, as well as on temperature, pressure or magnetic field. No simple relation exists between the paramagnetic H = 0 behaviour and the low temperature magnetic moment, whenever it exists. Magnetic moments occur continuously or discontinuously. The results are discussed within the framework of theories for weak ferromagnetism and for external field or exchange induced magnetism.

High pressure resistivity experiments performed on CeAl2 and CeCu6 show that the residual resistivity ϱ0 intervening in ϱ=ϱ 0 [1+α( T T c ) 2 −…] is a function of the characteristic temperature Tc. A scaling form is proposed to fit the low temperature resistivity of heavy fermions.

Disordered FexCo1 - xSi alloys are known to be ferromagnetic in an iron concentration range starting at x = 0.2. We have measured their magnetization in magnetic fields up to 40 kOe at 4.2 K and under hydrostatic pressures up to 8 kbar, as well as their zero pressure susceptibility at 4.2 K in high magnetic fields of 150 kOe. We have also determined the Curie temperatures of the ferromagnetic samples under hydrostatic pressure up to 8 kbar from the temperature variation of the initial susceptibility. We determined a critical iron concentration of x ≃ 0.95 for ferromagnetism in FeSi rich alloys. From some of our results, the ferromagnetism in the CoSi rich alloys seems to appear in a rather homogeneous way, whereas the FeSi rich alloys in contrast exhibit local effects.

High pressure magnetization and Curie temperature measurements have been performed on several amorphous Y,Ni,-, alloys. The results seem to indicate that ferromagnetism disappears in a rather inhomogeneous way.

Sintered powders of SmBa 2 Cu 3 O 7−x have been prepared and characterized. The physical properties, superconducting temperature transition, up to 92 K, magnetic susceptibility, electrical resistivity and thermopower have been studied as a function of the preparation conditions and therefore of oxygen stoichiometry. The upper critical field has been estimated from resistivity measurements in fields up to 8 T and the slope (dBc 2 /dT) Td c found to be close to 4 T K −1 for the highest T c samples. Studies of the electrical resistivity under hydrostatic pressures up to 18 kbar show an increase of T c under pressure with dT c /dp∼0.5 K kbar −1 . Finally measurements of voltage-current characteristics indicate that either Josephson junctions or superconductor-insulator-normal metal junctions can be found inside the samples, as expected in inhomogeneous materials Mesures de la conductivite electrique, du pouvoir thermoelectrique et de la susceptibilite magnetique; influence des conditions de preparation et donc de la stœchiometrie en oxygene. Effets de la pression et des conditions de preparation sur la temperature de transition supraconductrice; estimation du champ critique et de la pente (dHc 2 /dT) Td c. Mesures de la caracteristique I-V et mise en evidence de l'existence de jonction de type Josephson ou SIN

Among the different Y-Ni compounds three different behaviours, characteristic of the occurrence of d ferromagnetism in intermetallics, have been observed: (i) YNi 3 is a very weak itinerant ferromagnet; (ii) in the ferromagnetic compound Y 2 Ni 17 some Ni sites exhibit collective electron metamagnetism; (iii) a thermal spontaneous magnetization is observed in Y 2 Ni 7 . In this paper we present the effect of pressure on the magnetic properties of these compounds. We observe a decrease of ferromagnetism when pressure is increased. Whereas these effects are smaller than in some R-Co (R = rare earth) compounds they are stronger than in pure Ni. These results are well accounted for in the itinerant model of magnetism.

The magnetic properties of ferromagnetic Ni-Pt alloys containing 42.9, 45.2, 47.6, and 50.2 at.% Ni were measured in magnetic fields up to 60 kOe, at temperatures between 4 and 60 K, and at pressures up to 7 kbar. The results are presented in terms of Arrott and Mathon plots whose essential straightness over a broad range of alloying contents and external influences provides evidence for the close applicabilities of weak-ferromagnetic theory based on the intinerant-electron model. This model is used to discuss the results quantitatively and the pressure dependence of the critical concentration is estimated, in good agreement with observation. The effective interaction $U$ is found to be 0.44 eV for these alloys, while the ratio of $U$ to the bare interaction is found to be 0.65. The results are compared quantitatively with those for other similar materials, in particular Zr${\mathrm{Zn}}_{2}$.