Search

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.

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.

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.

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 .

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.

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 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 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.

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 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.

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.

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.

High T c superconductors and heavy fermions have some similar properties. Intermediate valency state exists or can be induced by impurities or by off-stoichiometry composition. All these compounds have a Fermi liquid behaviour below a characteristic temperature T SF corresponding to the band width of quasiparticles. Strong spin correlations could also exist in the high T c superconductors. Evidence is given that all these materials have one energy scale T SF as observed in 3 He which is known as being a strongly correlated Fermi liquid. All the critical temperatures T c are nearly proportional to this energy scale T c ∼ 1 γ∼ T SF , γ being the electronic specific heat coefficient per unit of volume. The superconductivity could have the same origin in the high T c oxides and in the heavy fermions systems.

We discuss the weak ferromagnetic properties of the two series of alloys Fe x Co 1− x Ti and Fe x Co 1− x Si in the light of new specific heat and resistivity studies as well as high field and high pressure magnetic measurements.

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

High pressure electrical resistance experiments performed on a single crystal of CeAl2 allow one to determine a characteristic line in the pressure-temperature plane where a continuous passage from weak to strong Kondo behaviour is observed. This line is not associated with any sizeable volume anomaly as shown by X-ray experiments.

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.

At 300 and 4 K, the CeSn 3 anomalous compressibility indicates a pressure induced valence change. The thermal contraction from 300 to 4 K is equivalent to that produced by a pressure of 8 kbar. High pressure susceptibility data show that the thermal expansion does not drive the “magnetic-nonmagnetic” transition of CeSn 3 .

Recent results are reported on the Kondo lattice compound CePt2Si2. Specific heat data present two anomalies in the range 20–100 K, associated with crystal field and incoherent Kondo effect. A maximum in C/T is observed around 1.4 K, which could be attributed to coherence effects. Resistivity and susceptibility measurements under pressure are also presented.

An experimental study on amorphous electrodeposited Ni 1− c P c alloys was performed using measurements of magnetic properties, coercive force, NMR and resistivity at low temperatures. The results show that weak ferromagnetism takes place for 0.15 c

We report here results from a study of superconductivity in the Sm 1.85 (Ce 0.15−x Th x )CuO 4−y system. We observe resistive superconducting transitions for all values of x, with transition midpoints ranging from 19.3 K for x =0 to 1.9 K for x =0.15. This is the first reported observation of superconductivity in the compound Sm 1.85 Th 0.15 CuO 4−y . Magnetic susceptibility measurements performed on this compound verify the presence of superconductivity. Additionally, electrical resistivity measurements done under pressure show a decrease in T c in Sm 1.85 Th 0.15 CuO 4−y at a rate dT c /dP=-0.043 K/kbar.

The electrical resistivity of CeMg has been measured under pressures up to 105 kbar. The Neel temperature continuously decreases, while the amplitude of the magnetic term as well as the negative slope at high temperature increase. We deduce the variation of the coupling constant Γ as a function of pressure.

We have measured the ordered moment μ s of UN under pressure up to 30 kbar. It is found to decrease rapidly at exactly the same rate as T N = d log μ s /d log V = d log T N /d V = 19. This is taken as good experimental evidence that UN is a band antiferromagnet.

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.

The electrical resistivity of Li 0.9 Mo 6 O 17 single crystal has been studied between 1.5 K and 300 K under hydrostatic pressures up to 20 kbar. A large increase of the superconducting transition temperature, from 1.7 K to 2.5 K, is associated to a sharp decrease of the temperature T m of the electronic, probably CDW instability.

Powdered Ba 0.6 K 0.4 BiO 3−y was synthesized using a two steps technique. The samples are cubic with a= 4.281(1). The transition temperature (Tc) is determined from a c susceptibility (Tcζ) as well as from resistivity measurements on pellets (Tc ϱ ). Tc is found to be dependent (25.5–27 K) related to the annealing temperature. Besides, Tc ζ > Tc ϱ is unusual and anomalous magnetoresistive effects are measured between Tc ϱ and Tc ζ . Both effects are discussed in terms of weakly coupled superconducting grains.

The magnetic properties of disordered Pd-Ni alloys, with low nickel content have been studied at high magnetic fields and high pressures. The variation of the saturation magnetization and Curie temperature with the concentration and pressure are described.