V. Pop

Effect of Fe substitution for Cu in the mixed state of (Bi, Pb):2223 superconductor

The effect of the partial substitution of x = 0.02 Fe for Cu in the stabilized (Bi, Pb):2223 phase has been investigated by magnetoresistivity measurements up to 0.7 T. The intra- and intergranular transitions as a function of magnetic field were analysed using versus T curves. The Arrhenius plots are represented by a family of straight lines. The activation energy is proportional to . The main dissipation process is related to the vortex cutting mechanism.

Effect of Fe substitution on the magnetic and electrical properties of the (Bi1.6Pb0.4)(Sr1.8Ba0.2)Ca2(Cu1−xFex)3Oy superconductor

Abstract The partial substitution of Fe for Cu on the electrical and magnetic properties of (Bi 1.6 Pb 0.4 )(Sr 1.8 Ba 0.2 )Ca 2 (Cu 1− x Fe x ) 3 O y (0 × 0.05) has been investigated by electrical resistance, magnetic a.c. and static susceptibilities measurements. The superconducting transition temperature T c decreases with increasing Fe concentration and the samples are characterized by a linear temperature dependence of the electrical resistance above 2T c . The excess conductivity was analyzed. The inter- and intragrain critical temperatures are affected differently. Static susceptibility yielded the effective magnetic moments corresponding to the high spin state of Fe.

Effects of Y, Er and Lu substitution upon superconductivity in (Bi,Pb):2223 system

Abstract The effect of partial substitution of Ca by x=0.02Y, 0.02Er and 0.02Lu in the (Bi,Pb):2223 superconductor was investigated by resistivity and AC susceptibility measurements. The rate of depression for Tc in x=0.02 samples is found to be almost the same for all substituents. Residual resistivity varies according to the type of substituting ions. The linear dependence of intergranular temperature Tp as a function of AC field amplitude agrees with Muller critical state model.

Intergranular properties of (Y{sub 1{minus}x{minus}y}Zr{sub x}Ca{sub y})Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} compounds

The intergranular properties of (Y1-x-yZrxCay)Ba2Cu3O7-δ ceramics were studies by using temperature-dependent ac susceptibility measurements at zero dc magnetic field. The intergranular critical current density was determined from ac susceptibility data by varying the field amplitude Hac. From the imaginary part of the complex susceptibility, χ″(T), we have determined the temperature dependence of the critical current density by using the Beans model. The ac field dependencies of the intergrain χ″-peak temperatures Tp are linear and agree with Muller critical state model. The results were analyzed in terms of superconductor-insulator-superconductor (SIS)-and superconductor-normal-superconductor (SNS)-type models for granular superconductors.

PARACONDUCTIVITY AND HALL EFFECT MEASUREMENTS IN THE SUPERCONDUCTING (BiPb)2(SrBa)2Ca2(Cu1−xCrx)3Oy

The effect of the Cu substitution by Cr in the “2223” superconductor was studied by resistivity and Hall effect measurements. The excess conductivity was analyzed by using the linear and Zou-Anderson extrapolation methods on the electrical resistivity. A crossover temperature is observed only for the x=0 sample. The sheet hole concentration per Cu-site, nHVcell is found to vary linearly with the temperature for all Cr doped samples in the 170–265 K temperature range. Deviations from linearity in the T2 dependence of the inverse Hall mobility above 250 K were evidenced.

Statistical representation of the excess conductivity in a Bi-Sr-Ca-Cu-O superconductor

The onset of superconductivity may be described by the product of a Gaussian with a Student distribution. The excess conductivity properties of many specimens are numerically fitted and prove the accuracy of this model. The number of degrees of freedom yielded by the Student distribution corresponds to the number of independent variants of percolation of the Cooper pairs above . This number depends linearly on the type of 3d substituents of Cu (Zn, Fe, Ni).

AC SUSCEPTIBILITIES AND ELECTRICAL RESISTANCE OF (BI1.6PB0.4)(SR1.8BA0.2) CA2(CU1-XZNX)3OY SUPERCONDUCTOR

Ac susceptibility and electrical resistivity measurements as a function of temperature have been carried out on the samples with x=0.00, 0.02 and 0.05 Zn. The decrease of inter- and intragrain critical temperatures with increasing x is in agreement with the increase of the residual resistivity. The intergrain properties are more influenced than the intragrain ones. The granularity parameters p and ρct were evaluated from resistivity data.

Superconductivity and Normal-State Properties of (Y1-x-yZrxCay)Ba2Cu3O7-δ Compounds

The electrical and magnetic properties of (Y1-x-yZrxCay)Ba2Cu3O7-δ ceramics are investigated by using resistivity, ac and dc susceptibility measurements. X-ray powder diffraction has been used to find the lattice parameters of the samples. The samples are orthorhombic, except the sample with x=0.15 and y=0.2 which is tetragonal. All the samples are superconducting. There is no significant change in Tc for the Zr-doped (Y1-xZrx)Ba2Cu3O7-δ samples, while for the (Y0.85-yZr0.15Cay)Ba2Cu3O7-δ compounds the Tc decrease with the dopant content and they show temperature dependent dc magnetic susceptibility, in the normal state.

MAGNETIC PROPERTIES OF CaxLa1 - xMnO3 (x > 0.5) PEROVSKITES

The magnetic properties of CaxLa1 - xMnO3 perovskites with x = 0.6 and x = 0.8 were studied in a large temperature range. A transition from mictomagnetic to antiferromagnetic state is evidenced at low temperatures for the samples with high lanthanum content. As the calcium content increases an essential antiferromagnetic behaviour is shown. The valences states of Mn ions were determined. The interactions inside the magnetic sublattices dominate those between the two sublattices as evidenced by the positive paramagnetic Curie temperature.

Magnetic studies on Mn-substituted YBaCuO

Abstract Magnetic d.c. and a.c. susceptibility and lattice constant measurements were performed in the normal state of YBa 2 Cu 3− x Mn x O 7−δ with Mn concentrations x = 0.075, 0.15, 0.2, 0.25 and 0.3. The orthorhombic structure is preserved throughout the substitution range and some amount of impurity phases were evidenced in the samples with x > 0.2. The inter- and intragrain properties from a.c. susceptibility measurements show different behaviour above and below the x = 0.2 Mn in the YBCO. The measurements of the d.c. susceptibility were analysed using a Curie-Weiss law with a temperature-independent background. The effective magnetic moment obtained per Mn atom, p Mn = 3.29 μ B , suggests the presence of a 4+ oxidation state. The variation of the Curie paramagnetic temperature θ for different x is associated with an antiferromagnetic exchange interaction between the Mn ions. A regular trend of p eff.tot 2 and T c as a function of x suggest that below x = 0.2 Mn ions are substituted in the Cu(1) positions.