U. Yaron

Effect of Zn, Ga and Ni substitution on the superconducting properties of the electron doped system Nd-Ce-Cu-O

Abstract We report experimental data about the influence of Zn, Ga and Ni substitution of T c of the electron doped system Nd 1.85 Ce 0.15 Cu 1- y M y O 4- δ , 0≤ y ≤0.15. The relative suppression of T c for a given dopant concentration decreases from Ni to Ga to Zn. Superconductivity is totally suppressed above y =0.01, 0.03 and 0.05 for M= Ni, Ga, Zn, respectively. The solubility of M=Fe in the matrix is extremely small. The effect of M=Ga 3+ on T c is similar to that of Ce 4+ . We compare doping results for magnetic and nonmagnetic ions and discuss the origin of the depression of T c . The effect of these dopants on T c in the Nd-Ce-Cu-O system is completely diffrent from that observed in the La-Sr-Cu-O system.

Critical current density, reversible and irreversible magnetization in YBa2Cu4O8 and Y0.9Ca0.1Ba2Cu4O8

Abstract DC magnetic measurements on the high- T c superconductors YBa 2 Cu 4 O 8 (124, T c =80 K) and Y 0.9 Ca 0.1 Ba 2 Cu 4 O 8 ( T c =87K) are reported. The irreversible lines of 124 and ceramic YBa 2 Cu 3 O 7 (123) are quite similar. The results show that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase. Conversions of 124 into 123+CuO does not change the grain size of the material but enhances its critical current density.

Temperature dependence of Fe hyperfine field in RBa2Cu3Oz

Abstract Following the discovery that the Cu(2) sublattice in some RBa2Cu3O7 (R=rare earth) systems orders magnetically a series of 57Fe-probe Mossbauer studies have been performed aiming at the Cu(2) magnetisation and its Neel temperature. It is shown here that the observed temperature dependences of the Fe hyperfine field are identical for all systems studied to date, if the normalized field Heff(T)/Heff(OK) is plotted versus the reduced temperature (T/TN). This universal experimental curve can be reproduced within a molecular-field model calculation for a spin- 1 2 system, if it is assumed that the iron is trivalent ( S = 5 2 ) and the Cu(2)-Fe exchange is only 0.26(1) of the Cu(2)-Cu(2) exchange in all cases.

Magnetic properties, specific heat studies and Mössbauer measurements of PrBa2NbCu2O8

Abstract PrBa2NbCu2O8 has been studied by DC susceptibility and specific heat measurements to determine the magnetic behavior of the Pr sublattice, and by a 1% 57Fe-doped sample to determine the magnetic behavior of the Cu(2) sites. The Pr sublattice and the CuO2 planes are both antiferromagnetically ordered at 11.6 and 360 K, respectively. The Pr contribution to the specific heat ΔCp(T) was calculated by subtracting the data of LaBa2NbCu2O8 as background. The entropy associated with the magnetic transition is 6.7 J/mol K, intermediate between the expected values for Pr4+ and Pr3+. At low temperatures the magnetic contribution has the form of ΔCp(T)=MT3.1 characteristic of 3D antiferromagnetic magnons. In contrast to PrBa2Cu3Oz, no linear term for the electronic specific heat is observed in PrBa2NbCu2O8.

Alternative approach to the description of magnetic properties of granular superconductors

Abstract A model, complementary to the critical state models, is proposed for the description of the magnetic properties of the granular superconductors after both zero-field-cooled and field-cooled procedures. An application of the model to the magnetic properties of the field-cooled granular superconductors is given. Experimental data are shown to be consistent with the model.

Coexistence and competition between superconductivity and magnetic order in YBa2Cu4O8

Abstract Magnetic susceptibility and 57 Fe Mossbauer spectroscopy were used to study superconductivity and magnetic order in YBa 2 (Cu 1−x Fe x ) 4 O 8+δ . T c is decreasing with x, disappearing for x > x c 0.04. For x c the crystal structure is orthorhombic and Mossbauer spectra show that iron substitutes Cu, predominantly in the Cu(1) site. Moreover, for the superconducting compound with x=0.025 and T c =27 (2) K, the Cu (1) site orders magnetically at T N =30(2) and H eff (Cu(1), 4.2 K)=461(2) kOe, thus superconductivity and magnetism coexist. For x>x c the crystal structure changes to tetragonal, and magnetic order is observed in the Cu (2) site,T N =380(5) K for x=0.1 and H eff (Cu(2), 4.2 K)=510(2) kOe. The coexistence of superconductivity and magnetic order in the Cu(1) site and the competition between superconductivity and magnetic order in the Cu(2) sites in YBa 2 Cu 4 O 8 are discussed in terms of the previously observed phase diagrams for Y 1−x Pr x Ba 2 (Cu 1−y Fe y ) 3 O 7+δ .

Field dependence of the magnetization of melt textured and ceramic Y-Ba-Cu-O

Abstract We report on the field dependence of the zero-field-cooled magnetization of cylindrical melt textured and ceramic samples, both having similar packing factors, ≈80%. We define an effective demagnetizing factor, N eff , which depends on the demagnetizing factors of the sample and of the grains, N and n , respectively, and on the fraction of superconducting volume, ⨍, which is itself a field dependent quantity. The structures of the two samples differ in two main properties. The ceramic sample is intrinsically isotropic and contains a significant amount of intergranular material, while the melt textured sample is intrinsically anisotropic and does not contain an appreciable amount of intergranular material. These structural observations lead to significant differences in the field dependence of ⨍ and of the ratio of the magnetic moments, measured with the applied field parallel and perpendicular to the axis of the cylinder.

Magnetism and superconductivity in Y1−χCaχSr2GaCu2O7−δ

Abstract Samples of Y 1−χ Ca χ Sr 2 GaCu 2 O 7−δ ( χ =0, 0.4), prepared under various oxygen pressures, have been studied by magnetometry. Samples doped with 57 Fe have been studied also by Mossbauer spectroscopy. The majority of the iron ions (Fe 3+ ) enters the Ga (Cu(1)) site, a minority enters the Cu(2) site. For the χ =0 sample, the Mossbauer spectra of the iron nuclei in the Cu(2) sublattice display magnetic order of the Cu with T N=370 K. The moments order along the c -axis. The iron in the Ga site displays magnetic order only at low temperatures, probably induced from the Cu(2) site. At temperatures above 90 K, these iron ions display a single pure quadrupole doublet Mossbauer spectrum, with a splitting of 2.0 mm/s. The sample χ =0.4 prepared under ambient pressure of oxygen (APO) also displays magnetic order of the Cu(2) site, with T N ≈370 K. The iron nuclei in the Ga site display paramagnetic long spin relaxation times phenomena at 4.2 K. The χ =0.4 sample, prepared under 110 atm oxygen pressure (HPO), displays superconductivity according to the magnetometry measurements ( T c≈50 K). The Mossbauer spectra give evidence of the presence of two phases in this sample. One displays magnetic order like the APO sample, the other is paramagnetic. The latter is probably associated with the superconducting phase.

New magnetic phases in the La-Sr-Cu-O system

Abstract The competition between superconductivity (SC) and magnetic order in the CuO 2 planes of (La 1− z Gd z ) 2− x Sr x Cu 1− y M y O 4 , M=Fe, Zn, Ga and Ni was studied by magnetometry and 57 Fe Mossbauer spectroscopy. In the superconducting ( x =0.15−0.2) samples substitution of La by Gd or Cu by Fe causes SC to disappear and magnetic order at T N =13 K to be formed. On the other hand, substitution in the CuO 2 planes by Zn, Ga or Ni suppresses SC andinhibits magnetic order in the plane. This behavior is similar to that found in Y 1− x Pr x Ba 2 (Cu 1− y M y ) 3 O z .

Angular dependence of the field-cooled, zero-field cooled and remanent magnetization in YBA2Cu3O7 and Bi2Sr2CaCu2O8 single crystals

Abstract We have studied the angular dependence of the zero-field-cooled (M ZFC ), field-cooled (M FC ) and remanent magnetization (M rem ) of YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 single crystals, by rotating the crystal relative to the applied field direction. Two compatible mechanisms, namely shape anisotropy and intrinsic anisotropy are used to describe the angular dependence of these quantities. At sufficiently low temperatures, the pinning forces are strong and the relation M FC =M ZFC +M rem holds for all orientations. The angular dependence of M ZFC is due only to variation of the demagnetization factor by rotation; whereas the angular dependence of the remanent magnetization can be interpreted by using the two mechanisms. For larger fields or higher temperature the pinning forces are weaker and |M FC | ZFC +M rem|.