AC susceptibility hysteresis in granular superconductors

Physica C-superconductivity and Its Applications, 1999

A comparative study of the hysteretical behavior of the zero resistance critical temperature for YBa Cu O , 2 3 7 yd Ž. Ž. BiPb Sr Ca Cu O and HgRe Ba Ca Cu O superconducting ceramics was performed at low magnetic fields 2 2 2 3 10qd 2 2 3 8qd Ž. under 400 Oe. The results were compared with those obtained through the magnetic hysteresis of the critical current density, showing good agreement in most cases. The qualitative explanation of the observed hysteresis in terms of intragranular flux trapping of the superconducting grains satisfies YBaCuO and BiSrCaCuO materials, but modifications might be necessary in the case of HgBaCaCuO.

Physica C: Superconductivity, 2000

Ž. Ž. The hysteresis of the transport critical current of YBa Cu O Hg,Re Ba Ca Cu O and BiPb Sr Ca Cu O 2 3 7 yd 2 2 2 8qd 2 2 2 3 10qd polycrystals was studied. While the first two systems display qualitatively similar hysteretical patterns, the third one behaves differently. The experimental results are interpreted in terms of a phenomenological model which considers the effects of the magnetization in the grains on the intergranular weak links as responsible for the observed hysteresis. The hysteresis of the transport critical current is discussed on the light of the microstructural features of the samples, concluding that the shape and disposition of the grains do not account in a straightforward fashion for the differences in the hysteretical behavior.

Journal of Superconductivity and Novel Magnetism, 2013

The magnetovoltage measurements (V-H curves) with different sweeping rates (dH/dt) of the external magnetic field in Ag-added polycrystalline YBa 2 Cu 3 O 7−x sample (YBCO/Ag) were investigated. The measurements of V -H curves were carried out as functions of the transport current (I ) and temperature (T ). Upon cycling H , all V -H curves measured for different values of I exhibit a clockwise hysteresis effects in forward region. The hysteresis effects in the V -H curve were interpreted in terms of two-level magnetic system, which considers the superposition of the external magnetic field and the local magnetic fields in the intergrain boundaries induced by magnetic dipole moment of neighbor superconducting grains. The analysis of magnetovoltage data showed that the flux trapping in the junction network has a negligible effect on the evolution of the V -H curves and the irreversibilities arise mainly from the flux trapping inside the grains. It is shown that the width of V -H curves shows a universal scaling behavior with respect to the applied current below the critical temperature T c . The comparison of V -H curves of the YBCO/Ag sample with those of YBCO shows that adding Ag to the superconducting structure weakens the pinning properties of Josephson medium and provokes the instabilities in measured dissipation. The presence of Ag in the superconducting matrix causes marked decrease in hysteresis effects and makes the V -H curves dH/dt dependent. At high values of dH/dt, the instabilities and plateau regions in V -H curves increase significantly as compared to those of observed in YBCO. In addition, the interrelation between the evolution of V -H curves obtained for different values of I and the critical current I c was demonstrated experimentally.

Physical Review B, 2008

We report on magnetization and magnetoresistance measurements of single-crystal, melt-processed, and polycrystalline YBa 2 Cu 3 O 7−␦ samples with the purpose of disclosing the connection between the magnetic irreversibility limit and the zero resistance temperature point as a function of applied field in the above samples with very different superconducting granularities. Another goal is to find out how much the applied fields degrade the grain couplings and the electric conductivity for the different field-current configurations. In homogeneous superconductors, the magnetic irreversibility line is well known as a limit below which an electric current different from zero can flow without resistance. Our data for the single crystal with a weak superconducting granularity closely follow this rule. However, the results on the other samples are quite different. Normally, good-quality melt-processed YBa 2 Cu 3 O 7−␦ samples do not exhibit the signature of superconducting granularity. Nevertheless, x-ray analysis of our melt-processed sample, containing 30 wt % of the Y 2 BaCuO 5 phase, shows considerable misalignment of the crystallite c axis, which weakens the grain couplings and hence is expected to result in superconducting granularity. Effectively, the magnetic irreversibility lines for H ʈ c axis as well as for H ʈ ab plane exhibit, in the low-field region, the usual signature of superconducting granularity. On the other hand, the zero resistance data for increasing fields along the c axis and J ʈ ab plane split away from the irreversibility line toward the lower-temperature side. Nevertheless, for H ʈ J ʈ ab plane, the zero resistance data closely follow the irreversibility line up to the highest applied fields. In the polycrystalline YBa 2 Cu 3 O 7−␦ sample, in which the grain junctions are much weaker, the effect of an increasing applied field on the grain couplings is much stronger. The zero resistance line is already split from the irreversibility line and is lower by more than 17 K at 8 kOe. Our present measurements provide good quantitative data for the discrepancy between the zero resistance and magnetic irreversibility lines. We explain these features in terms of the superconducting glass model.

Physica C: …, 1998

The low-field magnetic response of a physical system consisting of eight superconducting spherical grains in a cubic arrangement is studied by means of a three-dimensional Josephson junction network. The lower threshold field for this ™ system is numerically studied as a function of the inclination of the externally applied magnetic field H with respect to the z-axis. q 1998 Published by Elsevier Science B.V. All rights reserved. PACS: 74.80.B; 74.25.Ha; 74.50.q r

Europhysics Letters (EPL), 1996

PACS. 74.60Ge -Flux pinning, flux creep, and flux-line lattice dynamics. PACS. 74.72Bk -Y-based compounds. PACS. 74.80Bj -Granular, melt-textured, and amorphous superconductors; powders.

Physica C: Superconductivity, 1992

Results from AC susceptibility x(H, T) and resistivity p(H, T) measurements on YBa2Cu307 single crystals, in the mixed state by applying magnetic fields H parallel to the c-axis, strongly suggest that the occurrence of the dissipation peak in the imaginary part of the complex susceptibility X" (to, T) is due to the skin effect known for electrical transport in metals in the normal state. At fixed field H, the peak temperature Tp(tn, H) increases with increasing frequency of the superimposed AC probing field, and does not coincide with the irreversibility temperature T* (H) < Tp for magnetization obtained in the static limit to-. 0. A phase diagram is presented which includes the DC irreversibility line H*(T), the peak positions Hp (to, T), the experimentally resolved zero-resistivity fields Hp=0(T), and the crossover fields from activated to diffuse resistive behavior, Hk(T), all measured on the same YBa2Cu307 single crystal.

Journal of Superconductivity and Novel Magnetism, 2012

The objective of the work is to establish the contribution of superconductive granules and grain boundaries (weak links) in magnetoresistance ρ value of YBa 2 Cu 3 O 7−δ granular high-temperature superconductor (HTS) at T < T c. The current-voltage characteristics (CVCs) E(j) H ext =const of YBa 2 Cu 3 O 6.95 ceramic samples were measured in H ext (0 ≤ H ext ≤≈ 500 Oe) magnetic fields. The CVCs E(j) H trap =const of the samples magnetized in H treat magnetic fields were measured at H ext = 0. Based on the CVCs, ρ(j) H ext =const , ρ(H ext) j =const and ρ(j) H treat =const dependences were reestablished. The comparative analysis of ρ(j) H ext =const and ρ(H ext) j =const dependences indicates the magnetic field redistribution between grain boundaries and superconductive granules influence on transport and galvanomagnetic properties of granular HTS. The superconductive grain magnetoresistance ρ g was established to be significantly lower than ρ J value of Josephson medium.

Applied Superconductivity, 1995

We present measurements of hysteresis in the magnetoresistance of sintered YBaCuO. In analogy with hysteresis in current density and in harmonic generation, this is attributed to trapping of flux within the grains. This feature is used to track the penetration of magnetic field inside the superconducting granular regions.

Europhysics Letters (EPL), 1998

We propose the existence of an electric-field induced nonlinear magnetization in a weakly coupled granular superconductor due to time-parity violation. As the field increases the induced magnetization passes from para-to dia-magnetic behavior. We discuss conditions under which this effect could be experimentally measured in high temperature superconductors.