H.L. Ji

Characterization of Hg-Ba-Ca-Cu-O 1223 superconductor

Abstract The HgBaCaCuO 1223 superconductors have been characterized in this work. XRD and SEM analyses indicate that Hg-Ba2Ca2Cu3O8+δ is the major superconducting phase in the shape of a bar or plate in the samples. The transition temperature Tc of one sample is 129 K by resitivity, AC and DC susceptibility measurements. Magnetic measurements including the magnetic critical current density and magnetization relaxations indicate that a strong thermally activated flux motion also exists in the Hg-based system of high-temperature superconductors.

EQUIVALENT APPROACHES TO STUDYING FLUX-CREEP - TIME RELAXATION AND FIELD SWEEP RATE DEPENDENCE

Starting from the extended rate equation for flux motion in high-temperature superconductors, it is shown in this work that the investigation of field sweep rate (H) dependence of magnetization is an approach equivalent to the magnetization time relaxation as far as the flux creep is concerned. The determination of effective activation energy with the data from both time relaxation and magnetic hysteresis loops is described and applied to one melt-textured YBCO sample. The time scale tau0 in the magnetization relaxation formula M = M0 + M1 In (1 + t/tau0) depends on the field sweep rate as well as the slope of the magnetization relaxation curve M(t), mainly via an exponential term exp(U(eff)(J0)/kT). Further support for the extended rate equation comes from the correlation between the effective activation energy U(eff)(J, T) and hysteretically determined critical current density J(c)(T). Finally, the implication of this equation for explaining the fishtail-type magnetization loop and an experimental way to test the validity of this equation are discussed.

Dependence of flux-creep activation energy upon current density in HgBa2Ca2Cu3Oy

Abstract We have investigated the magnetic relaxation on a HgBa 2 Ca 2 Cu 3 O y superconductor over the range 4.2–60 K. Using a linear relation between U ( J ) and kT ln(1 + t / t 0 ), we extracted the effective activation energy and the time-window constant t 0 . A nonlinear dependence of U eff on J , U eff ∝ J − μ , was found. μ is about 0.98 in the temperature range 4.2–60 K, close to the value of 7 9 in the regime of flux creep in large vortex bundles predicted by the three-dimensional collective-creep theory. The time scale t 0 depends exponentially on the initial conditions. The relationship between U ( J ) and the magnetic relaxation curve is discussed. With logarithmic accuracy one can construct the U ( J ) curve simply using several long-time relaxation curves at different temperatures.

Magnetic pinning effect in high Tc superconductor GdBaCuO

Abstract The samples of GdBa2Cu3O7−δ and Y Ba2Cu3O7−δ were prepared by standard solid state reaction. Their pinning potentials were deduced from flux-creep studies. Despite the similarity of their microstructures, the pinning potentials of GdBaCuO are higher than that of YBCO at different applied magnetic field. In this paper we present our experimental results and discuss the physical mechanism of the differences based on the interaction between the isolated magnetic moments of gadolinium ions and flux lines in the sample. Theoretically, we argue that this interaction may be a new pinning mechanism in high Tc superconductors with magnetic rare earth ions.

Paramagnetism and scaling behavior of volume flux pinning force density in a GdBa2Cu3O6+x thin film

Magnetic measurements have been performed on a GdBa2Cu3O6+x superconducting thin film. The paramagnetism carried by rare earth Gd3+ ions in the film tilts the magnetic hysteresis loop and broadens the width of the magnetic hysteresis Delta M, then the magnetization critical current density and the volume flux pinning force density based on the Bean critical state model deviate from intrinsic values. Therefore, in order to get useful information on the pinning mechanism, correction for the paramagnetism is essential. And after correction for the paramagnetism, a scaling law of the volume flux pinning force density is obtained as f(b)=(b/4)(0.5)(1-b/4)(1.5), based on which the possible pinning mechanism in the film is discussed. In the end, deviation from the scaling law at high fields is interpreted by the collective pinning theory

Critical current densities of melt-textured Y Ba2Cu3O7−y

Abstract Three kinds of critical current densities of a melt-textured Y Ba2Cu2O7−y sample were obtained by measuring its magnetic hysteresis loops. The method proposed by Gyorgy et al was extended to evaluate the critical current densities.

Distorted hysteresis loops and flux creep in YBa2Cu2O7−4 ceramics

Abstract The relaxation of magnetic moment of high T c superconductors in an applied field is several orders of magnitude larger than that of conventional ones, so it is worthwhile to discuss the relationship between the hysteresis loops and the magnetic relaxation in high T c superconductors. Experimentally we used a vibrating sample magnetometer measure the hysteresis loops, and then at different points of this loop, the field sweep was stopped while the external field was maintained constant. The signals of magnetic moment were recorded at 10 s and 100 s after each stop, forming several distorted hysteresis curves. The dependence of their shapes on the magnetic relaxation, the pinning potential and other parameters in ceramic YBa 2 Cu 2 O 7−4 samples was discussed.

Correspondence between magnetic time relaxation and field sweeping rate dependence of magnetic hysteresis

Abstract The dependence of magnetization in flux creep experiments M(t) and field sweep rate dependence of magnetic hysteresis M(dH/dt) are derived and found to correspond with each other in Taylor expansions of the effective activation energy U eff (J). The agreement of U eff (J) extracted from the magnetic relaxation and magnetic hysteresis loops of a polycrystalline HgBa 2 Ca 2 Cu 3 O 8+δ sample is a strong support of this correspondence.

Study of magnetization relaxation on zone‐melted YBa2Cu3O6+x superconductors

The analytical magnetization versus time expressions for several flux‐pinning models for high‐Tc superconductors are deduced. Magnetic relaxation and hysteresis studies have been performed on a zone‐melted YBa2Cu3O6+x sample by SQUID. Nonlogarithmic magnetization decay is observed at relatively high temperatures; we attribute this to the nonlinear U(J) relationship which may arise from the local pinning potential Up(x), rather than collective pinning. At 70 K, the experimental data fits extremely well to the expression M(t)=M0+a(T)ln ln(t/τ) which is the result of the exponential U(J) relationship. Based on this U(J) law, the local pinning potential is determined to be Up(x)=U0(x/x0)[1−ln (x/x0)]. The voltage–current E(J) resulting from the observed U(J) relationship, as well as the role of the Y2BaCuO5 (211) phase in zone‐melted YBCO is discussed.

FLUX-PINNING IN HIGH-T-C SUPERCONDUCTIVE FILMS

Magnetization measurements have been performed onc-axis oriented Y- and Gd-based superconductive films in a wide range of the temperaturesT (4.2–85 K) and magnetic fieldsH (0–8 T) withH∥ c-axis. The influence of flux creep on both the temperature dependence of critical current densityJcm and the scaling behavior of flux pinning forceFp has been discussed in detail. The experimental results show that Y and Gd films have different pinning mechanism. Flux pinning-force peaks in high fields are observed in Gd film at high temperatures and can be considered as evidence for collective pinning.