X.S. Rong

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.

The fish-tail effect and the flux pinning induced by the spatial fluctuation of critical temperatures in melt-textured YBa2Cu3O7−δ bulk samples

Abstract The irreversible magnetization loops and the relaxation of the remnant magnetization were measured for a melt-textured YBa2Cu3O7−δ bulk with two different oxygen states. It is found that introducing the extra oxygen deficiencies into the sample will enhance the critical current density in the low-temperature region and the fish-tail effect in the intermediate-temperature region, but lower the critical current density in the high-temperature region. Further investigation shows that the fish-tail is not due to the crossover between different collective pinning and creep regions, but rather to the presence of the flux pinning induced by the critical temperature fluctuation on the background pinning induced probably, by the mean free path fluctuation.

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.

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

Magnetization relaxation in YBa2Cu3O7−δ

The time and field sweep rate dependencies of magnetization in high temperature superconductors are derived from a combined view based on the thermally activated flux creep. The τ0 in the magnetization relaxation expression M=M0+M1 ln(1+t/τ0) is a macroscopic quantity related primarily to the sample geometry and other experimental conditions via the effective activation energy U(J0). This activation energy provides an alternative interpretation to the nearly inverse proportionality of τ0 to the field sweep rate. The values of τ0 in the magnetization relaxation of single crystal YBa2Cu3O7−δ are consistent with the calculated ones. As the nonlinear U(J) is taken into account, the curvature in the ‖M‖‐ln t plot may change from negative to positive over a sufficiently long time window, and the magnetization hysteresis M(H) depends on the field sweep rate H by const+A ln H+B ln2 H, where A is slightly larger than M1 and B≳0. A discussion of the temperature dependency of the normalized flux creep rate is at ...

Scaling behavior of the flux-pinning force in YBaCuO/PrBaCuO superlattice films☆

Abstract Magnetization measurements have been performed on C-axis oriented superlattice films of YBaCuO/PrBaCuO in wide ranges of temperature T (4.2–70 K) and magnetic field H (0–8 T) with H∥C-axis. Pinning-force scaling has been found with the form ƒ p = A (1−t) 2.7 h m (1−h) n , m = 0.9, n = 1.8 for sample 28# (48 A/100 A), and m = 0.7, n = 3 for sample 29# (48 A/24 A). This result indicates that flux pinning centers in YBaCuO/PrBaCuO films are strong in a wide range of the field h so that Fp has a domelike dependence on magnetic field. The dependence of m and n on the samples with different thickness of the PrBaCuO layer manifests that the dynamic property (C44 and C66) of the flux-line lattice plays an important role in the volume pinning-force density Fp (T, H). The deviation of Fp from the scaling law in high reduced field h may be explained by giant flux creep or collective pinning.

Temperature dependence of the critical current determined from magnetic relaxation in HgBa2Ca2Cu3O8+δ

Based on the U(j) relation of the collective pinning theory, U(j) = [U-c/mu][(j(c)/j)(mu) - 1], we introduce a direct method to derive the temperature dependence of the characteristic exponent mu, the collective pinning energy U-c and the critical current density j(c) from magnetic relaxation measurement. By utilizing a linear relation between 1/R = 1//partial derivative ln j/partial derivative ln t/ and ln t at a fixed temperature, mu and U-c are easily obtained. Inserting these parameters into U(j) we can extract j(c). So mu(T), U-c(T) and j(c)(T) are able to be derived from relaxation measurements at different temperatures. This method was applied to a HgBa2Ca2Cu3O8+delta superconductor. The results were discussed within the collective pinning and creep theory

Flux-creep mechanism studied in extended windows for the HgBa2Ca2Cu3Oy superconductor

Flux-creep mechanisms are examined by the current density J dependence of the effective barriers U(J), which were determined to be U(J) alpha J(-mu) by three independent measurements i.e. AC susceptibility (ACS), magnetic relaxation (M-t) and hysteresis loop (M-H) for polycrystalline HgBa2Ca2Cu3Oy. It is found that, as J is increased, the U(J) of M-N decreases monotonically and gradually crosses over from mu = 0.43, which connects smoothly with the ACS one with J approximate to 10(2) A/cm(2), to the one with mu = 1.03, which just overlaps with the M-t one with J approximate to 10(6) A/cm(2). This result demonstrates the consistency of the data of the three methods and is considered to be evidence of a crossover between the CDW type and large bundle creep in the vortex-glass phase or collective-creep theories.

Study of the paramagnetism on the magnetic relaxation in MTG GdBa2Cu3O6+y superconductors

Magnetic measurements have been performed on a GdBa2Cu3O6+y sample prepared by melt-textured growth method. Because of the paramagnetic moments carried by the rare earth Gd3+ ions in the sample, the magnetization relaxation rate is increased by a factor of 4 pi chi. The effect of the paramagnetism on the U(J) relationship and on the relaxation law of the sample has also been discussed