Z.Y. Zeng

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.

Flux creep during time t ≈ t0 detected by AC susceptibility measurements

Abstract Complex AC susceptibility was measured systematically as a function of temperature at different AC field amplitudes H AC and frequencies ƒ for a melt-grown YBa 2 Cu 3 O 7−δ sample. Based on the flux creep model of Feigelman et al., the dependence of the peak temperature T p in χ′ on ƒ and H AC can be well explained and the important parameter t 0 of 10 −5 s ( ƒ 0 = 1/t 0 = 10 5 Hz ) was extracted. Because we have measured flux creep over the times t(= 1/ƒ) 10 −6 s ≤ t ≤ 10 −2 s , the condition t ⪢ t 0 was proved to be invalid for some of the measurement frequencies higher than ƒ 0 . Therefore, we have detected the creep at times t ≈ t 0 (ƒ ≈ ƒ 0 ) .

Resistance relaxation resulting from current diffusion in superconducting thin films

The current diffusion process in superconducting thin films was investigated numerically. The attention was concentrated on the influence of current diffusion on experimental results of transport measurements. It was found that the voltage and the resistance of the film sample in transport measurements were time dependent. The calculated relaxation curves of resistance differed obviously from those observed in magnetic relaxation measurements. Transport measurements on a GdBawCu3O7 thin film were carried out to examine our numerical investigation. Good agreement between the experimental results and our calculation was obtained

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

Dependence of the Glassy Exponent on Field and Temperature in Textured YBCO Specimen

The glassy exponent Μ of a melt-textured YBa2Cu3O7-x sample is derived from measurements of the magnetic hysteresis loops with various applied field sweep rates and at different temperatures and magnetic fields. Thus, the dependences of the exponent Μ on temperature and field are obtained. Furthermore, we report for the first time the crossover-line of the vortex creep regime from a single vortex to a small bundle.

Glassy exponent of melt textured YBCO extracted from magnetic hysteresis loops with various field sweep rates

Abstract The glassy exponent μ of the melt-textured YBCO sample is extracted from the magnetic hysteresis loops with the various applied field sweep rates at the different temperatures. Our results show that the field and temperature dependence of μ is consistent with the predictions of the collective pinning theory.

Frequency dependence of ac magnetic response in type-II superconductors

Abstract The ac magnetic response in type-II superconductors was analyzed on the basis of flux creep theory. A modified model was proposed to describe the variation of current density in a sample immersed in an alternating field, which simultaneously took into account the sweeping applied field and the flux creep inside the sample. The frequency dependent ac susceptibility of a textured YBa 2 Cu 3 O 7 bulk sample was measured and explained in this model. Also discussed was the relation between our model and other models that appeared in literature.

Surface pinning and influence of field reduction on magnetic relaxation in YBa2Cu3O7 bulk sample

Abstract Differences between the magnetic relaxation due to surface barriers and bulk pinning is studied. It is shown that a small field reduction can affect the two relaxation processes quite differently. Experimental results on a melt-gown YBa 2 Cu 3 O 7 bulk sample and theoretical calculations based on surface pinning support this conclusion. We suggest that field-reduction relaxation experiments can be used to probe surface pinning.

Surface pinning effects in YBa2Cu3O7 bulk sample

Abstract The difference between thermal activation of vortices over surface barrier and over bulk pinning centers is discussed. It is concluded that small reduction of the applied field can affect these two relaxation proceedings quite differently. Based on this conclusion, a method for detecting surface pinning effect is proposed. Experimental results on a melt grown YBa 2 Cu 3 O 7 bulk sample support our proposal.

Flux creep determined ac magnetic response in type-II superconductors

Abstract The ac magnetic response in type-II superconductors was analyzed on the basis of flux creep theory. A model was proposed to describe the variation of current density in alternating field, which simultaneously took into account the sweeping applied field and the flux creep inside the sample. The frequency dependent ac susceptibility of a textured YBa 2 Cu 3 O 7 bulk sample was measured and explained in this model.