M. A. R. LeBlanc

Coexistence of critical and normal state magnetostrictions in type II superconductors : A model exploration

Exploiting the simple formula proposed by Ikuta et al. [Phy. Rev. Lett. 70, 2166 (1993)] to describe the magnetostriction of high TC superconductors and introducing a B2 dependent magnetic flux density component, as proposed by Koziol and Dunlap [J. Appl. Phys. 79, 4662 (1996)], we reproduce families of magnetostriction curves measured by Nabialek et al. [Physica B 319, 286 (2002)] and de Visser et al. [Phys. Rev. B 45, 2962 (1992)], using a simple expression for the dependence of the critical current density JC on the magnetic field in each analysis.

Flux line cutting and cross-flow in a high-Tc superconducting YBCO tube

We present strong evidence observed in a YBCO superconducting tube that helical flux lines can concurrently enter and leave across the inner surface of the wall of a hollow cylinder of sintered high-Tc superconductors at 77 K, and hence cut and traverse each other. The evidence for flux line cutting and cross-flow is examined in the perspective of basic concepts. The traffic of the flux lines is also described mathematically. The penetration fields across the wall and into the grains of the sintered ceramic tube are seen to correlate with salient features (peaks and valleys) of the flux line cutting and cross-flow phenomena. This enables us to claim that we are witnessing flux line cutting and cross-flow in the weak links as well as in the interior of the grains in the YBCO tube. The Clem/Perez-Gonzalez phenomenological theory is exploited in a simplified framework to describe the crucial features of the data semi-quantitatively. This analysis confirms the above conclusions and provides estimates of jc||m and jc||g, the critical current densities for intergranular and intragranular flux line cutting and their dependence on the magnetic flux density.

Contribution of the Meissner current to the magnetostriction in a high Tc superconductor

We show that the magnetostriction hysteresis curves, measured at different temperatures by Chabanenko et al (1998 Supercond. Sci. Technol. 11 1181) in a La1.85Sr0.15CuO4 single crystal, can be well reproduced by exploiting the critical state framework proposed by Ikuta et al (1993 Phys. Rev. Lett. 70 2116) to describe this phenomenon in high Tc superconductors, provided that the role of the Meissner current circulating at the surface of the crystals is taken into account.

Flux-line cutting in granular high-Tc and semi-reversible classical type-II superconductors

The magnetic responses of granular (NdBa2Cu3O7-x and YBa2Cu3O7-x) and monolithic (PbIn) superconducting plates, cooled in a fixed magnetic field Hz parallel to their plane and, later, subjected to cycles of a transverse magnetic field Hy with an amplitude Hy,max>Hz, are investigated both experimentally and theoretically. Magnetization curves, generated as Hy with Hz constant applied and of the order of the full penetration field Hp, are interpreted by employing two competing models (the double critical-state model and the two-velocity hydrodynamic model) for describing flux-line cutting effects on the magnetic behaviour of superconductors. Both models qualitatively and semi-quantitatively reproduce the experimental curves. Unlike the double critical-state model, the two-velocity hydrodynamic approach predicts the appearance of zones with homogeneous magnitude of the magnetic induction B at relatively small values of Hy as |Hy| is increased. However the effect of these zones is not appreciable since they disappear when |Hy| reaches a sufficiently large value: |Hy| Hz≈Hp.

Effect of the configuration of the trapped flux on the decay rate of remanent magnetic moments in type II superconductors

Several workers have measured the rates of decay of remanent magnetic moments in granular high- superconductors where the temperature-field histories generated trapped flux profiles comprising two concentric regions of countercirculating persistent currents. Exploiting a simple model based on the critical state for bulk specimens and the conservation of flux for the redistribution of vortices, and neglecting , we account for the major features of these observations although in some cases the number of trapped vortices is small. The model makes readily testable predictions of the relative magnitudes of the initial rates of magnetic relaxation for various configurations of flux density profiles.

Magnetic relaxation on bridges between the envelopes of major hysteresis loops of type II superconductors

The initial decay rate along the envelope of the major hysteresis curves and along bridges between these envelopes is examined in the framework that the current density of the relaxing field profiles initially exists in a critical state and can be described by a product function where with assumed independent of H. This simple model is shown to reproduce the pertinent observations of Beasley et al and Kwasnitza and Widmer. The model also predicts interesting structure in the curves of R versus H along bridges where H changes sign during its swing between the magnetization envelopes when descends steeply with H.

Observations of a hysteresis loss valley in yttrium barium copper oxide superconductors

We present measurements of the ac susceptibility of sintered yttrium barium copper oxide pellets prepared by a melt processed solid state reaction method, with different amounts of ZnO added. The measurements were performed versus temperature T for several different ac magnetic field amplitudes Hac in the presence of static bias magnetic fields Hb directed along Hac. For all our samples, for each choice of Hac, the imaginary part of the ac susceptibility versus T traces a peak, denoted by χ″max, centered at a temperature denoted by Tmax.We also find that plots of the values of χ″max vs Hac and of χ″max vs Tmax generate curves that display a valley. Calculations of hysteresis losses, exploiting a critical-state model where the critical current density jc=α(T)∕Bn, reproduce the experimental data quite well.

Magnetic relaxation in a commercial Bi:2223/Ag tape at 77 K

The evolution of the rate of flux creep along the major hysteresis envelopes and along bridges between the envelopes in a commercial Bi:2223/Ag tape at 77 K has been investigated. Magnetic relaxation rates depend strongly on the prior magnetic history. It was found that the Bi:2223/Ag tape sample has large flux creep along the hysteresis envelope. Flux creep activation energy was calculated to be 0.12 eV from the rate of decay for the remanent magnetization of 6 mT.

Enhancement of the Meissner effect upon the warming of fast field cooled weak-linked granular high Tc superconductors: a simple model

Several workers have observed that, after fast field cooling of weak-linked granular high Tc type II superconductors, the diamagnetic magnetization will increase and trace a valley, hence a peak of enhanced Meissner effect, as the temperature approaches Tc during slow warming to the normal state with the applied field Ha kept fixed throughout the temperature excursion. We present a simple extension to interconnected granular systems of the established model for isotropic single crystals which qualitatively and quantitatively reproduces our observations of this phenomenon over a large range of Ha, the extensive measurements of Jung et al on YBCO samples and the results of Hyun on compacted Nb3Sn polycrystals.

Vortices in high temperature superconductors: role of the centrifugal and Lorentz forces

It is shown that the centrifugal force plays the dominant role in dictating the configuration of the electric charges and associated field near the vortex core in type II superconductors in weak magnetic fields.