We have calculated the initial magnetization curves and complete hysteresis loops for hard type‐II superconductors. The critical‐current density Jc is assumed to be a function of the internal magnetic field Hi according to Kim’s model, Jc(Hi)=k/(H0+‖Hi‖), where k and H0 are constants. As is the case for other critical‐state models, additional assumptions are that bulk supercurrent densities are equal to Jc, and that the lower critical field is zero. Our analytic solution is for an infinite orthorhombic specimen with finite rectangular cross section, 2a×2b (a≤b), in which a uniform field H is applied parallel to the infinite axis. Assuming equal flux penetration from the sides, we reduced the two‐dimensional problem to a one‐dimensional calculation. The calculated curves are functions of b/a, a dimensionless parameter p=(2ka)1/2/H0, and the maximum applied field Hm. The field for full penetration is Hp=H0[(1+p2)1/2−1]. A related parameter is H*m=H0[(1+2p2)1/2−1]. Hysteresis loops were calculated for the di...

Kim model for magnetization of type‐II superconductors

Anisotropy of critical current in layered oxide superconductors

THE defects in as-grown bulk YBa2Cu3O7 have been shown to act as weak flux pinning sites, leading to a reversible magnetization1, magnetization decay2 and non-zero resistivity3 in moderate magnetic fields at temperatures near the transition temperature, Tc. These effects limit the technological utility of the as-grown material3. This is not entirely surprising, as both single crystals and polycrystalline materials are grown under conditions expected to minimize the defect density, but the weak pinning in these materials has nevertheless raised the issue of a possible intrinsic limitation to the critical current density. Here we report the attainment of a critical current density, Jc, of ∼ 6 x 105 A cm-2 at 77 K and 9 kOe, in a single crystal of Yba2Cu3O7 irradiated with fast neutrons to introduce defects. (In both irradiated and unirradi-ated crystals, the critical current density was inferred by measuring the magnetic moment resulting from an induced persistent current.) This hundredfold enhancement of Jc relative to unirradiated crystals demonstrates that artificially induced defects can act as strong flux pinning sites, and analysis of the defects should increase our understanding of flux pinning in high- Tc superconductors.

Critical currents near 106 A cm−2 at 77 K in neutron-irradiated single-crystal YBa2Cu3O7

The behavior of vortex matter in high-temperature superconductors (HTS) controls the entire electromagnetic response of the material, including its current carrying capacity. Here, we review the basic concepts of vortex pinning and its application to a complex mixed pinning landscape to enhance the critical current and to reduce its anisotropy. We focus on recent scientific advances that have resulted in large enhancements of the in-field critical current in state-of-the-art second generation (2G) YBCO coated conductors and on the prospect of an isotropic, high-critical current superconductor in the iron-based superconductors. Lastly, we discuss an emerging new paradigm of critical current by design-a drive to achieve a quantitative correlation between the observed critical current density and mesoscale mixed pinning landscapes by using realistic input parameters in an innovative and powerful large-scale time dependent Ginzburg-Landau approach to simulating vortex dynamics.

Vortices in high-performance high-temperature superconductors

Thin films of YBa2Cu3O7−x have been prepared epitaxially on 〈100〉 SrTiO3 by laser evaporation in a single‐step process. The resistivity of the films decreases linearly with temperature and is measured to 80 μΩ cm at 100 K. The samples are completely superconducting at 88 K. The critical current densities are as high as 5×106 A/cm2 at 77 K in strip lines of 8 μm width. Irradiation with 25 MeV 16O ions at 77 K combined with in situ measurements was applied to study the influence of lattice disorder on superconducting properties. The resistivity ρ increases, whereas the critical superconducting temperature Tc0 and the critical current density jc in the absence of a magnetic field decrease proportionally to the ion fluence. Contrary to this behavior, we observe an enhancement of jc in magnetic fields larger than 1 T. The maximum enhancement is about a factor of 2 at a fluence of 1014 16O/cm2. This increase is due to pinning at irradiation‐induced pinning centers. Simultaneously the upper critical magnetic fie...

Irradiation‐induced enhancement of the critical current density of epitaxial YBa2Cu3O7−x thin films

Magnetization measurements were performed on single crystals of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ to study the effect of fast neutron irradiation on flux pinning and the critical magnetization current. Neutron irradiation up to a fluence of 8.16\ifmmode\times\else\texttimes\fi{}${10}^{17}$ n/${\mathrm{cm}}^{2}$ systematically enhanced the magnitude and reduced the anisotropy of the critical magnetization current compared to unirradiated samples. The largest increases in the critical magnetization current occurred at 77 K. There was a slight linear decrease in the transition temperature with neutron fluence.

Enhanced critical magnetization currents due to fast neutron irradiation in single-crystal Y Ba 2 Cu 3 O 7 − δ

Magnetization measurements at 11 K are used to obtain anisotropic ${H}_{c1}$ values for fields along the $c$ direction and in the basal plane of single crystals of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. The anisotropic magnetic penetration depth and equilibrium shielding currents at ${H}_{c1}$ are derived and analyzed within effective-mass theory.

T. J. Moran

Papers

Enhanced critical magnetization currents due to fast neutron irradiation in single-crystal Y Ba 2 Cu 3 O 7 − δ

Anisotropy of the lower critical field, magnetic penetration depth, and equilibrium shielding current in single-crystal YBa2Cu3O7- delta.