Nb films containing extended arrays of holes with 45-nm diameter and 100-nm spacing have been fabricated using anodized aluminum oxide as substrate. Pronounced matching effects in the magnetization and Little-Parks oscillations of the superconducting critical temperature have been observed in fields up to 9 kOe. Flux pinning in the patterned samples is significantly enhanced as compared to unpatterned reference samples in applied fields exceeding 5 kOe. Matching effects are a dominant contribution to vortex pinning at temperatures as low as 4.2 K due to the extremely small spacing of the holes.

Superconducting transition and vortex pinning in Nb films patterned with nanoscale hole arrays

In this paper, we report the results of field (H) and temperature (T) dependent magnetization (M) measurements of a pellet of uniform, large-grain sintered MgB2. It was found that iron is compatible with MgB2 and can be used as sheath material for fabrication of Fe-clad MgB2 wires. We show that at low temperatures the size of the pellet and its critical current density, Jc(H) – i.e. its M(H) – ensure low-field flux jumping, which of course ceases when M(H) drops below a critical value. With further increase of H and T the individual grains decouple and the M(H) loops drop to lower lying branches, unresolved in the usual full M(H) representation. After taking into account the sample size and grain size, respectively, the bulk sample and the grains were deduced to exhibit the same magnetically determined Jcs (e.g. 105 A/cm2, 20 K, 0 T) and hence that for each temperature of measurement Jc(H) decreased monotonically with H over the entire field range, except for a gap within the grain-decoupling zone.

Flux jumping and a bulk-to-granular transition in the magnetization of a compacted and sintered MgB2 superconductor

Nb3Sn superconductors have significant applications in constructing high-field (> 10 T) magnets. This article briefly reviews development of Nb3Sn superconductor and proposes prospects for further improvement. It is shown that significant improvement of critical current density (Jc) is needed for future accelerator magnets. After a brief review of the development of Nb3Sn superconductors, the factors controlling Jc are summarized and correlated with their microstructure and chemistry. The non-matrix Jc of Nb3Sn conductors is mainly determined by three factors: the fraction of current-carrying Nb3Sn phase in the non-matrix area, the upper critical field Bc2, and the flux-line pinning capacity. Then prospects to improve the three factors are discussed respectively. An analytic model was developed to show how the ratios of precursors determine the phase fractions after heat treatment, based on which it is predicted that the limit of current-carrying Nb3Sn fraction in subelements is ~65%. Then, since Bc2 is largely determined by the Nb3Sn stoichiometry, a thermodynamic/kinetic theory was presented to show what essentially determines the Sn content of Nb3Sn conductors. This theory explains the influences of Sn sources and Ti addition on stoichiometry and growth rate of Nb3Sn layers. Next, to improve flux pinning, previous efforts in this community to introduce additional pinning centers (APC) to Nb3Sn wires are reviewed, and an internal oxidation technique is described. Finally, prospects for further improvement of non-matrix Jc of Nb3Sn conductors are discussed, and it is seen that the only opportunity for further significantly improving Jc lies in improving the flux pinning.

A review and prospects for Nb3Sn superconductor development

Magneto-optical imaging and magnetization measurements performed on thin films of the borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon reducing the applied magnetic field towards the remanent state. In contrast to other low-Tc materials such as Nb and MgB2, where similar instabilities occur in both increasing and decreasing magnetic fields, dendritic flux patterns are observed in YNi2B2C for decreasing fields only. Also in the magnetization measurements, a distinct asymmetry is evident between increasing and decreasing fields. The effect does not depend on the sweep rate of the field, but is strongly dependent on the maximum field applied before reduction. The observation of spontaneous flux instabilities in this additional family of low-temperature superconductors suggests that the responsible mechanism is universal to this class of materials.

Observation of dendritic flux instabilities in YNi2B2C thin films

Using magneto-optical imaging the phenomenon of dendritic flux penetration in superconducting films was studied. Flux dendrites were abruptly formed in a 300-nm-thick film of ${\mathrm{MgB}}_{2}$ by applying a perpendicular magnetic field. Detailed measurements of flux density distributions show that there exists a local threshold field controlling the nucleation and termination of the dendritic growth. At 4 K the local threshold field is close to 12 mT in this sample, where the critical current density is ${10}^{7}{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2}.$ The dendritic instability in thin films is believed to be of thermomagnetic origin, but the existence of a local threshold field and its small value are features that distinctly contrast with the thermomagnetic instability (flux jumps) in bulk superconductors.

Local threshold field for dendritic instability in superconducting MgB 2 films

The isothermal critical‐state model of hard superconductors is extended to include the effects of heating when the applied field is changed suddenly and magnetic flux enters adiabatically into the bulk. We consider the following specific situation. A semi‐infinite slab of superconductor is cooled in a magnetic field lying in its surface plane. Next, the external field is raised isothermally by an amount Hs. This excess field decreases linearly to a depth δ= 10Hs/4πJc from the surface. Finally, the field is raised by an infinitesimal amount ΔH in a time short compared to the thermal diffusion time and long compared to the electromagnetic diffusion time. Each element of volume exposed to the changing field receives a thermal impulse proportional to the local‐flux‐change times Jc. This thermal impulse, in turn, lowers the critical current and allows more flux to penetrate. We find that if Hs exceeds some critical value Hfj, then the isothermal critical state is not the only allowed state of the superconductor. This instability field is given in terms of the critical current density Jc, derivative of the critical current density with temperature, ∂Jc/∂T, and the volume specific heat C by the formula Hfj= [−π3CJc/(∂Jc/∂T)]1/2. The application of the incremental field ΔH can initiate an avalanching process, or a flux jump, that terminates in an adiabatic critical state. Immediately following the flux jump the internal field, the induced supercurrent, and the temperature rise at each position are associated in a self‐consistent way with the avalanche of flux that has entered the superconductor. In this framework a flux jump is viewed as a switching from the isothermal critical state to an adiabatic critical state. The magnitude of the jump is related to Js and is calculated.

P. S. Swartz

Papers

A Model for Magnetic Instabilities in Hard Superconductors: The Adiabatic Critical State