Depairing critical current achieved in superconducting thin films with through-thickness arrays of artificial pinning centers

TL;DR

This study demonstrates that creating through-thickness nanoscale pores in superconducting Nb films significantly enhances the critical current, approaching the theoretical depairing limit, and explores the effects of magnetic back-filling on flux pinning.

Contribution

The paper introduces a novel fabrication method for large-area artificial pinning centers in Nb films and investigates their impact on critical current enhancement and magnetic pinning behavior.

Findings

Achieved critical current densities close to the Ginzburg-Landau depairing current at 5 K.

Observed a matching field effect corresponding to pore density in Jc measurements.

Back-filling pores with magnetic material like Co reduces Jc but influences flux vortex behavior.

Abstract

Large area arrays of through-thickness nanoscale pores have been milled into superconducting Nb thin films via a process utilizing anodized aluminum oxide thin film templates. These pores act as artificial flux pinning centers, increasing the superconducting critical current, Jc, of the Nb films. By optimizing the process conditions including anodization time, pore size and milling time, Jc values approaching and in some cases matching the Ginzburg-Landau depairing current of 30 MA/cm^2 at 5 K have been achieved - a Jc enhancement over as-deposited films of more than 50 times. In the field dependence of Jc, a matching field corresponding to the areal pore density has also been clearly observed. The effect of back-filling the pores with magnetic material has then been investigated. While back-filling with Co has been successfully achieved, the effect of the magnetic material on Jc has been found to be largely detrimental compared to voids, although a distinct influence of the magnetic material in producing a hysteretic Jc versus applied field behavior has been observed. This behavior has been tested for compatibility with currently proposed models of magnetic pinning and found to be most closely explained by a model describing the magnetic attraction between the flux vortices and the magnetic inclusions.