Magneto-optical imaging reveals impacts of niobium preparation for superconducting performance

The exact preparation of superconducting materials affects their ultimate performance, where systems like particle accelerators need superconducting radio-frequency (RF) cavities to expel magnetic flux effectively. Pinning centers, that trap quanta of magnetic flux known as vortices, can form due to tiny and unavoidable ambient magnetic fields during cooling. These residual microtesla-scale fields are critical as they can increase RF losses.

Macroscopic approaches have identified a correlation between high grain size and low dissipation, leading to proposals that grain boundaries act as pinning centers. Researchers have now magneto-optically examined magnetic flux expulsion and pinning in superconducting niobium, and report a microscopic view in the Journal of Applied Physics.

The researchers’ first steps optimized a high-resolution magneto-optical technique to examine niobium samples at the 10-micrometer scale. Before now, the imaging method had only been used above the magnetic fields strengths where transition to the superconducting state occurs. But their new method operated below 10 millitesla, capturing flux information during field cooling, and was combined with electron backscatter diffraction to map grain boundaries. An indicator material visualized the magnetic field produced from the niobium, as niobium itself does not interact with photon spin.

To identify pinning centers, the group compared two samples of niobium, one an untreated ingot and the other a high-temperature heat-treated sample. The results revealed that niobium hydrides, that form during the cryogenic cool-down, increase trapping and play a bigger role in pinning than grain boundaries. “The nice thing about this imaging technique is that you have all the information in one image, to directly compare grain boundaries, hydrides and other defects,” said co-author Julia-Marie Köszegi.

With the technique established, the researchers want to systematically analyze samples of niobium with different preparation histories, helping to define the optimal parameters of superconductor preparation.

Source: “A magneto-optical study on magnetic flux expulsion and pinning in high-purity niobium,” by J. Köszegi, O. Kugeler, D. Abou-Ras, J. Knobloch, and R. Schäfer, Journal of Applied Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4996113 .