Complete Suppression of Thermomagnetic Instabilities in Nb Superconducting Films by Combined Metallic Layers and Ion Irradiation

Abstract

Thermomagnetic instabilities in superconducting films can trigger flux avalanches that disrupt the critical state and impair the performance of superconducting devices. Here we investigate the stabilization of Nb thin films subjected to a perpendicular magnetic field by two complementary approaches: the addition of normal-metal overlayers and Ar ion irradiation. Magnetization measurements and magneto-optical imaging show that the metallic layers suppress the onset of avalanches at low applied fields, whereas ion irradiation reduces the high-field portion of the instability region by shifting the upper threshold for avalanche activity to lower fields. When combined in the same sample, these two partial stabilization effects lead to complete suppression of thermomagnetic instabilities. In particular, a Nb film coated with a 1~\(μ\)m-thick Cu layer after irradiation at a fluence of \(5\times10^{16}\)~ions/cm\(^2\) exhibits smooth magnetization curves and avalanche-free flux penetration. This combined treatment restores stable critical-state behavior and provides a practical route for stabilizing Nb superconducting films in thin-film superconducting technologies.