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Description
Nb3Sn, among high Tc conventional superconductors, is the A15 compound class material currently under focus for the development of next-generation superconducting radio-frequency (SRF) cavities. Its high critical temperature (18.2 K) and lower surface resistance compared to Nb, in fact, potentially enable high quality factors while maintaining cryogenic operation at 4.5 K instead of 2 K. Only applicable in the form of film, due to its brittleness, coating Nb3Sn on copper substrates (instead of bulk Nb) is attracting because copper offers good thermal conductivity and reduced material cost for SRF applications. However, optimization of the coating process remains challenging, especially regarding the influence of deposition temperature on superconducting and structural properties. Previous studies on superconducting thin films have shown that parameters such as critical temperature (Tc), phase purity, and critical current density (Jc) are strongly linked [1] to microstructure and film quality. Increasing growth temperature generally improves crystallinity and phase formation of Nb3Sn, leading to enhanced superconducting transition properties. Nevertheless, for copper-based cavities, high deposition temperatures are undesirable because of induced degradation of the copper substrate mechanical properties above approximately 400 °C. In addition, magnetic hysteresis and Jc measurements provide valuable insight into flux pinning, defects and phase homogeneity, all of which may influence RF dissipation and vortex-related losses in practical cavity operation.
In this work, Nb3Sn thin films co-sputtered on copper substrates at different growth temperatures are investigated through superconducting and structural characterization. Samples grown between 450 °C and 700 °C were analysed using magnetic susceptibility [2] and magnetization measurements to estimate the critical temperature (Tc), lower critical field (Hc1) and critical current density (Jc), together with X-ray diffraction (XRD) analysis to evaluate phase purity and crystallinity. The results show an improvement in Tc and Nb3Sn phase formation with increasing growth temperature. XRD measurements also reveal improved crystalline characteristics for films deposited at higher temperatures. However, magnetization hysteresis measurements indicate significant variation in Jc among the samples up to 600 °C, with the films grown at 600 °C exhibiting the highest critical current density, while the 700 °C sample, despite improved phase quality, shows reduced Jc. These results suggest that optimal superconducting performance arises from a balance between phase formation, microstructural defects, homogeneity, and flux pinning rather than from growth temperature alone. The study provides insight into the processing window for Nb3Sn on copper coatings and highlights the importance of combining structural and magnetic characterization for future SRF cavity optimization.