Fabrication and characterization of micropillar Josephson junction with Niobium Nitride and Permalloy/Samarium Cobalt exchange spring magnet

The superconducting proximity effect occurs when the superconductor is adjacent to non-superconducting material since the superconducting condensate, pairs of electrons known as Cooper pairs, leak into a non-superconducting material. We fabricated micropillar exchange spring coupled Josephson junctions (ESCJJs) and attempted to observe the odd triplet effect in a superconductor (S)/ferromagnet (F)/superconductor (S/F/S) system. First, niobium (Nb) was used for S and a ferromagnetic exchange spring system of Permalloy (Py) and Samarium Cobalt (SmCo) bilayer was used for F. In this work, we introduce a new superconductor, niobium nitride (NbN), which has a much higher bulk Tc than Nb. We found the Tc increased from ~3.3 K for 100 nm Nb thin film to ~9.3 K for 100 nm NbN thin film. To optimize the Tc of NbN thin film N2 gas was introduced during the magnetron sputtering process. NbN thin films at different ratios of N2/Ar gas were made from both Nb and NbN sputtering targets. It turned out that there is only a very narrow window where NbN thin film can be optimized. Also, it shows that the Tc of NbN thin film is sensitive to the structure and stoichiometry of NbN. The critical current of micropillar ESCJJs is measured to investigate the odd triplet effect.