Magnetic flux avalanches in Nb/NbN thin films

Technological applications of NbN thin films may be threatened by the development of magnetic flux avalanches of thermomagnetic origin appearing in a large portion of the superconducting phase. In this work, we describe an approach to substantially suppress the magnetic flux avalanche regime, without compromising the upper critical field. This procedure consists of depositing a thin Nb layer before the reactive deposition of NbN, thus forming a bi-layered system. We use ac susceptibility and dc magnetometry to characterize both the single-layer films, Nb and NbN, and the bi-layered specimen, as well as calibrated magneto-optical imaging to map the instability regime of the studied samples. Magnetic flux imaging reveals interesting features of the dendritic flux avalanches in the bi-layer system, including halo-like patterns and crossing avalanches.