We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data are complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer.

Abstract

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.