Quantitative magneto-optical imaging of S/F hybrid structures

Magneto-optical imaging, based on the Faraday rotation of polarized light, has proven to be a successful and widely used technique for mapping the magnetic field generated by either permanent magnets or superconductors. Nonetheless, its performance may be reduced when dealing with hybrid systems combining both types of materials. The primary reason behind this limitation is the difficulty to discern the contribution of each sub-system to the total magnetic field. In particular, when using hard ferromagnets and conventional superconductors, the former can lead to an overwhelming signal making it rather challenging to unveil the weak signal from the superconductor.

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 is 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.