Microstructure and surface composition of ferromagnetic thick films prepared with NiCo polyol-derived powders

Interest in ferromagnetic thick films stems from their magnetoresistive properties which open new opportunities for achieving reliable and cheap contactless physical sensors. In previous work we have studied the properties of different classes of ferromagnetic metal-based thick films, either prepared from commercial Ni-based pastes, or from prototype NiCo-based pastes, including those prepared with NiCo powders obtained with a polyol process. The latter has been found to give comparatively better magnetoresistive and magnetic responses; nevertheless the information on the real microstructure of the films was incomplete, especially with regard to the surface composition and contamination. In this paper results of an investigation aimed at clarifying these features and their evolution with the firing conditions are reported. The study included analyses of powders and films with scanning electron microscopy (SEM) and X-ray photoemission spectroscopy (XPS). Results show the evolution of sintering and grain growth with the peak firing temperature and give evidence of residual contamination of carbonaceous residues on the films together with an extensive surface oxidation, although the firing process was carried out in nitrogen flow. Moreover the film surfaces are Co rich. These results can guide in the choice of composition and processing conditions for further improvement of the magnetic and magnetoresistive properties of the films.