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

Interest in ferromagnetic thick films stems from their magnetoresistive pro perties which open new opportunities for achieving reliable and cheap conta ctless physical sensors, In previous work we have studied the properties of different classes of ferromagnetic metal-based thick films, either prepare d from commercial Ni-based pastes, or from prototype NiCo-based pastes, inc luding those prepared with NiCo powders obtained with a polyol process. The latter has been found to give comparatively better magnetoresistive and ma gnetic responses; nevertheless the information on the real microstructure o f the films was incomplete, especially with regard to the surface compositi on and contamination. In this paper results of an investigation aimed at cl arifying these features and their evolution with the firing conditions are reported. The study included analyses of powders and films with scanning el ectron microscopy (SEM) and X-ray photoemission spectroscopy (XPS). Results show the evolution of sintering and grain growth with the peak firing temp erature and give evidence of residual contamination of carbonaceous residue s on the films together with an extensive surface oxidation, although the f iring process was carried out in nitrogen flow. Moreover the film surfaces are Co rich, These results can guide in the choice of composition and proce ssing conditions for further improvement of the magnetic and magnetoresisti ve properties of the films. (C) 2000 Elsevier Science S.A. All rights reser ved.