The work reports on theoretical and nanoscale characterization of Au/Co/Au multilayers used as transducers in Magneto-optical Surface Plasmon Resonance (MOSPR) biosensors. Different aspects related to the optimization of transducers are discussed. In particular, optimized sensitivity is demonstrated to depend on the full multilayer total thickness, on the Co layer thickness and its position within the film, as well as on the discrepancy between the optical constants in very thin layers with respect to bulk values. Co layer thickness and position in the trilayer are optimized to provide the best compromise between magneto-optic (MO) activity and optical losses as well as to ensure large electromagnetic (EM) fields at the Au-dielectric interface. In this sense surface sensitivity can be maximized with respect to variations in the bulk properties of the measuring fluid and ensure higher performances with respect to …