A method for manufacturing magnetic field detection devices comprises the operations of manufacturing a magneto-resistive element comprising regions with metallic conduction and regions with semi-conductive conduction. The method comprises the following operations: forming metallic nano-particles to obtain regions with metallic conduction; providing a semiconductor substrate; and applying metallic nano-particles to the porous semiconductor substrate to obtain a disordered mesoscopic structure. A magnetic device comprises a spin valve, which comprises a plurality of layers arranged in a stack which in turn comprises at least one free magnetic layer able to be associated to a temporary magnetisation (MT), a spacer layer and a permanent magnetic layer associated to a permanent magnetisation (MP). The spacer element is obtained by means of a mesoscopic structure of nanoparticles in a metallic matrix produced in accordance with the inventive method for manufacturing magneto-resistive elements.