The aim of this work is to elaborate nanocomposite thin films sensitive to a magnetic field by incorporating magnetic nanoparticles in a symmetrical diblock copolymer matrix organized in lamellae on plane substrates. On the one hand, the symmetrical diblock copolymer P(n-BMA)-b-PS is synthesized by ATRP and characterized by several methods. On the other hand, nanoparticles made of maghemite γ-Fe2O3 are functionalized with PS using the "grafting from " technique in order to make them compatible with polystyrene and to insert them in PS lamellae. Composite films are elaborated by "spin coating" a solution containing both the γ-Fe 2O3@PS core-shell nanoparticles and the copolymer onto a silicon substrate. After annealing, these films are characterized mainly by AFM and neutron reflectivity to investigate the lamellar order.