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 Gamma-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 Gamma-Fe2O3@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.