Two fabrication schemes of magnetic metal-polymer nanocomposites films are described. The nanocomposites are made of graphene-coated cobalt nanoparticles embedded in a polystyrene matrix. Scheme 1 uses non-covalent chemistry while scheme 2 involves covalent bonding with radicals. Preservation of the net-moment of cobalt and electrical insulation are achieved by means of a core double-shell structure of cobalt-graphene-polystyrene. The graphene shell has two functions: it is a protective layer against metal core oxidation and it serves as the functionalization surface for polymer grafting as well. The polystyrene shell is used as an insulating layer between nanoparticles and improves nanoparticles dispersion inside the polystyrene matrix. The theoretical maximum volume filling ratio estimated at similar to 30 % is almost reached. The nanocomposites are shown to undergo percolation behavior but retain low conductivity (<1 S/m) at the highest filling ratio reached similar to 25 % leading to extremely low losses (10(-3)) at high frequency. Such low conductivity values are combined with large magnetization, as high as 0.9 T. Ability for radiofrequency applications is discussed in regards to the obtained magnetization.