Methods are disclosed for synthesizing nanocomposite materials including ferromagnetic nanoparticles with polymer shells formed by controlled surface polymerization. The polymer shells prevent the nanoparticles from forming agglomerates and preserve the size dispersion of the nanoparticles. The nanocomposite particles can be further networked in suitable polymer hosts to tune mechanical, optical, and thermal properties of the final composite polymer system. An exemplary method includes forming a polymer shell on a nanoparticle surface by adding molecules of at least one monomer and optionally of at least one tethering agent to the nanoparticles, and then exposing to electromagnetic radiation at a wavelength selected to induce bonding between the nanoparticle and the molecules, to form a polymer shell bonded to the particle and optionally to a polymer host matrix. The nanocomposite materials can be used in various magneto-optic applications.