- Nanocomposites of magnetic nanoparticles and polymer matrices combine the properties of their components, and as such are good examples of functional nanomaterials with excellent application potential. Against this background, experimental and theoretical studies of such composites are of great interest. In this study we aim to provide insight into the static and dynamic magnetic response, as well as the dielectric response, of magnetic nanocomposites subjected to external magnetic and electric fields. We directly compare the behavior of polyurethane films doped with superparamagnetic Fe 3 O 4 , and blocked ferromagnetic CoFe 2 O 4 nanoparticles. While a reversible, Langevin magnetization curve is observed for Fe 3 O 4 @PU films, hysteretic magnetic behavior is found in case of CoFe 2 O 4 @PU films. The hysteresis observed for CoFe 2 O 4 nanoparticles can be explained by interactions at the interface between particles and polymer matrix in conjunction with its ferromagnetic nature. The results of dielectric spectroscopy experiments revealed different effects of Fe 3 O 4 and CoFe 2 O 4 nanoparticles on polymer dynamics.