Two different concentrations of nanocrystalline material: α-Fe/C were prepared by the carburization of nanocrystalline iron and characterized by XRD and SEM. The nanoparticles were next used as fillers in polymer nanocomposites using the in situ polycondensation reaction in a poly(ether-ester) matrix with two concentrations: 0.1 wt. % and 0.3 wt. %. The temperature dependence of the ferromagnetic resonance (FMR) spectra was investigated to study magnetic interactions in the compounds. The introduced FMR parameters (intensity and position of the right peak) describe well the temperature dependence of FMR spectra of strongly interacting magnetic nanoparticles. The FMR spectra depend strongly on the concentration of magnetic nanoparticles, which influence the magnetic interactions between them. Two main critical points of the matrix (the glass state and the freezing of benzene rings) influence the behaviour of the FMR spectra.