Low Concentration Effect of Fe$\text{}_{3}$O$\text{}_{4}$ and Fe$\text{}_{3}$C Magnetic Nanoparticles in Non-Magnetic Matrix on the FMR Spectra

PTMO-block-PET polymer filled with a low concentration (0.1 wt.%) of binary magnetic system: magnetite (Fe$\text{}_{3}$O$\text{}_{4}$) and iron carbide (Fe$\text{}_{3}$C) with carbon (C) was prepared. Characterization of the sample was done by using X-ray diffraction and scanning electron microscopy methods. Temperature dependence of the FMR spectra was carried out in the 10-300 K temperature range. The extraordinary temperature dependence behaviour of FMR spectra was recorded. The FMR spectra were fitted by two Lorentzian lines originating from magnetite and iron carbide. The anomalous behaviour of FMR spectra below 100 K could be explained by the skin effect or by the freezing of the matrix benzene rings at about 115 K. The temperature dependence of the g parameter below 75.5 K (opposite shift of the resonance field to usually observed) was interpreted as a result of strong antiferromagnetic interaction of some portion of agglomerates.