| PTMO - block - PET polymer filled with a low concentration (0.5 wt.%) of nanoparticle mixture: Fe3O4-35 wt.% (magnetite), Fe3C-29 wt.% (iron carbide or cementite) and C-36 wt.% (carbon), was synthesised. The sample has been characterised by using XRD and SEM method. Ferromagnetic resonance (FMR) investigations of the sample were carried out in the 30 - 300K temperature range. A slightly asymmetric FMR line has been recorded at higher temperatures, while a broad FMR line at lower temperatures was registered. A strong, extraordinary temperature dependence of FMR spectrum was observed. The resonance field shifted toward lower magnetic fields, a decrease in intensity and an increase of the linewidth with decreasing temperature was detected. This low-temperature line is very anisotropic and its low-field shoulder spreads to negative magnetic fields. With decreasing temperature, the linewidth of this line decreases slightly, in contrast to the behaviour of the high-temperature line. The intensity of the low-temperature line is weakly dependent on temperature. The FMR spectrum has been fitted by two Lorentzian lines: one centred at g = 2.259(2) with the linewidth ΔB = 146(1) mT, and the other at near zero applied magnetic field with the linewidth ΔB = 307(1) mT, at room temperature. It is believed that the FMR spectrum at higher temperatures is dominated by magnetite signal, while at lower temperatures it is dominated by iron carbide. The temperature dependence of FMR spectrum of magnetite at higher temperatures suggests the presence of thermal fluctuations and the interparticle interaction similar to the superparamagnetic resonance. The observed change of the FMR spectrum at 90K could be explained by Verwey transition and the skin effect. |
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