Temperature dependence of the FMR spectrum of magnetic nanoparticle agglomerates in a nonmagnetic matrix

Sample composed of 60 wt.% α-Fe and 40 wt.% Fe₃C nanoparticle agglomerates (with a typical size of α-Fe nanocrystallites of 17 nm and Fe₃C of 46 nm) dispersed at a concentration of 0.1 wt.% in a nonmagnetic matrix (wax) has been prepared. The agglomerates have been characterized by XRD method. Ferromagnetic resonance (FMR) measurements of the sample have been carried out at different temperatures. An intense and broad magnetic resonance absorption line has been recorded. With decreasing temperatures the resonance line has shifted in the direction of higher magnetic fields and its intensity and linewidth displayed an unusual behaviour. The resonance line could be fitted by a superposition of two Lorentzian lines: one centred at lower magnetic field and the other placed at higher resonance magnetic field, showing a strongly decreased integral intensity with decreasing temperatures. At higher temperatures (over 100K) a magnetic resonance absorption from α-Fe nanoparticle agglomerates dominates the spectrum and the integral intensity strongly decreases with decreasing temperature. Below 100K the magnetic resonance absorption arising from Fe₃C nanoparticle agglomerates is recorded. The matrix freezing phenomena has a prominent influence on the FMR spectra of α-Fe nanoparticle agglomerates.