Ferromagnetic resonance of nanocomposites based on iron oxides

Abstract

The present work has been focused on the interpretation of variable temperature ferromagnetic resonance spectra of mixed α-Fe2O3 and γ-Fe2O3–SiO2 nanocomposites, combined with previous magnetization and electron microscopy results. These materials have been shown to exhibit very promising applications as magneto-optical sensors. Sol–gel processing has been employed to synthesize in situ iron oxide nanoparticles within a transparent SiO2 matrix. Monolithic samples have been obtained employing formamide as drying control chemical agent, following a specific processing route aimed to generate γ-Fe2O3 nanoparticles. Although γ-Fe2O3 is the main phase, the formation of some amount of α-Fe2O3 phase is unavoidable. Variable temperature ferromagnetic resonance spectra are composed of: (i) a high field peak attributed to γ-Fe2O3 nanoparticles. This peak comes from the overlapping of the two expected peaks for ellipsoidal particles. When temperature is decreased, this peak broadens as a consequence of size and shape distributions; (ii) at medium fields (ca. 1525 Oe) and low temperatures, the ferrimagnetic peak from uncompensated magnetic moments in γ-Fe2O3 shows up; and finally, (iii) at low fields and temperatures, the anti-resonance peak due to normal antiferromagnetic α-Fe2O3 phase is also visible.