Thermal evolution of the ferromagnetic resonance in Fe2O3/SiO2 nanocomposites for magneto-optical sensors

The present work has been focused on the interpretation of variable temperature ferromagnetic resonance spectra of mixed α/γ-Fe2O3–SiO2 nanocomposites, combined with previously reported magnetization and electron microscopy results. These materials have been shown to exhibit very promising properties for applications as magneto-optical sensors. Sol–gel processing has been used to synthesize in situ iron oxide nanoparticles in a transparent SiO2 matrix. Monolithic samples, i.e., free from cracks and fissures, have been obtained employing formamide as a drying control chemical agent, followed by 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. Thus, variable temperature ferromagnetic resonance spectra of these compounds are mainly composed of three lines: (i) a high-field line (3350 Oe at room temperature, i.e., g ≈ 2), attributed to the uniform mode of individual and clustered γ-Fe2O3 superparamagnetic nanoparticles (which arises from two peaks expected for ellipsoidal particles, overlapping at high temperature, but separated as temperature decreases); (ii) at medium fields (ca. 1600 Oe or g ≈ 4.2), the electron paramagnetic resonance line of Fe3+ ions in tetrahedral sites with rhombic distortion of the silica amorphous matrix, more clearly seen as temperature is lowered; and finally, (iii) at low fields and temperatures, a shoulder in the region of g ≈ 9.7 (approximately 700 Oe), also characteristic of the appearance of axial distortion in the crystal field of Fe3+ ions in many oxide glasses.