Influence of annealing treatment on magnetic properties of Fe2O3/SiO2 and formation of ε-Fe2O3 phase

Magnetic properties of Fe2O3/SiO2 samples were studied after being produced by sol-gel synthesis and formation of ε-Fe2O3 polymorph. Samples were thermally treated, using different annealing temperatures and annealing times. The size and morphological characteristics of the iron oxide nanoparticles were examined using a TEM microscope. We used the “ellipticity of shapes”, which is a measure of how much the shape of a nanoparticle differs from a perfect ellipse, in order to quantitatively describe morphological properties of nanoparticles. Coercivity measurements were used to identify and monitor the formation of the epsilon-iron oxide phase during the thermal treatments (annealing). Coercivity values were in the range from 1.2 to 15.4 kOe, which is in accordance with previous experience regarding the existence of ε-Fe2O3. We have determined the optimal formation conditions for the ε-Fe2O3 polymorph (t=1050 °C for 7 h, HC=15.4 kOe), as well as the narrow temperature interval (1050-1060 °C) in which the polymorph abruptly vanished (HC=2300 Oe), on the basis of results of the magnetic properties. The threshold temperature for the ε-Fe2O3 phase transformation was measured as 1060 °C. We found that different annealing temperatures and annealing times significantly affected magnetic properties of the examined samples.