STRUCTURAL AND MAGNETIC-PROPERTIES OF FE2O3 NANOPARTICLES DISPERSED OVER A SILICA MATRIX

The structure and the magnetic properties of a series of Fe2O3-SiO2 na nocomposites (9-33 wt % Fe2O3), prepared by a sol-gel method and submi tted to thermal treatments in the temperature range 300-900 degrees C, were investigated through XRD, TEM, EPR, and magnetic susceptibility measurements. Superparamagnetic iron(III) oxide nanoparticles with a n arrow size distribution, dispersed over the amorphous silica matrix, a re present in all the samples. They are mostly amorphous, antiferromag netic in the samples treated at low temperatures. At T > 700 degrees C , a lot of gamma-Fe2O3 crystalline ferrimagnetic nanoparticles (4-6 nm ) are formed, while a further increase of the temperature results in t he gamma- to alpha-Fe2O3 transformation. The variation of iron oxide c ontent affects the abundance of gamma-Fe2O3 formation, which reaches t he maximum percent values in the more dilute samples. In the more conc entrated samples, while the amount of maghemite is still growing, anti ferromagnetic alpha-Fe2O3 begins to form. As a consequence, the satura tion magnetization lowers in the samples with higher Fe2O3 content. Al so, interparticle interactions, evidenced by fitting susceptibility va lues versus temperature and by EPR observations, contribute to such a decrease.