Anomalous magnetic behaviour of Cr2O3-SiO2 nanocomposites

Nanocomposites containing Cr2O3 particles having dimensions in the range 13-35 nm dispersed in a silica matrix have been prepared by a chemical method. X-ray diffraction investigation and high resolution electron microscopy substantiate the formation of Cr2O3 particles. X-ray photoelectron spectroscopy studies show the presence of a small amount of CrO2. The samples exhibit ferromagnetic property below 300K with the hysteresis loop showing an exchange bias. The latter is explained as arising due to exchange interaction between the spins associated with Cr3+ and Cr4+ ions. Dc electrical resistivity measurements were analysed by a small polaron hopping model confirming the presence of Cr4+ ions at the surface of the particles. At temperatures above 100K the magnetic hysteresis loop shift was such that zero magnetization was observed only for the negative magnetic field. This was explained as being due to a disorder effect on the magnetic spins of Cr4+ ions.