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 300 K with the hysteresis loop showing an exchange bias. The latter is explained as arising due to exchange interaction between the spins associated with Cr ³⁺ 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 100 K 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.