Electron paramagnetic resonance and other allied studies of sol-gel derived nanocrystalline chromium oxides in a silica glass matrix

Monolithic silica gels doped with various concentrations (1, 5, 10 mol%) of chromium oxide are prepared by a sol–gel process and are subjected to calcination at different temperatures up to 1200 °C. This has principally led to the formation of Cr2O3 nanocrystals and also of CrO3 nanocrystals at the higher calcination temperatures. The sizes of the Cr2O3 nanocrystals have been determined by x-ray diffraction (XRD) and transmission electron microscopy (TEM) studies and have been found to lie in the range 5–30 nm depending upon the dopant concentration as well as the calcination temperature. The formation of nonmagnetic CrO3 crystals in 10 mol% chromium oxide doped silica glass calcined at 1200 °C as a result of oxidation of Cr5+ ions to Cr6+ ions has been confirmed from XRD studies. Optical, infrared and X-band electron paramagnetic resonance (EPR) spectra of these nanocrystals have also been studied. EPR behaviours such as the g-factor and linewidth of Cr2O3 nanocrystals investigated at room and liquid nitrogen temperatures have been found to be quite distinct from those of antiferromagnetic (AF) bulk Cr2O3 crystals and may be manifestations of superparamagnetism/ferromagnetism in these nanocrystals. A narrow anisotropic EPR line (g_{\parallel }=1.955; g_{ \perp }=1.976 ) has been detected for chromium oxide doped silica gel calcined at 800 °C and has been assigned to isolated Cr5+ ions in a tetragonally distorted tetrahedral environment. Studies of the thermal dependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetization (σ) 10 mol% chromium oxide doped silica glasses calcined at 800 °C by a SQUID magnetometer in the 5–300 K temperature range have revealed the occurrence of the blocking temperature TB at about 250 K, which is a characteristic feature of superparamagnetism in Cr2O3 nanocrystals. The superparamagnetic effects are almost quenched in the samples calcined at 1000 and 1200 °C because of the presence of larger nanocrystals in these samples. Magnetic hysteresis effects in these calcined samples at 300 K as well as at 5 K in the ± 6 T magnetic field range have also been investigated. The existence of ferromagnetic order at low temperature (5 K) as well as at room temperature (300 K) (for samples calcined at higher temperatures) has been confirmed.