Size-dependent magnetic properties ofVO2 nanocrystals dispersed in a silica matrix

Abstract

Magnetic nanocrystalline VO2 particles have been successfully synthesized in a silica glassmatrix by the sol–gel method at calcination temperatures of700 °C and above. The presence of small quantities of diamagneticV2O5 nanocrystals in glasssamples calcined at ≥800 °C has been confirmed. The thermal behavior of magnetization(zero-field-cooled and field-cooled) and magnetic hysteresis ofVO2 nanocrystals in the 10–300 K temperature interval have demonstrated that theVO2 nanocrystals present in these glasses display superparamagnetic–ferromagnetic transition atlow temperatures. X-band electron paramagnetic resonance (EPR) spectra ofVO2 nanoparticles have depicted broad symmetric lines[ΔHpp∼104 mT at room temperature (RT) and 250 mT at liquid nitrogen temperature (LNT)].The increase in EPR line intensity on going from RT to LNT suggests thatVO2 nanocrystals have undergone ferromagnetic-like ordering in thetemperature interval RT–77 K. The presence of small numbers of isolatedVO2+ ionsin the SiO2 matrix has also been confirmed from EPR studies. From EPR and optical spectral studies it has been shownthat the VO2+ ion has square pyramidal conformation with tetragonal compression havingC4v symmetry. It is established that the in-planeσ bonding andout-of-plane π bonding are predominantly ionic in nature.