Remarkable magnetic properties of undoped NiO-based nanocomposites: size and surface/interface effects

摘要

Nanosized and surface/interface effects on the room temperature ferromagnetic properties of NiO-based nanocomposites have been investigated. Pure NiO nanoparticles and NiO-based nanocomposites composed of NiO/ZnO, NiO/SnO_2, NiO/α-Fe_2O_3 and NiO/CuO were synthesized by ultrasonic assisted sol-gel method. XRD, HRTEM, SAED and FTIR analyses confirmed the formation of highly pure NiO nanocrystals with cubic structure and clearly proved the coexistence of two oxide phases in all composites. TEM images demonstrated the formation of well dispersed spherical nanoparticles in all samples. Particle size distribution histograms of the prepared samples possessed small and narrow size distribution with average particles sizes range from 15 to 26 nm. UV-Vis absorption spectra displayed strong absorption at wavelengths of 370, 298, 302, 326 and 295 nm for pure NiO, NiO/ZnO, NiO/SnO_2, NiO/α-Fe_2O_3 and NiO/CuO samples, respectively. The band gap energy of the pure NiO nanoparticles was found to be 3.3 eV and the higher value was recorded in NiO/CuO nanocomposite (4.18 eV). The electrical conductivity of the pure NiO nanoparticles and NiO/CuO nanocomposite with p-p type structure was increased over the entire temperature range of 30-410 °C. On the other hand, NiO/ZnO, NiO/SnO_2 and NiO/α-Fe_2O_3 nanocomposites having p-n type structure displayed curves with three different electrical conductivity regions. The M-H hysteresis loops of the pure NiO and NiO-based nanocomposites exhibited room temperature ferromagnetic-like character with different S-loop shapes. The saturation magnetization and coercivity of the pure NiO was estimated to be 0.22 emu/g and 181 Oe, respectively. Except NiO/CuO nanocomposite, sensible jumps in the magnetic parameters including saturation magnetization (Ms), retentivity (Mr) and coercivity (Hc) were observed in NiO/ZnO, NiO/SnO_2 and NiO/α-Fe_2O_3 nanocomposites. Modification of ferromagnetism in NiO-based nanocomposites draws attention to the importance of nanosized oxide-oxide interface interactions.