Influence of interparticle interaction on the structural, optical and magnetic properties of NiO nanoparticles

In this work, we have synthesized nickel oxide (NiO) nanoparticles using a simple wet chemical method. The synthesized nanoparticles were annealed at different temperatures ranging from 573 to 973 K. The structural, magnetic, and optical properties were characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM), Superconducting Quantum Interference Device (SQUID), UV-VIS and Photoluminescence (PL) spectrophotometer. The optical properties depict that as NiO particles grow bigger, the intensity of the UV emission increases. The broad emission band can be attributed to the direct recombination of photogenerated charge carriers from the conduction band with the holes in the valence band. This kind of emission due to the transition of 3d8 electrons of the Ni2+ ions was also observed in the PL spectra. NiO nanoparticles exhibit antiferromagnetic behaviour with varied magnetic moment values of 3.08-5.72 μB. The ZFC curves of all samples show cusp and sharp decrease in magnetization at temperatures in the region of 10-21 K. This is due to the surface cluster spins whose thermal fluctuations freeze in a cluster-glass-like state. The bifurcation temperature between ZFC-FC curves increases with respect to the particle size due to the exchange field which is related to the overlap of electron wave functions of two magnetic ions (Ni2+). This field acts indirectly through the neighbouring anions O2−, as Ni2+ ions are too far apart for direct exchange forces mechanism called super exchange and originates from Pauli exclusion principle and electrostatic force.