Magnetic properties of NiO (nickel oxide) nanoparticles: Blocking temperature and Neel temperature

Crystalline nickel oxide (NiO) nanoparticles dispersed in an amorphous silica matrix have been prepared by a sol-gel combustion synthesis method. The sample was characterized using X-ray powder diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy and superconducting quantum interference device magnetometry. The nanoparticles were spherical with an average size of about 5 nm and narrow particle size distribution. The magnetization measurements revealed two maxima in the zero-field cooled magnetization curve at 5 K and 56 K. An analysis of DC and AC magnetic susceptibility measurements revealed that the higher temperature maximum corresponded to the onset of antiferromagnetic ordering within the particles. The Neel temperature of T-N = 56 K is much lower than in bulk NiO. The 5 K maximum corresponded to the particle blocking temperature TB. The frequency dependence of TB was successfully fitted using the Vogel-Fulcher law, indicating the presence of weak inter-particle interactions (C-1 = 0.08 and C-2 = 0.76). A Curie-Weiss law fit to the magnetic susceptibility above T-N revealed antiferromagnetic exchange interactions with a Curie-Weiss temperature of theta = 24 K and an effective magnetic moment of mu(eff) = 2.87 mu(B) per NiO unit. We conclude that these magnetic properties can be attributed to the smallness of the NiO nanoparticles and to crystal lattice defects. (C) 2015 Elsevier B.V. All rights reserved.