Anomalous magnetic properties of antiferromagnetic CoO nanoparticles

Antiferromagnetic CoO nanoparticles ranging from 10 to 80 nm were prepared by the sol–gel process. The morphology, size and structure of the particle were characterized by TEM microscopy, electron diffraction and X-ray diffraction. Magnetic properties were measured using VSM and MPMS magnetometers. The nanoparticles has an FCC structure with a lattice parameter of 4.258 Å. Anomalous magnetic properties, such as hysteresis, were observed in CoO nanoparticles comparing to the course grain materials. The coercive force increases as the particle size is reduced, but decreases when the size is less than 20 nm. The magnetization increases below 100 K for the nanoparticles. However, no significant shifts of Neel temperature were observed. This magnetic behavior could be interpreted in terms of a core-shell model, in combination with the magnetic interaction between particles.