Neutron diffraction study of microstructural and magnetic effects in fine particle NiO powders

Nickel oxide powders with grain sizes ranging from 100 to 1500 nm have been studied by high-resolution neutron diffraction. We have found that the atomic structure, the antiferromagnetic ordering, and the value of the nickel magnetic moments inherent in the bulk material of NiO are still preserved and are nearly independent of the average size of the grains. The sizes of the coherently scattering atomic and magnetic domains were estimated independently owing to a complete separation of the nuclear and magnetic peaks in the neutron diffraction patterns. It is shown that the finite-size and surface disorder effects in particles at the submicron scale have a more pronounced influence on the magnetism than on their structural properties. We conclude that the core–shell model suggested earlier for nanosized particles can be successfully extended to particles whose sizes are in the submicron range.