Core-shell model of the vacancy concentration and magnetic behavior for antiferromagnetic nanoparticle

The local defect structure of NiO nanoparticles was determined by extended x-ray absorption fine structure method. By using the bulk and surface sensitive characterization techniques, we are able to show that the vacancies mostly reside on the surface of the particles and the distribution of vacancies can be considered within a core-shell model. We argue that these observations can give a suitable explanation for the venerable problem of observed magnetic behaviors of antiferromagnetic nanoparticles and other functional properties of the material. The observed magnetic moment has been attributed to the interacting vacancies inside the antiferromagnetic host and the distribution of vacancies over the particle volume determines behaviors such as size-dependent different nature of spin-glass freezing and exchange bias.