Effect of spin disorder on magnetic properties of nanostructured Ni-ferrite

This work presents a structural and magnetic study of NiFe2O4 nanoparticles. Powder samples with different particle sizes were made by annealing the fine powder at different temperatures. X-ray diffraction, transmission electron microscopy, and low temperature SQUID magnetometry experiments were carried out to characterize the samples. Results support a previously proposed model that the NiFe2O4 nanoparticle has a core/shell structure due to surface disorder. However, a detailed analysis of the data reveals that although the structural disorder in the surface of the particle is the major cause for forming a shell, the superexchange coupling is the driving force for the magnetic configuration, which creates a magnetically immobilized region near the surface region with thicknesses significantly larger than that of the structurally inhomogeneous layer on the surface. Also, the ease of magnetization of the core can be significantly affected. Since the crystalline and magnetic disorders commonly exist in the surface region of magnetic nanoparticles, one has to wisely utilize the exchange coupling in the development of nanomagnetic materials.(C) 2004 American Institute of Physics.