Strongly interacting superspins in Fe3O4 nanoparticles

In this paper we report structural and magnetic properties of Fe3O4 nanoparticles synthesized by thermal decomposition of ball milled iron nitrate and citric acid in N2 and air ambient. The XRD pattern of samples which are prepared in air shows some impurity phases, while the samples synthesized in the N2 atmosphere are almost pure Fe3O4 phase. The result shows that by increasing the particle size, the magnetization of the samples increases. The increase of magnetization by increasing the particle size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel -Brown model for superparamagnetic relaxation, but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass like phase in these samples.