Size and surface effects on magnetic properties of Fe3O4 nanoparticles.

In this study, size and surface effects on temperature and frequency dependent magnetic properties of superparamagnetic Fe3O4 nanoparticles in a size range of 1.1-11 nm are investigated by SPR technique. We used a theoretical formalism based on a distribution of diameters or volumes of the nanoparticles following lognormal proposed by Berger et al. The nanoparticles are considered as single magnetic domains with random orientations of magnetic moments and thermal fluctuations of anisotropic axes. The individual line shape function is derived from the damped precession equation of Landau-Lifshitz. Magnetic properties of the samples were strongly temperature and size dependent. The increase in SPR line width, the decrease in the resonance field and also increase in anisotropy filed by decreasing the temperature core-shell type structure of the nanoparticles and disordered magnetic structure (spin-glass like phase) of the particle surface. A linear microwave frequency dependence of the resonance field and the increase in the blocking temperature of the particles by the particle size were also observed.