Magnetite nanoparticles with diameters of 7, 9, and $12\mathrm{nm}$ have been prepared by a chemical coprecipitation method. The transmission of light through magnetic fluid containing these nanoparticles has been investigated as a function of film thickness with wavelength between 400 and $750\mathrm{nm}$, and applied magnetic fields up to $275\mathrm{Oe}$. The transmission threshold shifts to the lower wavelength side with decreasing magnetic fluid film thickness as well as the particle size. For a given film thickness, the transmittance increases with increasing magnetic field for films with a particle size of 7 and $9\mathrm{nm}$, but decreases in the $12\text{−}\mathrm{nm}$ film. This is attributed to the competition between the van der Waals and dipole-dipole interaction.

• Received 18 February 2005

DOI:https://doi.org/10.1103/PhysRevE.72.031408