Magnetooptical Response of Gold-Magnetite Nanocomposite Films

We present experimental evidence of an enhanced magnetooptical response in chemically synthesized noble metal-magnetic oxide nanocomposites. Colloidal gold and magnetite (Fe₃O₄) nanoparticles were chemically linked using a bifunctional organic ligand creating clusters of the two nanoparticle components. The linked gold-magnetite clusters were blended with a transparent polymer and cast as films. Three samples were produced with a different ratios of 30-nm gold and 8-nm magnetite nanoparticles. The localized surface plasmon peak of the gold nanoparticles observed in the optical absorption data at 538 nm. Spectral Faraday rotation and ellipticity data show changes in both spectra which correlate to this surface plasmon resonance wavelength. Specifically, the inclusion of gold nanoparticles results in a change in sign of the Faraday rotation as well as an enhancement of the ellipticity of about 1.8 in the spectral region corresponding to the surface plasmon absorption peak of the gold nanoparticles. Standard models which ignore optical coupling between particles cannot account for the observed effects. The results suggest that the effects are caused by near-field coupling between the optical field concentrated near the gold nanoparticle surface and the magnetite nanoparticles.