Finite Size Effect on Magneto-Optical Responses of Chemically Modified Fe₃O₄ Nanoparticles Studied by MCD Spectroscopy

A magneto-optical study of chemically modified magnetite (Fe₃O₄) nanoparticles is presented with magnetic circular dichroism (MCD) spectroscopy. The magnetite nanoparticles are synthesized using the high-temperature solution reduction of Fe­(III) acetylacetonate in the presence of oleylamine. The size of the magnetite nanoparticles examined is 3–4 or 7–8 nm. In contrast to broad and featureless UV–vis–NIR absorption spectra for these magnetite nanoparticles, the MCD responses are much more structured. MCD essentially corresponds to electronic transitions in the absorption spectrum, so to decode electronic transitions in the magnetite nanoparticles and to compare them with those in the bulk phase, simultaneous deconvolution analyses of both the electronic absorption and MCD spectra are conducted, giving accurate transition energies with enhanced spectral resolution. The decomposed transitions are successfully assigned on the basis of the theoretical band structure calculations previously reported. Then the relative absorption and MCD intensities of some deconvoluted bands associated with LMCT or ISCT transitions are found to be size dependent, which can be due to partial surface hydration and/or oxidation of the magnetite nanoparticles. We also compare the magneto-optical responses of magnetite nanoparticles and those of bulk Fe₃O₄ calculated