Faraday rotation spectra of Co nanoparticles embedded in polymethylmethacrylate (PMMA) matrix were investigated by the theoretical and experimental approaches. Co nanoparticles were synthesized by means of the thermal decomposition method, which makes it possible to control the particle concentration without changing the particle size distribution. The amplitude of Faraday rotation is dependent on the particle concentration, and the Faraday rotation spectra show the blueshift accompanied by the decrease in size of Co nanoparticles. The experimental results of Faraday spectra were consistent with the calculation using the Maxwell-Garnett model which is considering the size-dependent free-electron contribution and size independent bound-electron contribution. It was found that the spectral shift was caused only by the size-dependent diagonal element in the dielectric constant tensor, which originates from the confinement of the free electrons in nanoparticles.

The Faraday rotation in Bi-substituted yttrium-iron garnet thin films, in which an artificially fabricated Au nanoparticle array is embedded, is studied as a function of lattice spacing compared with the extinction spectra. With decreasing lattice spacing in the Au array, the wavelengths corresponding to the enhanced Faraday rotation and the extinction peak showed blueshifts in the same manner. This indicates that Faraday rotation can be manipulated by means of the wavelength shift of localized surface plasmon resonance that originates from the change in electromagnetic interaction between Au nanoparticles.

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