Optical absorption tail and superparamagnetism of gamma-Fe2O3 nanoparticles in a polymer matrix

The magnetic and optical properties of small particles are crucially dependent upon their size. Well defined spherical particles of gamma-Fe2O3 have been synthesized in the pores of a polymer matrix in the form of beads by an ion exchange and precipitation reaction. The particle size distribution is a gaussian with an average diameter of 80 A. The DC magnetic susceptibility and the magnetization of the nanocomposite have been measured between 4 and 300 K using a Faraday balance and a vibrating sample magnetometer, respectively. The magnetic measurements demonstrate that the particles are superparamagnetic with a blocking temperature T-b about 55 K. A small susceptibility peak at the blocking temperature, appearing at small heating rates only, is attributed to the magnetocaloric effect. The temperature dependent magnetic susceptibility and the field dependent magnetization of the nanocomposite is modeled by using Neel's theory of superparamagnetism and the particle size distribution. The optical absorption edge of the mesoscopic system is red shifted with respect to single crystal films of gamma-Fe2O3 with an absorption tail extended deeply in the gap. Lattice distortion and the existence of excitonic states in the gap can explain the absorption behavior.