QUANTUM SIZE EFFECTS IN LIGHT-ABSORPTION SPECTRA OF MAGNETITE FINE PARTICLES DISPERSED IN MAGNETIC FLUIDS

Light absorption spectra of magnetite fine particles of about 7 nm in mean diameter dispersed in magnetic fluid were measured as functions of photon energy, E, from 0.5 eV through 1.8 eV and of particles' temperature from 294 K up to 383 K. The light absorption coefficient alpha of the particles is smaller than that of the bulk magnetite. With increasing temperature, alpha decreases for E lower than 1.378 eV, but it increases for E higher than 1.378 eV. In addition, the spectra are shifted towards higher energy region by 0.136 eV compared with that of the bulk magnetite. If concentration of oscillator strengths on the bottom of 4s band of iron is assumed, the energy shift can be interpreted in terms of the quantum size effect of the fine particles, and the effective mass of the 4s electron is evaluated to be 0.37 times as much as that of a bare electron.