Optical and magneto-optical properties of nanocrystalline cobalt ferrite films

The absorption coefficients and the Faraday rotation spectra of nanocrystalline cobalt ferrite thin films were determined in the visible–near infrared range. The films prepared by rf sputtering in an Ar–O2 atmosphere display magnetic and structural properties which differ from those of the bulk material. Optical and magneto-optical transitions observed in the films are identified by comparing our spectra with those obtained with well characterized bulk materials. In the bulk, cations with an octahedral environment (point group 3̄m) locate on a center of symmetry and the corresponding optical transitions have a very small oscillator strength. In the films, these transitions may lead to a large absorption coefficient. One absorption band appears that cannot be identified on the basis of the symmetry properties of the bulk material. The observed absorption spectra may be explained, however, if some of the cations have one of the following symmetry point groups: 4, 422, and 32. The Faraday rotation spectra show that: (1) the magnetic disorder increases when the oxygen to argon ratio is increased, (2) the disorder influences the cations on both tetrahedral and octahedral sites of the spinel structure.