Optical and magneto-optical study of nickel and cobalt ferrite epitaxial thin films and submicron structures

Abstract

Epitaxial films and ordered arrays of submicron structures of nickel and cobalt ferrites were deposited on Nb doped SrTiO3 by pulsed laser deposition. X-Ray diffraction and atomic force microscopy showed that the films have a good crystalline quality and smooth surfaces. A larger number of phonon bands was observed in the polarization dependent Raman spectra of the ferrite films than expected for the cubic spinel structures. This is explained by short range ordering of the Ni2+ (or Co2+) and Fe3+ cations at the octahedral sites inducing a lowering of the symmetry. The same behavior was also observed in the Raman spectra measured for the submicron structures, suggesting the same cation distribution as in the films. The diagonal components of the dielectric function for nickel and cobalt ferrites are determined from ellipsometry in the 0.73–5 eV photon energy range. The absorption edge was analyzed using a bandgap model and the energies for the indirect and direct optical transitions were calculated. It was found that both nickel and cobalt ferrites are indirect bandgap materials with bandgaps of 1.65 eV and 1.42 eV, respectively, while the first direct transitions lie at 2.69 eV and 1.95 eV, respectively. Magneto-optical Kerr effect spectroscopy in combination with spectroscopic ellipsometry allowed the off-diagonal elements of the dielectric tensor to be determined in the energy range from 1.7 eV to 5 eV.