Infrared optical properties of Mn1.56Co0.96Ni0.48O4 spinel films sputter deposited in an oxygen partial pressure series

Mn1.56Co0.96Ni0.48O4 spinel films were sputter deposited onto silicon substrates using a series of oxygen partial pressures. Fourier transform infrared transmission and reflectance, and Raman scattering measurements were made. The 1-25 mu m wavelength range was examined using these optical techniques. The complex index of refraction was calculated for this entire wavelength range. Infrared active vibrations were analyzed using multiple oscillator analysis, Kramers-Kronig analysis, and derivative reflectance spectroscopy. The Raman and infrared active lattice vibrations were observed to shift with increasing oxygen partial pressure during film deposition, and were consistent with the earlier published shift in Debye frequency calculated from resistivity data. The films were shown to have an optically transparent window from 6 to 14 mu m wavelength, with the multiphonon cutoff occurring at 14 mu m. The frequency of the multiphonon cutoff was also observed to shift to higher frequency with the oxygen partial pressure during sputtering. These studies, and the earlier work on the variation of the thermopower and resistivity with oxygen partial pressure and film temperature, are consistent with a change in the ratio of Mn3+ to Mn4+ cations with oxygen, with small polaron hopping as the charge transport mechanism. (C) 1999 American Institute of Physics. [S0021-8979(99)01417-6].