Optical and electronic properties of NiFe2O4 and CoFe2O4 thin films

We report the optical and electronic properties of the inverse spinel ferrite NiFe2O4 and CoFe2O4 thin films deposited on single crystal sapphire by electron beam deposition. We carried out variable temperature (78-500 K) transmittance measurements on the thin films to investigate the optical properties and electronic structures of these ferrites. The absorption spectra of both NiFe2O4 and CoFe2O4 thin films show insulating characters with Ni (Co) d to d on-site transitions below 3 eV. The energy bands above 3 eV are mainly due to the O 2p to Fe 3d charge transfer transitions. The observed electronic transitions have been assigned based on the first principles calculations and comparisons with structurally similar Ni and Co-containing compounds. The Co2+ d to d transition in the CoFe2O4 thin film shows a strong temperature dependence, likely due to the spin-charge coupling effect.