Optical and electronic properties of NiFe2O4 and CoFe2O4 thin films
Spinel Ferrites Thin Film Research Highlight
Understanding the electronic structures of these ferrite thin films is very important to improve thin film quality as well as to enhance the physical properties, particularly the magnetic properties for potential applications. In previous magneto-optical studies on NFO and CFO, the electronic transitions between 0.5 and 5.0 eV were controversially assigned to inter-valence charge transfer (IVCT) and inter-sublattice charge transfer (ISCT) transitions. The electronic transitions between two metal ions, such as between Co2+ and Fe3+ ions, and between Ni2+ and Fe3+ ions are referred to as IVCT transitions, and the electronic transitions between Fe3+ ions on different crystallographic sites are referred to as ISCT transitions.
NiFe2O4 (NFO) Thin Film Excitations
(a) Electronic absorption spectra of a 120 nm NFO film on (0001) sapphire at representative temperatures. The spectra show a weak excitation centered at ~1.5 eV and a strong excitation peak at ~4.5 eV. Transmittance spectra of NFO films display high transparency (<75%) for photon energy up to 3 eV. The inset highlights the ~1.50 eV band, which becomes even weaker below 175 K.
(b) Schematic energy level diagram for the Ni2+ (3d8) ion in an octahedral field and possible electronic transitions within Ni d states. In an octahedral crystal field, the degeneracy of Ni2+ states is lifted, allowing 3 possible on-site Ni d to d transitions. These three spin-allowed transitions have weak oscillator strengths and overlap each other, resulting in a single broad band. Temperature dependence suggests the local environment of NiO6 changes with cooling.
CoFe2O4 (CFO) Thin Film Excitations
Absorption spectra of a 100 nm CFO thin film at 78, 300, and 400 K. The spectra show a typical insulating character with three distinct low energy peaks centered at ~0.75, 1.25, and 1.75 eV. These peaks are attributed to spin-allowed Co2+ d to d on-site transitions, as depicted in the energy level splitting of the Co2+ (3d7) ion in an octahedral field. Electronic excitations above 3.00 eV are attributed to O 2p to Fe 3d charge transfer excitations.
Conclusion
Absorption spectra of NFO and CFO thin films show insulating characters with metal d to d on-site transitions below 3 eV and O 2p to Fe 3d charge transfer transitions above 3 eV. Optical spectra display weak oscillator strengths for electronic transitions below ~3 eV, characteristics of spin-allowed but parity-forbidden metal d to d on-site electronic transitions. Previously assigned IVCT and ISCT transitions between two metal ions in these ferrites would have very small oscillator strengths compared to observed metal d to d on-site and O 2p to Fe 3d charge transfer transitions. Therefore, IVCT and ISCT transitions in absorption spectra are ruled out in inverse spinel ferrites.
Reference: R. C. Rai et al., Appl. Phys. A 106, 207 (2012).
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