Optical band gap hierarchy in a magnetic oxide: Electronic structure of NiFe2O4

We combined first-principles calculations with optical spectroscopy and variable-temperature film growth techniques to comprehensively investigate the electronic structure of NiFe2O4. We find this system to be an indirect-gap material in the minority channel, with two higher-energy direct-gap structures in the minority and majority channels, respectively. An analysis of states near the band edge simultaneously exposes both the charge transfer and Mott limits of the Zaanen-Sawatzky-Allen classification scheme. The gap hierarchy is well suited for spintronics applications.