Chemical tuning of the optical band gap in spinel ferrites: CoFe2O4 vs NiFe2O4

We measured the optical properties of epitaxial CoFe2O4 thin films and compared our findings with complementary electronic structure calculations and similar studies on the Ni analog. Our work reveals CoFe2O4 to be an indirect band gap material (1.2 eV, X -> Gamma in the spin-down channel) with a direct gap at 2.7 eV. The latter is robust up to 800 K. Compared to NiFe2O4, the indirect gap is approximate to 0.5 eV lower, a difference we discuss in terms of size and covalency effects in spinel ferrites. (C) 2013 AIP Publishing LLC.