Chemical tuning of the optical band gap in spinel ferrites: CoFe_2O_4 vs NiFe_2O_4

摘要

We measured the optical properties of epitaxial CoFe_2O_4 thin films and compared our findings with complementary electronic structure calculations and similar studies on the Ni analog. Our work reveals CoFe_2O_4 to be an indirect band gap material (1.2eV, X→Γ in the spin-down channel) with a direct gap at 2.7 eV. The latter is robust up to 800 K. Compared to NiFe_2O_4, the indirect gap is ≈0.5eV lower, a difference we discuss in terms of size and covalency effects in spinel ferrites.