Electronic Structure of NiFe$_2$O$_4$ using screened Hybrid Functionals

As an insulating ferrimagnet with a high Curie temperature, NiFe$_2$O$_4$ (NFO) may be a promising candidate for future spin-based applications. Recent demonstration of spin-Seeback effect in magnetic insulators indicates that important new phenomena may be discovered in such materials. Unfortunately, density functional theory cannot give a full account of its properties; most notably, LDA calculations find it to be metallic. LDA+U yields an insulator but underestimates the band gap. The recently-implemented screened hybrid functionals method (HSE06) represents only a moderate increase in computational effort compared to traditional DFT. This method allows one to modify the PBE-approximated exchange potential with a portion of the Hartree-Fock exchange. We present LDA, LDA+U, and HSE06 calculations of the density of states and band structure of NFO. We show that hybrid methods greatly improve agreement with the experimental band gap over LDA +U. We find that NFO is an indirect band gap system with the spin-down channel having the lower, indirect gap, whereas the majority channel possess a direct gap with over a 0.5eV difference with the minority gap. Comparison of our theoretical results with recent optical measurements on NFO thin films is also presented.