Influence of the cation distribution, atomic substitution, and atomic vacancies on the physical properties of CoFe2O4 and NiFe2O4 spinel ferrites

CoFe2O4 and NiFe2O4 are well-known insulating and ferrimagnetic spinel ferrites with high Curie tempera-tures, an important characteristic for electronic and spintronic applications. We used first-principles calculations to investigate how their electronic and magnetic properties can be altered or tuned by the presence of structural point defects. We considered successively the effects of cation distribution in the spinel lattice for stoichiometric compounds and of atom substitutions or vacancies. Our calculations demonstrate that a deviation from the perfectly inverse distribution of cations increases the magnetization and decreases the width of the band gap at the Fermi level. In contrast to cation vacancies, oxygen vacancies are not expected to strongly affect the magnetization. We show that NiFe2O4 crystals with an excess of Ni cations can display a spin-polarized hole conductivity. We finally calculated the formation energy of the different defects and we give details on their gap states.