Influence of the cation distribution, atomic substitution, and atomic vacancies on the physical properties of CoFe2O4 and Ni<

Abstract

CoFe 2 O 4 and NiFe 2 O 4 are well-known insulating and ferrimagnetic spinel ferrites with high Curie temperatures, 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 NiFe 2 O 4 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.

Interplay between oxygen vacancies and cation ordering in the NiFe2O4 spinel ferrite

“…Electronic and magnetic properties are also likely to be modified by the presence of point defects such as atom vacancies. In a recent study, 10 we showed that charged Ni vacancies and neutral O vacancies (V 0 O ) are the most probable of these defects, under an oxygen-rich or oxygen-poor growth condition, respectively. Oxygen vacancies are a common type of defects in oxides.…”

“…The cation distribution is the first structural parameter which can drastically influence the electronic properties of a spinel oxide. 8–10 Even if so-called inverse spinel ferrites are mostly found with an inversion degree lower than 1, it has been shown that NFO can be grown with a perfectly inverse structure and an ordering of the Fe/Ni cations inside the Oh sublattice corresponding to the space group P 4 1 22 (no. 91).…”

“…13,14 A lowering of the inversion parameter is expected to have for consequences a decrease of the band gap as well as an increase of the magnetization as Fe 3+ cations possess a higher spin magnetic moment (∼5 μ B in the isolated-ion limit) than the Ni 2+ (∼2 μ B ). 10,13…”

Interfaceless Exchange Bias in CoFe2O4 Nanocrystals

“…This fact hints that, besides inducing magnetic disorder, some short-range correlated spin disorder occurs. This situation, particularly in terms of the effects stemming from the presence of Co 3+ and the iron vacancies, inducing a charge reorganization, with local modifications of the valence charge states and possible creation of defect gap states, can explain the changes not only in the magnetization but also in the electronic, ionic, and tunnel conductivities . Such effects require a more in-depth investigation that falls out of the scope of this study.…”

“…This situation, particularly in terms of the effects stemming from the presence of Co 3+ and the iron vacancies, inducing a charge reorganization, with local modifications of the valence charge states and possible creation of defect gap states, can explain the changes not only in the magnetization but also in the electronic, ionic, and tunnel conductivities. 76 to a normal spinel configuration, 77,78 would lead to a magnetization enhancement and can be therefore discarded. In summary, whereas the HAADF-STEM images show NCs with a uniform crystalline structure up to the surface and without the presence of defects or strain, both EELS and Raman spectroscopy, jointly with the magnetic properties, point to the presence of a pseudo core−shell structure with no physical interface.…”