Multiplicity of Zn coordination sites at cubic spinel ferrites: magnetism and influence of the Zn d band

Zinc substituted nickel ferrite (ZnxNi1 - xFe2O4) is investigated under density functional theory (DFT) within the DFT + U approximation for x = 0.50, with particular interest in understanding the effect of Zn on the net magnetization. Using as a reference ZnFe2O4, the localization of the Zn d band is proved to have a large impact on the preference for Zn to occupy either tetrahedral (ZnA) or octahedral (ZnB) coordination sites, which in ZnFe2O4 is equivalent to the relative stability of the direct and inverse spinel forms. This affects the lattice volume, with ZnA favoring larger lattice expansions. Additional important consequences emerge on the magnetism of the system, as ZnA and ZnB alter the balance of atoms at the magnetic sublattices in a different way: while ZnA enhances the global magnetization by reducing the minority spin contribution, the opposite occurs for ZnB. On the other hand, the dominant magnetic exchange interactions are not significantly altered by Zn independently of its distribution, while the magnetic anisotropy of soft NiFe2O4 is further weakened. Our simulations support the presence of a significant ratio of Zn atoms at octahedral positions at ZnxNi1 - xFe2O4, mainly as the Zn concentration increases, putting limits to the ability to increase the magnetization of NiFe2O4 by Zn substitution. © 2023, The Author(s).