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

Abstract Zinc substituted nickel ferrite (Zn_xNi_{1 − x}Fe₂O₄) 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 (Zn_A) or octahedral (Zn_B) coordination sites, which in ZnFe₂O₄ is equivalent to the relative stability of the direct and inverse spinel forms. This affects the lattice volume, with Zn_A favoring larger lattice expansions. Additional important consequences emerge on the magnetism of the system, as Zn_A and Zn_B alter the balance of atoms at the magnetic sublattices in a different way: while Zn_A enhances the global magnetization by reducing the minority spin contribution, the opposite occurs for Zn_B. 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 Zn_xNi_{1 − x}Fe₂O₄, mainly as the Zn concentration increases, putting limits to the ability to increase the magnetization of NiFe₂O₄ by Zn substitution.