Multiplicity of Zn coordination sites at cubic spinel ferrites: magnetism and influence of the Zn d band
C. Tejera-Centeno, Rubén Rico, S. Gallégo
Abstract
Abstract Zinc substituted nickel ferrite (Zn x Ni 1 − x Fe 2 O 4 ) 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 ZnFe 2 O 4 , 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 ZnFe 2 O 4 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 NiFe 2 O 4 is further weakened. Our simulations support the presence of a significant ratio of Zn atoms at octahedral positions at Zn x Ni 1 − x Fe 2 O 4 , mainly as the Zn concentration increases, putting limits to the ability to increase the magnetization of NiFe 2 O 4 by Zn substitution.