Atomic-level cation occupation and magnetic properties of Ce³⁺-doped ZnFe₂O₄ spinel ferrite

Rare-earth doping has been proven to be an effective strategy for tailoring the magnetic properties of ferrites. In this study, we report a direct experimental observation of the precise occupation of cations in Ce-doped spinel ZnFe₂O₄ ferrites. We demonstrate that divalent Zn²⁺ cations and trivalent Fe³⁺ cations respectively occupy all tetrahedral A sites and all octahedral B sites in ZnFe₂O₄ nanofibers, which aligns well with the normal spinel structure. Rare-earth Ce³⁺ ions preferentially occupy the octahedral sites of the ZnFe₂O₄ lattice, while the excess Fe³⁺ ions are displaced to the tetrahedral sites. The observed behavior is possibly due to the greater bond energy of Ce³⁺-O^2- relative to Fe³⁺-O^2-, requiring additional energy for Ce³⁺ to substitute into the B-sites. This cation redistribution leading to the appearance of 4f-3d orbital couplings results in the changes in magnetic performance at room temperature. With increasing Ce³⁺ doping concentration, the saturation magnetization (M _s) first increases before reaching a maximum and subsequently decreases. ZnFe₂O₄ nanofibers doped with 0.05 mmol Ce³⁺ exhibit the highest M _s value due to the enhanced A-O-B super-exchange interaction.