First-principles Study On The Influence Of Doping On Properties Of Spinel Cobalt Ferrite And Zinc Ferrite

The spinel ferrite as an important class of magnetic materials, its unique optical,electronic and magnetic properties,is widely used in electronic devices, information storage systems, magnetic devices, communication devices and microwave devices,and so on. Therefore, we use first-principles calculations and the generalized gradient approximation（GGA） + U method based on the density function theory to systematically study the influence of doping on the crystal structure, electronic structure and magnetic properties of cobalt ferrite and zinc ferrite, and their associated physical mechanisms.In this dissertation, the structural stability, magnetic properties and electronic structure of CoFe₂O₄ were studied. The results showed that the Co ions tend to locate in the octahedral sites, i.e. the inverse structure of CoFe₂O₄ is the most stable. From the analysis of density of states（DOS）, the inverse and normal structure are insulating,and the band gap of inverse structure are wider than that of normal structure. It also indicated that Co and Fe ions are chacacterized to be in their high spin configuration,and the total magnetic moment of normal structure is larger than that of the inverse structure.The results of the Ni doping on CoFe₂O₄ show that Ni ions tend to substitute the Co ions located in the octahedral sites, and with the increase of Ni content, the lattice constant decreased. The DOS shows that the band gap changes as the Ni-doping concentration changes, and most of system are insulating. Magnetic properties depend on the doping amount of Ni ions, the change of atomic magnetic moment is not obvious with the increase of Ni content, but the total magnetic moment shows linear decreasing trend, mainly because the unpaired 3d orbital electrons of Ni ion is smaller than that of Co ions.The calculated results for the structural stability, magnetic properties and electronic structure of CoFe₂O₄ with Mn doping show that the Mn ions prefer substituting Co ions and locate in the octahedral sites with +3 valence state when the doping amount of Mn is less than 37.5%. However, partial Mn ions prefer locating in the tetrahedral sites with +2 valence state when the doping amount of Mn is larger than 37.5%,i.e. the Mn²⁺ tend to locate in the tetrahedral sublattice, while Mn³⁺ tends to locate in the octahedral sublattice. The lattice constant increases linearly with theincrease of Mn doping concentration due to the ionic radius of Mn is larger than that of Co. The DOS shows that the band gap of system decreases with Mn doping. The results of magnetic moments show that the total magnetic moment increases with the increase of Mn content when the doping amount of Mn is larger than 37.5%.Spinel Zinc ferrite（ZnFe₂O₄） is widely used in treatment of dye wastewater due to its high activity. In present paper, impurity levels have been introduced in the band with doping to improve the ability of light absorption for ZnFe₂O₄ by adjusting the band structure. And the photocatalytic efficiency, to a certain extent, can be improved. The results show that the normal spinel structure of ZnFe₂O₄ is the most stable, and the band gap is 1.92 eV. The influence of two valence elements doping on the magnetic properties of ZnFe₂O₄ is obvious. The contribution of doping ions(e.g.Be²⁺、Mg²⁺、Ca²⁺、Sr²⁺and Ba²⁺) without unpaired electrons on the magnetic properties of ZnFe₂O₄ is very small. While the doping ions(e.g. Cr²⁺、Mn²⁺、Co²⁺、Ni²⁺and Cu²⁺)possessing unpaired 3d electrons have different degree influence on the magnetic properties of ZnFe₂O₄. The band gap changes obviously with the different trivalent elements doping, and the doping of different trivalent elements make the total magnetic moment decrease.