First-principles Study On Spinel Cobalt Ferrites And M-type Hexagonal Strontium Ferrites

Ferrites are important magnetic functional materials. Spinel ferrites, such as CoFe₂O₄, can be applied in varied fields such as information storage systems, spin electronic devices, a variety of magnetic devices, and communication equipments due to their good electromagnetic properties. Furthermore, CoFe₂O₄ is a good magnetic oxide candidate for multiferroic systems. The hexagonal ferrites such as SrFe₁₂O₁₉ have been intensively investigated due to its considerable importance to the electronic material industry.As a traditional permanent magnet, SrFe₁₂O₁₉ is widely used for many applications such as traditional permanent magnets, micro wave devices, magneto optical devices, high density magnetic recording media, and so on.In this dissertation, the electronic structure and magnetic properties of CoFe₂O₄ are studied by first principles calculations. We mainly discuss the influence of the cation distributions,strains from the substrates, defects and impurities on the electronic structure and magnetic properties of CoFe₂O₄. Meanwhile, we studied the effect of the intrinsic defects, Co doping and La Co co doping on the magnetic properties and electronic structure of M type SrFe₁₂O₁₉, in addition to its bulk properties.Following a brief introduction to the magnetic phenomena and classification of magnetic materials, we reviewed the current status of researchs on the spinel ferrites and magnetoplumbite ferrites. In addition, the theoretical basis of the density functional theory was also introduced.Regarding the CoFe₂O₄ related studies, we discussed its electronic structure and magnetic properties based on our first principles calculations, with a focus on the roles of the cation distributions, strains from the substrates, defects and impurities. The results showed that the size effect of ions plays an important role in CoFe₂O₄, resulting in Co ions tend to occupy on the octahedral sites. Thought Co and Fe ions are characterized to be in their high spin states at either tetrahedral or octahedral sites, the electronic structure and magnetic properties depend on the cation distributions remarkably.The films of CoFe₂O₄ were simulated on various substrates with a realistic biaxial strain model. The results showed that the inverse spinel is still energetically favored under strains （in a range of 9.35%¹2.12%）, but the energy difference between the inverse and normal spinels decreases and thus Co²⁺ ions are expected to incorporated into the tetrahedral sites easier. As the strain increases, the band gap becomes narrower, while the net magnetic moment changes little.Using a 56 atom supercell, we have studied the intrinsic defects on CoFe₂O₄, which indicated that the oxygen vacancies are easy to form under the metal rich and cobalt rich conditions. The cation vacancies located in the octahedral sites have lower formation energy under the oxygen rich condition, indicating that cation vacancies are easy to appear under the oxygen rich condition and possibly introduce the p type carriers. The magnetic properties of CoFe₂O₄ are affected by the defects clearly, especially under the oxygen rich condition, as the growth temperature increases. The density of states （DOS） and charge density contour showed that the defect energy level appeared in the band structure of defective cell are usually attributed to the obitals from the neighboring atoms of the defects.The lattice parameters of CoFe₂O₄ increase as the Zn （Cd） doping concentration increases except for the Zn doping at x =0.5. The net magnetic moments of the compound were found to increase linearly with the Zn （Cd） doping concentration, though it was reported to drop after increasement in some experiments. It was also found that the magnetic moments of Co/Fe ions increase with Zn （Cd） doping. The DOS is sensitive to the concentration of Zn （Cd） and the spinel becomes half metallic at some degree of doping.The results of the RE ions substituting the Fe ions in CoFe₂O₄ showed that the lattice constant of CoFe1.875RE0.125O4 （RE=La, Ce, Pr, Nd, Eu and Gd） decreases due to the decreasing ionic radius of RE as the atomic number increases. The magnetic properties depend on the unpaired 4f electrons of RE³⁺ ions. The net magnetic moment of CoFe₂O₄ will increase with Eu and Gd doping, mainly because there are more unpaired 4f electrons for Eu³⁺ and Gd³⁺. The contribution from the doped La, Ce, Pr, and Nd is not remarkable to the magnetic properties, since these RE ions, with larger ionic radii, could distort the crystal structure of CoFe₂O₄.Regarding the M type SrFe₁₂O₁₉ related studies, we discussed the effects of the intrinsic defects, Co doping, and La Co co doping on its magnetic properties and electronic structures based on our first principles calculations. In order to accurately describe the conductivity behavior, we predict the band gap of 1.44eV for M type Sr ferrite from the sol method. Then we employ GGA+U to describe the strongly correlated effect between the 3d electrons of Fe ions.Using the crystal cell containing 64 atoms, we have studied the intrinsic defects and substitutional defects of Co at Fe sites in M type Sr ferrite. It was found that the vacancies of metal ions are difficult to form under the oxygen poor conditions, while the oxygen vacancy is easy to form. Under the oxygen rich conditions, the vacancies of metal ions are still difficult to form when the Fermi level close to the valence band maximum （VBM）, but it gets much easier to form when the Fermi level is close to the conduction band minimum （CBM）, especially for the Fe vacancy at the 2a sites. We also found that CoFe defects have relatively low formation energy under the oxygen rich conditions, implying that Co can be doped at Fe site by a large amount. The CoFe defects favor at the 4f2 sites followed by 2a sites, which are stabilized with the isovalent doping.The lattice constant of La Co co doped M type Sr ferrite is smaller than that of pure bulk, as the radius of La³⁺ is smaller than that of Sr²⁺. The total net magnetic moments change little upon doping since the moments of Fe ions keep the same although those at the 2a and 12k sites are reduced slightly. Co ions are at their ³⁺ valence states with a electronic configuration of t_2g3↑e_g2↑t_2g↓for the 3d electrons. La Co co doped Sr ferrite shows the n type semiconductor character, in line with the available experimental observation.