First-principle Calculation On Magnetic And Electric Properties Of New Spinel Half-metals

Spintronics has been emphasized very recently because the charge and spin of electrons can be probably controlled in spintronic materials. A new electronic age will be opened with the quick development of spintronics. Half-metal materials are very important spintronic materials. The cubic spinel half-metal materials are thought to have larger application probability than other half-metal materials because they have high Curie temperature, large room-temperature spin-polarization and they can be prepared simply. However, researches on them have been carried on for short time so that their room-temperature magnetoresistance is still not enough to satisfy their application and there are few kinds of spinel half-metal materials. Therefore, it is very necessary to probe more new spinel half-metal materials with good physical properties and to study their magnetic and electric properties, and then their microscopic mechanism.In this paper, the high spin-polarized material Fe3F4 and many new spinel half-metal materials such as ScFe₂O₄, LaFe₂O₄ and LiPr₂O₄ are prefaced from the first-principle calculation based on the density function theories. And their magnetic and electric properties including half-metallicity, charge distribution and molecular magnetic moments are calculated or analyzed in system. Then their electronic structures and the microscopic mechanism of magnetic and electric properties are analyzed based on the ligand field theories. At last, application probability of these half-materials is prefaced. The main works and results involve:The geometric structures of cubic spinel compounds including transition metals (TM) on A-sites or B-sites are optimized,where TM=Sc,Ti,V,Cr,Mn. Then their spin-polarized state densities and energy band structures are calculated in system. Many new half-metal materials including ScFe₂O₄, Fe₂ScO₄, TiFe₂O₄ and CrFe₂O₄ are prefaced from calculation. The molecular moments of ScFe₂O₄, Fe₂ScO₄, TiFe₂O₄ and CrFe₂O₄ are all larger than those of Fe₃O₄, but their resistivity is lower than that of Fe₃O₄. Therefore, the magnetoresistance of ScFe₂O₄, Fe₂ScO₄, TiFe₂O₄ and CrFe₂O₄ is larger than that of Fe₃O₄ so that they have wider application probability in spintronics. ScFe₂O₄, Fe2ScO4 and TiFe₂O₄ have weak ferromagnetic coupling, but CrFe₂O₄ and Fe₃O₄ has ferrimagnetic coupling. The mechanism is that there are both covalent bonds and ionic bonds between the transition ions (TM) and their ligands in the compound ML₄ and ML₆. However, the proportion of ionic bonds in ScFe₂O₄ and TiFe₂O₄ is larger than those in Fe3O4 and CrFe₂O₄ because Sc and Ti atoms have fewer electrons than Fe and Cr atoms.Fe-ions on A-sites or B-sites of Fe3O4 are substituted by doped La-ions or Pr-ions so that the cubic spinel compounds (RexNM1-x)A(ReyNM2-y)BO4 are designed, where Re=La, Pr and NM=Li, Co, Mn, Fe. Their geometric structures are optimized and their spin-polarized state densities and energy band structures are calculated. New rare-earth spinel half-metal materials including FeLa₂O₄, CoLa₂O₄, MnLa₂O₄ and LiPr₂O₄ are prefaced from calculation. Calculated results show that FeLa₂O₄ is a kind ofâ…¡B type half-metal material, which is similar with CoLa₂O₄ and Fe3O4, but MnLa₂O₄ and LiPr₂O₄ are bothâ…¡A type half-metal materials. FeLa₂O₄, CoLa₂O₄, MnLa₂O₄ and LiPr₂O₄ are weak ferromagnetic coupling compounds because the centric ions on A-sites and B-sites do not have magnetic moments at the same time. Therefore, the coupling can be interpreted by the Rude-Gmann-Kittel-Kasuya-Yosida Model (RKKY). Their molecular magnetic moments vary from 1.0μB to 5.0μB. Therefore, they have wider application area. The magnetic moments of FeLa₂O₄, CoLa₂O₄ and MnLa₂O₄ mainly come from transition ions. The mechanism is that the 3d-orbits of transition ions are splitted by strong crystal field in the tetrahedron compounds ML4 and octahedron compounds ML6. This results in that a kind of 3d sub-band are near fermi level, and then are mixed into hybrid orbits. However, the other kind of 3d sub-band are above the fermi level. La-ions have no contribution for the magnetism of half-metal materials because they have no 4f electrons, although there are hybrid orbits between O-ions and La-ions. The magnetic moments of LiPr₂O₄ mainly come from Pr-ions. The mechanism is that the Pr4f orbits are splitted by strong crystal field, and then there is relative moving between the up-spin and down-spin sub-band. This results in that the up-spin sub-band is near fermi face, but the down-spin sub-band is above the fermi level. On the other hand, the Pr4f orbits can not be mixed with other orbits, and then are localized in ions because there is static shielding of electrons around them.The O-ligands of Fe3O4 are substituted by F-ions, and then new cubic spinel materials are designed. Their geometric structures are optimized and their spin-polarized state densities and energy band structures are calculated in system. The mechanism of magnetic and electric properties and the electronic structures are analyzed based on the ligand field theories. Results show that spinel materials have no half-metallicity if their O-ligands around the Fe-ions on A-sites are substituted by F-ions. On the other hand, the concentration of F-ligand around Fe-ions on B-sites can cause important influence on the half-metallicity and stabilities of materials. Materials have no half-metallicity if x≤0.5. Materials have half-metallicity if x>0.5. The Fermi face move forward higher energy, and then the half-metallicity are more stable with increasing concentration of F-ions. The main mechanism is that the concentration of electrons in a primitive cell increases with the increasing of F-ions so that there are stronger crystal field, which cause stronger splitting of up-spin sub-band and down-spin sub-band. The stable crystal constant is about 0.643 nm and the molecular magnetic moment is about 10.68μB, which is higher than 4.0 of Fe3O4. The spin-polarization of Fe3F4 is a little lower than Fe3O4, but its conductance is much higher than Fe3O4 so that it has huger application potential than Fe3O4.