First-principles Study Of Magnetic Properties And Band Structures In Heusler Alloys

Electrons have charge and spin two unified properties. When the scale of materials matches the physical characteristic length, electrons with spin properties in transmission will show novel physical effects. Heusler alloys have two different spin sub-bands. The spin up sub-band is metallic, while the spin down sub-band might be semiconducting. If so, the alloy is called half-metallic ferromagnet. Therefore, ideal half-metal has 100% spin polarization near the Fermi level. So, Heusler alloys with half metallic properties will be an excellent semiconductor spin electrons inject source. Meanwhile, we should make great effort to do researches on Heusler alloys because it has significant scientific value and application prospect, which will definitely promote the development of spintronics. But, it is extremely hard to select Heusler with half-metallic properties among such enormous Heusler alloys. Also, there will be a lattice constant mismatch when growing Heusler alloys and its electronic and magnetic properties are very sensitive to the tiny change of lattice constant. Hence, it is very important to carry out stress analysis on Heusler alloys. At last, the transition metal elements that form the Heusler alloys will also have a great effect on electronic and magnetic properties of Heusler alloys.This dissertation predicted Heusler alloys with half-metallic properties by first-principles calculation. Meanwhile, due to the different experimental conditions in material growth process, the Heusler alloys will have lattice constant deformation. So, we carry out stress analysis on Heusler alloys. At the same time, we studied a series of Heusler alloys with the adjacent transition metal elements as components. We studied how the electronic and magnetic properties change with the different transition metal elements. We did find out the changing rules and it will be the theoretical foundation of developing spintronics devices. The main research aspects of this dissertation are as follows:1. We studied the electronic and magnetic properties of half-Heusler: Pd Fe Bi andPd Co Bi in all three lattice configuration at their optimized situation, which are optimized lattice constants, magnetic moments and band structures. We studied all three possible lattice configurations: α, β, and g phases and see how the influence will be made on lattice constants. One atom has different atom as its nearest neighbor, which will have effect on the strength of bonding and crystal field effect. We also studied the change of magnetic moments along with different lattice configurations. Meanwhile, we studied the influence of lattice constants on the band structures and magnetic moments at the same lattice configuration, which include the influence of lattice constant on the strength of crystal effect, orbital hybridization and exchange interaction. We found that the alloy in different lattice configuration has different optimized lattice constant and magnetic moment. Furthermore, the difference in lattice configuration will cause the variation of the crystal field effect so that the magnetic moment and the band structure changed.2. We studied a typical ternary compound, half-Heusler alloy Pd Mn Bi. First of all, we studied the electronic and magnetic properties at its optimized situation in all three difference lattice configurations. We studied the total energies and magnetic moments of different lattice configurations. Then, we change all the lattice constants by applying uniaxial hydrostatic pressure and we did find the half-metallic lattice constants range in α and β phases. There is no half-metal in g phase in studied lattice constant range. We chose two typical lattice constants to explain the half-metallic properties: crystal field effect, orbital hybridization and exchange interaction. We figured out why there is a semiconducting gap in the spin down sub-band. Due to the d states of Pd and Mn and the exchange interaction. Spin down states of Mn become unoccupied and form the semiconducting gap. Due to the fact that both Pd and Bi have big atomic weight and atomic radius, we studied the influence of spin-orbital interaction on two typical lattice constants of half-metals Pd Mn Bi. The results are consistent with the fact that the spin-orbital interaction is more significant in Heusler alloys with heavier elements. Finally, we gave the possible applications of this ternarycompound, which include the spintronics devices, large magnetic moments and well matching of most semiconductor devices due to optimized lattice constant is also large.3. We studied a series of half-Heusler alloys which contain gradational transition metal elements. By choosing typical transition metal elements X= Mn, Fe, Co and Ni, we studied ternary compound Pt XBi in all three possible lattice configurations: α, β, and ? phases. There are four different transition metal elements and three different lattice configurations, which makes twelve combinations. We calculated the optimized lattice constants, total energies and magnetic moments for these twelve combinations. From different transition metal elements and different lattice configuration point of view, we make detailed explanations on the differences of lattice constants, total energies and magnetic moments. Due to the changing of lattice constant, we found a wide half-metallic lattice constant range in α phase and the semiconducting gap is also wide. We chose a typical lattice constant in the range and we gave a detailed explanation on the formation of half-metallic properties by analyzing the total density of states, and partial density of states. Due to the fact that Pt and Bi are all heavy, we studied the half-metallic properties under the influence of spin-orbital interaction and calculated the spin-orbital splitting.4. We studied the non-magnetic 2p light element carbon-doped rutile Ti O2, which ais very appealing for spintronics and infranics. Ti O2 has three possible structures: rutile, anatase, and brookite, with rutile being the most thermodynamically stable. Thus, we chose the rutile structure. Ti O2 has a similar structure with common Heusler alloys. Besides, Ti O2 can have localized magnetic moment by dpoing non-magnetic transition metal elements, which is just like Heusler alloys. The results show that carbon dopants tend to couple ferromagnetically around the Ti atom in the rutile structure, and the magnetic moment per C is about 1.3 B. The ferromagnetism is predicted to be the collective effects from a p-d exchange-like p-p coupling interaction.