First-Principle Studies Of Some Spintronics Materials

In this thesis, we study the magnetic properties of some quaternary Heusler alloys, the magnetic properties in the surfaces of MP(M=K, Rb)with rock-salt structure, as well as the magnetic and thermodynamic properties of double perovskite Ba2 P r V O6. In our research,we find some half-metallic ferromagnets, spin-gapless semiconductors and half metallic antiferromagnets as well as discuss the origin of their magnetism.Since the theoretical prediction of half-metallicity of half-Heusler alloy NiMnSb by de Groot's group in 1983, spintronics has attracted more and more attention to researchers. Spintronics materials are the key to approach the next-generation semiconductor technology. In spintronics, not only the charge freedom but also spin freedom can be tuned. Comparing to traditional semiconductors, spintronics materials can realise the storage of information with non-volatile memory, low power consumption and high integration level. First-principle study is an efficient method to predict the properties of materials with high accuracy. So it is widely used to the explore new materials and new properties in spintronics research, which has provided much meaningful guidance for experiments.In the second chapter, we give a brief introduction to the development and basic concept of density function theory. At the beginning of foundation, quantum mechanics can only calculate simple H atom. After many years' development and effort, E. A. Hylleraas and D. R. Hartree calculated the ground state of He atom by means of variational and self-consistent method,respectively. After that, V. Fock introduced the famous Hartree-Fork method by modifying D. R. Hartree's self-consistent method. This method is the basic principle of many modern electronic structure calculation codes. In 1960's, density function theory was established. Since then, it is widely applied in fields of computational chemistry and material simulations because of its high precision and moderate computational consumption.In the third chapter, the electronic structures and magnetic properties of some quaternary Heusler alloys are investigated by using first-principle calculations. At first, the electronic,magnetic and mechanical properties of quaternary Heusler alloys CoFeScZ(Z=P, As, Sb) are studied. The results show that under equilibrium lattice condition CoFeScP is half metallic,while CoFeScAs and CoFeScSb are nearly to half metallic. The change of the properties of CoFeScAs and CoFeScSb with pressure are also studied, indicating they can be used as pressure-sensitive materials. Next, we study the properties of some Zr-based spin-gapless semiconductors in quaternary Heusler structure. In the aspect of formation energy and cohesion energy, these compounds are structurally stable and chemically synthetised. By analysing the possible hybridization structure, we get the origin of the magnetism for the spin-gapless semiconductors. It is very interesting that the number of valence electrons is 21 or 26 for theses alloys.In the fourth chapter, we study the properties of the surfaces for MP(M=K, Rb) in rock-salt structure by using first-principle calculations. The calculations show that the half-metallicity in bulk compounds are preserved in the(111) surfaces of MP(M=K, Rb). The calculations of surface energies indicate that all(111) surfaces can be easily fabricated and the K( or Rb)-terminated surface is more stable than the P-terminated one.In the fifth chapter, also based on first-principle study, the properties of double perovskite oxide Ba2 P r V O6 are investigated. It is found that within LSDA+U(considering Coulomb correlations) Ba2 P r V O6 is a half-metallic anti-ferromagnet, which is also named as compensated half metal. According to the total and partial density of states, the anti-ferromagnetism of Ba2 P r V O6 is originated from the spin down state P r+4(4f-2) and the spin up hybridized state between the partially filled t2 gstate of V and the partially filled triple-degeneration state4f-1,4f+1,4f+3of Pr. It is found that the compensated half metallic state is the ground state through fixed spin moment calculations. By employing the quasi-harmonic Debye model, the thermodynamic properties of Ba2 P r V O6 are also studied.At the final chapter, we make a summary of this thesis and a plan for the following work.