First-principle Study On Half-metals

Spintronics is a new emerging field of research at the cross of magnetism, electronics and informatics, etc. Using both electronic charge and spin, the spintronic devices have the advantage over traditional semiconductor electronic devices because of the nonvolatility, high-speed of data processing, low-consumption of power and considerable integration densities. At present, one of the key issues is to find spintronic materials with high spin-polarized conductive electrons, therefore, half-metals, of which the conductive electrons are 100% spin-polarized, would have potential applications in spintronics.Lots of materials have been theoretically predicted or experimentally proven to be half-metals. Among those materials, Heusler alloys are thoroughly studied. Besides, some new half-metallic ferromagnets, i.e., the IIA-VA, IA-IVA, IIA-IVA compounds with Zinc-Blende structure have been attrcted much attention. In this work, using the first-principle calculation based on density functional theory, the character of half-metallicity and electronic structure, etc in half-Heusler alloys CoMnZ (Z=P, As, Sb), full-Heusler alloys FeV₂Z (Z=Si, Ge, As, Sb) and Zinc-Blende CaC are studied.The effects of structural change on the half-mtallicity for CoMnZ (Z=P, As, Sb) alloys are investigated by first-principle study. The total energy calculations indicate that half-Heusler C1_b phase of CoMnZ is generally more energetically stable than Fm-3m supercell phase. Electronic structure calculations indicate that C1_b CoMnZ (Z=P, As, Sb) are half-metals, and the total magnetic moment per f.u. is 3μB, following the Slater-Pauling rule. CoMnP, CoMnAs and CoMnSb can maintain half-metallic for the volume alterations in the range of -16% to 16%, -20% to 11%, and -32% to 0%, respectively. Under tetragonal distortions the half-metallicity in CoMnP, CoMnAs and CoMnSb can be kept for the in-plane lattice variation in the range of 5.00 to 5.80 (A|°), 5.15 to 5.85 (A|°) and 5.45 to 5.85 (A|°), respectively. It seems that the half-metallicity in CoMnP and CoMnAs are more robust than CoMnSb. Therefore, CoMnP and CoMnAs are more promising for practical applications.For FeV₂Z (Z=Si, Ge, As, Sb), the equilibrium lattice constant is calculated based on the total energy calculation. The electronic structure and magnetic properties of FeV₂Z are investigated via first principle calculations. FeV₂Sb is predicted to be half-metallic ferrimagnet. Although FeV₂Si, FeV₂Ge, and FeV₂As are not half-metal, they still present considerably high spin-polarizison. Under volume distortions, the range in which the alloys can maintain half-metallicity is from 5.80 to 6.00 (A|°) for FeV₂Si, from 5.90 to 6.00 (A|°) for FeV₂Ge, from 5.90 to 6.00 (A|°) for FeV₂As, and from 6.00 to 6.15 (A|°) for FeV₂Sb, respectiely. The study on defects in half-metal FeV₂Sb indicates that the half-metallicity can be retained in the cases of As, Ge substitution for Sb site and vacancies at Sb sites, while the cases of Fe(V)-surplus enters in V(Fe) sites, Fe-V swaps and vacancies at transition metal sites destroy half-metallicity.The half-metallicity at surface and interface for the IIA-IVA compound CaC is investigated and the optimized orientation of eptaxial growth which can bring promising injected spin-polarized current is found. The total energy and electronic structure of CaC under Zinc-Blende structure, Wurtzite structure, NaCl structure and NaO structure are calculated via first-principle study. Though ZB-CaC is not energetically preferred, it is the only half-metal with integer magnetic moment per f.u. of 2μB. The results of the DOSs and PDOSs indicate that the half-metallicity results from the polarized p states of C atom. The investigation of surfaces and interfaces indicate that (111) ZB-CaC/GaAs interface with C-Ga contact can bring about promising spin injection.The calculated results agree well with the existed research results, proving the reliability of the computational method used. In addition, some new results and rules are obtained and tentatively explained in this work, which need to be verified via theories and experiments. However, they might be the guidance for the future studying and potential applications of half-metals.