Study On The Half-metallicity Of The Quaternary Heusler Alloys

Spintronic devices have the advantages of high speed, small size, and low powerconsumption. High spin injection efficiency is the key to achieve high performance of thespintronic devices. Half-metallic ferromagnets which have100%spin polarization, are theideal semiconductor spin injection sources.Using computer simulation to design new materials and improve material propertiesis one of the important research directions of the condensed matter physics and materialsphysics. In this thesis, using the first-principles based on the density functional theory, westudy the electronic structure, magnetic and surface properties of the quaternary Heusleralloys CoFeCrZ (Z=Al, Si, Ga, Ge) and CoMnCrSi.Our first-principles calculations show that, within generalized gradient approximationfor the electronic exchange–correlation functional, both CoFeCrGa and CoFeCrGe arenearly half-metals, while both CoFeCrAl and CoFeCrSi exhibit excellent half-metallicferromagnetism with the large half-metallic gaps of0.16and0.28eV, respectively. Thehalf-metallicity of CoFeCrAl and CoFeCrSi is robust against the lattice compression (upto7%and4%, respectively). We also reveal that the half-metallicity is lost for bothCoFeCrAl and CoFeCrGa but retentive for both CoFeCrSi and CoFeCrGe when theCoulomb interactions are considered. In addition, the (001) surfaces with and withoutantisite defects lose the bulk half-metallicity in CoFeCrSi.Our results show that, the type (I) structure of CoMnCrSi exhibits excellenthalf-metallic ferromagnetism with the large half-metallic gap of0.31eV, while the type (II)structure is a nearly half-metal. The half-metallicity is robust against the lattice constantchange from5.50to5.80for the type (I) structure and from5.6to5.80for the type (II)structure. In addition，the bulk half-metallicity of CoMnCrSi with type (I) structure isdestroyed at the (001) surface.