First-principles Study On The Half-metallicity Of Binary And Quaternary Heusler Alloys

Half-metallic ferromagnets, which have 100% spin polarization and high Curie temperatures, are the key to achieve high performance of the spintronic devices. In this thesis, using the first-principles based on the density functional theory, we firstly study the slectronic structure, magnetic property and surface stability of DO3-Mn3Ge and its (001) surface. On the other hand, we study the structural stability, electronic and magnetic properties of the quaternary Heusler alloys CoRuVSb and CoRuCrSi. We also study the surface properties of the CoFeMnGe(001) surface. We obtain the following main results:DO3-Mn3Ge shows half-metallic ferrimagnetism, and there is a band gap about 3.52eV in the spin down direction. The total magnetic moment of DO3-Mn3Ge is 1.00 ?B, which is a typical characteristic of half-metallic magnets. The bulk half-metallicity is preserved at the pure Mn-term(001) surface due to the large exchange split, but the MnGe-term (001) surface destroys the bulk half-metallicity. We compare the surface energies of DO3-Mn3Ge (001) surface and experimental D022-Mn3Ge (001) surface, and reveal that the feasibility to grow the Mn3Ge (001) films with DO3 phase other than DO22 one. The surface half-metallicity and stability make DO3-Mn3Ge a promising candidate for spintronic applications.In the three different structures of quaternary Heusler alloys CoRuVSb and CoRuCrSi, both them have the most stable type I structure, and both CoRuVSb and CoRuCrSi with type I structure exhibit excellent half-metallic ferromagnetism. The half-metallicty of CoRuVSb is robust against the lattice compression (up to 4.87%) and expansion (up to 4.87%). In addition, we also find that the half-metallicity of CoFeMnGe with type I structure is lost at its CoFe-and MnGe-terminated (001) surfaces, but the spin polarization is still high at the CoFe-terminated (001) surface (78%).