Surface Half-metallicity In Several Binary/Ternary Compounds

In1983, de Groot et al. discovered a new class of materials with the Clb type structure. They found that both the half-Heusler alloys NiMnSb and PtMnSb are metallic in one of the two spin channels while semiconducting or insulating in the other one. This property was called as half-metallic (HM) ferromagnetism. Because HM materials have the full spin polarization (100%) at the Fermi level, they have potential applications in spintronic devices. In this thesis, we study the structural, electronic and magnetic properties of the bulk, surface and interface of several binary and ternary HM compounds by employing the first-principles full-potential linearized augmented plane-wave method based on the density functional theory (DFT). Five main contributions are addressed.First, a detailed analysis of the bulk and (111) surface of rock-salt (RS) VPo is provided. It is found that the bulk VPo is a HM ferromagnet at an equilibrium lattice constant (5.9A) with the total magnetic moment3.00μB per formula unit. The half-metallicity is conserved at the V-terminated (111) surface, but lost at the Po-terminated (111) surface because of the different bond length and magnetic moment. It is also noticed that atomic spin magnetic moments decrease at the Po(V)-terminated (111) surfaces compared to the bulk RS VPo. The calculated total magnetic moment complies with the Slater-Paulig demeanor of the binary ZB ferromagnets Mlot=(Ztot-8)μB. We also discuss the surface stability.Second, we investigate the half-metallic properties of the (110) and (001) surfaces of RS VPo and found that the (110) and (001) surfaces of RS VPo preserve the bulk HM characteristics. In addition, we have further shown that, the spin magnetic moments of V and Po atoms at both the (110) and (001) surfaces are increased compared to those in the bulk VPo. It is noticed that both (110) and (001) surfaces in equilibrium conditions are not stable, and therefore the non-equilibrium growth techniques are necessary for the realization of RS VPo thin films.Third, we study the bulk and (001) surface of TiTe with the zinc blende (ZB) structure. Bulk ZB TiTe shows HM ferromagnetism at the equilibrium lattice constant (6.41A) with the total spin magnetic moment of2.00μB per formula unit. It is found that the half-metallicity confirmed in the bulk TiTe is kept on both Ti-and Te-terminated (001) surfaces. We have also shown that the atomic magnetic moments are increased (decreased) at the surface (subsurface) compared to the bulk values. According to the calculations of surface energies, one can deduce that the Te-terminated (001) surface is more stable than the Ti-terminated (001) one. Fourth, We dilate the previous study on the half-metallicity of ZB TiPo to the surface of TiPo(001) and the interface of TiPo/CdTe (001). It is found that both Ti-and Po-terminated (001) surfaces preserve the HM characteristics of the bulk ZB TiPo. The surface atomic magnetic moments increase (decrease) at the Ti(Po)-terminated (001) surface compared to the bulk values. From the calculated surface energies, it is revealed that the Po-terminated (001) surface is more stable energetically than the Ti-terminated (001) one. Furthermore, interfacial studies show that the Ti-Te configuration of the TiPo/CdTe(001) interface is most stable and HM. It is also revealed that the Ti-Te configuration of the TiPo/CdTe (001) interface based on the slab with the in-plane dimension of the2×1unit cell still maintains the half-metallicity.Fifth, for the bulk and (001) surface of Mn2CoSn Heusler alloy, the structural, electronic, and magnetic properties have been studied by using the first-principles calculations. The spin-polarization calculations exhibit that the Mn2CoSn Heusler alloy is a half-metallic ferrimagnet at the equilibrium lattice constant (5.85A) with a magnetic moment of3μB per formula unit. It is found that the Mn(2)Sn-and Mn(1)Co-terminated (001) surfaces of Mn2CoSn destroy the bulk HM characteristics. By calculating the atomic magnetic moment, we also disclose that the atomic magnetic moments increase at the Mn(2)Sn-terminated (001) surface compared to the bulk values, while the magnetic moments of the Mn(1)Co-terminated (001) surface are greatly different from the bulk values. Interestingly, the spin state of Mn(1) atom is turned from a low-spin state to a high-spin state at the Mn(1)Co-terminated (001) surface, which makes Mn2CoSn full-Heusler alloy a promising candidate for spintronic applications. The full-Heusler alloy Mn2CoSn exhibits a Slater-Pauling demeanor and the total magnetic moment (Mtot) with the total number of valence electrons (Ztot) complies with the rule Mtot=(Ztot-24)μB.