Electronic Structure, Half-metallicity And Magnetic Properties Of Inverse Full-Heusler Alloys Ti_2-xM_xCoAl（M=Nb, V）

Research on half-metallic ferromagnets is intensively growing in the decades since the half-metallicity was first proposed by de Groot and collaborators from the band-structure calculations for the half-Heusler alloys, NiMnSb and PtMnSb. Heusler alloy are attractive candidates for providing halfmetallic materials in spintronic application and are expected to exhibit100%spin polarization due to an energy band gap for the minority-spin electrons at the Fermi level (εF). This feature is of great importance in spin-dependent device such as spin-injection, spin-filters, tunnel junctions and giant magnetic resistance (GMR) devices.As promising candidate of half-metals, full-Heusler alloys attract much attention and a number of works has been implemented on them. However, the environment of the preparation such as temperature and ingredients proportion can cause some disorder and defects which could cause a slight shift of their Fermi levels, leading to considerable changes of crystal structure, Curie temperature, magnetic moment, spin polarization, and even the loss of half-metallicity. Some groups have shown that doping could be a method of choice to tune the Fermi level to obtain the desired property by adjusting the electronic structure of certain materials. Fermi level tuning has been successfully demonstrated in Co-based full-Heusler alloy CO2FeAl0.5Si0.5which exhibits a high spin polarization above0.9, while it is only around0.5(0.6) at a low temperature for Co2FeAl (Co2FeSi). Controlling of εF toward the center of the half-metallic gap by Al-doping to CMS (Co2MnSi) was clearly demonstrated in magnetic tunnel junctions with a Co2MnAlxSi1-x. The stability of half-metallicity against temperature can be shown by the position of εF in the band gap.In recent years, inverse Heusler alloy attracts more attention as a new family of the half-metallic ferromagnet since they combine coherent growth on semiconductors with large Curie temperatures which exceed the1000K as in the case of Cr2CoGa. More and more inverse Heuslers have been detected to be half-metallic. The recent study on inverse Heusler compound Ti2CoAl indicates that the minority-spin band gap is narrow (0.49eV) and the εF is very close to the bottom of the gap. Thus the behaviors such as crystal defect, atomic disorder, surface and interface effects, and the narrow energy separation between the Fermi level and the conduction/valence band edge might cause a shift of theεF, resulting in the significant changes in electronic and magnetic properties, even the destruction of the half-metallicity. To precisely control the properties and make better use of the material for spintronic device, in this work, within the framework of density functional theory (DFT), we study the doping effects and defect effects on the half-metallicity and magnetism of the inverse Heusler alloy Ti2CoAl.The conclusions can be summarized as the following two aspects:1. The study of the doping effects shows that the V/Nb doping at the Ti(A)/Ti(B) site is energetically favorable compared with the Ti(B)/T(A) site due to the lower total energy. For the V doped compounds, half-metallicity can be well retained regardless of doping sites and percentages except for the case of Ti(A)-site doping with x=1, while for Nb doped compounds, the half-metallicity persists only in Ti(B)-site doping with different percentages. For the doped compounds with half-metallicity, their Fermi levels are adjusted to the expected positions at a certain doping concentration, which might effectively inhibit the spin-flip excitation.2. The investigation on the disorder effects reveal that Ti(A/B)-Co and Co-Al swap defects, Ti(A/B) and Al vacancy defects as well as CoTi(A)/Al and AlTi(A)/T i(B) antisite defects, due to negative formation energy, are most likely to form in a concentration as high as12.5%. In this case, spin polarizations of only37-60%are obtained for Ti(A/B)-Co swap and Ti(B)/Al vacancy defects. However, a considerably high spin polarization around95%is detected in Co-Al swap and Ti(A) vacancy. Remarkably, our results show that all the antisite defects with negative formation energies almost retain the half-metallic character in a concentration of12.5%even though they have the possibility to form. However, induced by antisties, the Fermi levels shift to the edge of band gap with small peaks arising just above the Fermi level, which might destroy the halfmetallicity.