Study Of The Defect And Doping Of Ti₂NiAl Inverse Heusler Alloy On The Electronic, Half-Metallicity And Magnetic Properties

Half-metallic ferromagnets (HMFS) with special band structure, in which one spin channel is metallic while the other is semiconducting, leading to a complete spin polarization at the Fermi level (EF), have attracted continuously growing attention due to their potential applications in spin-dependent devices, such as spin-injection, spin-filters, tunnel junctions and giant magnetoresistance spin-valve.With the development of Heusler alloys, researchers discovered that the novel material with Hg2CuTi-type structure has half-metallicity. However, there are tremendous difficulties in experimentally demonstrating the half-metallicity of these compounds due to the sensitivity of spin polarization to several factors such as structural defects, surface and interface effects, and the narrow energy separation between the Fermi level and the conduction valence band edge. Some groups have studied (001) surface, structural defects and more d-electron doping effects on the half-metallicity of the inverse Heusler alloy Ti2CoAl. The results shown that the half-metallicity of Ti2CoAl alloy is destroyed by surface states in five terminations and structural defects, but the doping of Nb atom with appropriate concentrations can achieve a robust half-metallicity.Recently, theoretical and experimental studies of the disorder effects on the inverse Heusler alloy are still scarce. Here the Ti2NiAl inverse Heusler alloy is selected for this study due to its robust half-metallicity against interfering effects such as increasing temperature among the similar type of inverse Heusler alloys. In this work, we firstly introduce the background, and then describe the theoretical basis and calculation methodology. Finally, based on the density functional theory (DFT), we research the defect effects and doping effects on the magnetism and half-metallicity of the inverse Heusler alloy Ti2NiAl.Our results summarizes as the following:1. We have studied the effect of defects on electronic and magnetic properties of the inverse Heusler alloy Ti2NiAl. The formation energy calculations show that the and AINi antisites as well as Ni-Al/Ti(B) and Al-Ti(A) swaps have positive values of formation energy, which may be ruled out by annealing during the Ti2NiAl growth, while the rest of defects, namely Ni/AlTiA), Ni/AlTi(B), NiAi antisites as well as Ni-Ti(A) and Al-Ti(B) swaps are found to have negative values of formation energy, which indicates that these types of defects are likely to be formed spontaneously during the growth. Among them, the NiTi(A) antisite is found to be the most probable defect especially. Besides, we deduce from relative binding energy of the swap with respect to their antisites that the Ni atom prefers atomic antisite to site swap, while the Al atom prefers the site swap to atomic antisite. The band structure calculations exhibit that the spin polarization of Ti2NiAl are considerably reduced by the Ni/AlTi(B) antisite as well as Ni-Ti(A) and Al-Ti(B) swaps, while a very high spin polarization is obtained for Ni/AlTi(A) antisite and only the NiAi antisite retains the half-metallicity with a perfect spin polarizations.2. We study the effects of mono-doping and co-doping on Ti2NiAl inverse Heusler alloy. For the Si mono-doped systems, with the increase of doping concentration, the band gap is widened gradually, but the Fermi level gradually moves to the low-energy region, eventually destroying the half-metallicity. However, all of the Co mono-doped systems retain the half-metallicity, but the band gap is gradually narrowed with increasing Co content. For the co-doping systems, the half-metallicity is reserved and the band gap is widened, leading to a robust half-metallicity. Among the (Si0.5+Co0.5) co-doping compoud, the half-metallicity is most stable due to the Fermi level just locating at the middle of the energy gap. Therefore, we can infer that the co-doping at a certain concentration is a more efficient way to adjust the Fermi level in the positions of the band gap.