Half-metallicity And Magnetism Of Ti₂Ni_1-xCo_xAl_1-ySi_y Inverse Heusler Alloys From First-principles Calculations

Spintronics devices develop rapidly due to their more powerful data storage,faster information processing,and lower energy consumption than traditional microelectronics devices.The materials which application in spintronics need to possess high electronic spin polarizability,therefore the half-metallic ferromagnetic materials with 100% spin polarization in theory have a great application prospect in this field.At present,a lot of half-metallic ferromagnetic materials have been theoretically predicted or confirmed by experiments.Half-metallic Heusler alloys attract much attention due to their high Curie temperature and small lattice mismatch rate with semiconductor.However,a large number of experiments show that Heusler alloys often under the influence of preparation environment as temperature and ingredient proportion in the process of actual preparation,are likely to occur atomic disorder and structural defects,resulting in their band gap narrow,Fermi level moves slightly,etc.and making us not detect theoretically predicted 100% spin polarization in the actual preparation of Heusler alloys.In addition,it has been shown theoretically that doping the right proportion of impurity element can change Heusler alloy energy gap width and the position of the Fermi surface,and thus the doping is likely to be an effective way to develope half-metallic ferromagnetic materials with good performance.Since Cr2 CoGa alloy was applied to the heterojunction,half-metallic inverse Heusler alloys are increasingly favored by the researchers.The theoretical study in Ti-based inverse Heusler alloys indicated that Ti2 NiAl and Ti2 CoSi alloys were predicted to be half metallic ferromagnets,and the Fermi level of Ti2 NiAl is close to the bottom of the minority-spin band gap while that of Ti2 CoSi is approach the top of the gap.Thus their half metallicities both are vulnerable to atomic disorder and structure defects etc.,resulting in the decrease of spin polarization.Therefore in this work,within the framework of density functional theory(DFT),we study the Co+Si co-doping effect on the electronic and magnetic properties by substituting the Ni with Co as well as Al with Si simultaneously in inverse Heusler alloy Ti2 NiAl.The conclusions can be summarized as follows:1.For Co/Si mono-doped and Co+Si co-doped Ti2 NiAl inverse Heusler alloys,the optimized lattice constants all conform to the Vegard law as the increase of the impurity concentration,and the magnetic moments decrease with the increase of Co concentration while increase with the increased concentration of Si,obeying the Slater-Pauling rule when the half-metallicity is retained.2.The defect formation energy calculations indicate that the defect formation energies of the co-doped systems are lower than those of the mono-doped systems due to the charge compensation effects,thus the Co+Si co-doping is more favorable in energy than the Co/Si mono-doping.3.For the Co and Si mono-doped systems,the Co mono-doping retains the minorityspin bandgap unchanged,although the Fermi level moves towards high energy region,and the Si mono-doping leads to the minority-spin bandgap narrowing and even the loss of half-metallicity at the high concentration,while for the Co+Si co-doped systems,the minority-spin gaps of majority co-doped compounds get widened and obviously show more stable half-metallicity.4.In particular,the minority-spin band gap of the co-doped compounds Ti2Ni0.5Co0.5Al0.5Si0.5,Ti2Ni0.25Co0.75Al0.5Si0.5 and Ti2NiCoAl0.25Si0.75 are widened distinctly and their Fermi level are adjusted to the middle of the minority-spin gap,indicating that they possess robust half-metallicity and thus they are promising candidates for spintronics applications.