Effect Of Strain And Atomic Disorder On The Half-Metallic And Spin Gapless Semiconducting Properties Of Ti₂CoSi Compound:a First-Principle Study

In the last two decades, development of the spintronic device gained special attention for scientists in the field of condensed matter physics. A lot of materials which have a variety of physical properties have been predicted by the first-principles calculations. Ti-based full-Heusler compounds have been widely studied in recently years. Ti2CoSi compounds with Hg2CuTi-type structure has been predicted to be the half-metallic material and the spin gapless semiconductor, i.e., the compound has a zero-gap in the majority-spin band at the Fermi level, whereas it has a direct gap in minority-spin band.In the thesis, we have investigated the effects of strains and atomic disorder on the electronic structures, magnetic property, the half metallic and spin gapless semiconducting properties of full-HeusIer Ti2CoSi compound by using the full-potential linearized-augmented plane-wave (FPLAPW) method within the generalized gradient approximation (GGA). The results are as follows:1. The optimized lattice constant of full-Heusler Ti2CoSi compound with Hg2CuTi-type structure is calculated to be6.023A. The density of state and the band structure show that the majority-spin band has a zero-gap at the Fermi level, while the Fermi level located on the band gap in the minority-spin band. The total magnetic moment is3.000μB following the Slater-Pauling rule ofMt=Zt,-18. The atomic magnetic moments are1.347μB for Ti(A),0.677μB for Ti(B),0.388μB for Co and 0.012μB for Si, respectively.2. The investigations on the effect of uniform strain and the tetragonal distortion in the [001] direction show that:(1) The half-metallic property of the compound is half-metallic ferromagnet in the range of lattice constants from5.821A to6.826A, whereas the compound maintains the spin gapless semiconducting property in the range lattice constants from5.924A to6.027A.(2) The half-metallic property of the Ti2CoSi compound could be maintained for a tetragonal distortion in the range from-3.5%to2.8%, while the tetragonal distortions with as little as±0.1%distortion will destroy the spin gapless semiconducting property.3. We considered five types of atomic disorders that are the Co-Si, Ti(B)-Co, Ti(A)-Si, Ti(A)-Co and Ti(B)-Si disorders. The result show that the half-metallic property and spin gapless semiconducting property are destroyed by the atomic disorders, and the spin-polarization and magnetic moments change significantly. The spin-polarizations of Co-Si and Ti(A)-Co disordered are-0.1%and52%, respectively. The total magnetic moments of the atomic disordered systems are less than the total magnetic moment of the ordered structure. From the total energies, we found that the Ti(A)-Co disordered system has a lowest total energy indicating the atomic disorder is more likely to occur in the preparing of Ti2CoSi compound with Hg2CuTi-type.