First-principle Study For Ti-based Heusler Compounds

Spintronics is a topic that has been widely studied, much more attention is focused on Heulser alloys due to their potential value in this field. Especially, half-metallic Heusler alloys, which keep a±100%spin-polarization at the Fermi level, possibly have higher magnetoresis-tance effect than ordinary magnetic materials; they have similar crystal structure with the widely used binary semiconductors crystallizing in the zinc-blende structure, can be used as a source of spin-polarized carriers injected into semiconductors; the possible transformation between cubic structure and tetragonal structure lead to they might be used as sensor design materials, etc. Thus great efforts are taken in searching for new functional materials with half-metallicity. In this thesis, we study systematically the physical and chemical properties for Ti-based Heusler alloys by using density functional theory.In this thesis, using full-potential local-orbital minimum-basis code (FPLO), the Ti2Y Z (Y=Fe, Co, Ni and Z=Al, Ga, In) Heusler compounds are studied systematically. The results show that these compounds exhibit a half-metallic behavior, and their spin magnetic moments satisfy the Slater-Pauling rule:Mt-Zt-18, where Mt is the total spin magnetic moment and Zt is valence electron concentration per unit cell. There is a indirect gap existing for all com-pounds. Among these compounds, Ti2CoGa has the most stable half-metallic properties due to it has the biggest band gap and spin-flip gap. The half-metallic characters are insensitive to the lattice distortion and a fully spin polarization can be obtained within a wide range of lattice parameters, which offers a certain reference for crystal growth. The Curie temperatures of all compounds are considerable above room temperature, which is favorable in realistic ap-plications, except for Ti2FeZ (Z=Al, Ga, In). Low formation energy implies these Heusler compounds can be fabricated experimentally;The calculations of cohesive energy show that Ti2Y In (Y=Fe, Co, Ni) compounds are good candidate for shape memory alloys; while the obtained clastic constants confirm that all compounds meet the mechanical stability of cubic crystals except for Ti2FeZ (Z=Al, Ga, In). In general, the transformation between cubic and tetragonal structure may occur. We inves-tigate the possible transformation in a given volume, our results exhibit that only Ti2CoAl、Ti2CoIn and Ti2NiIn compounds have a global stable cubic structure, among them, Ti2CoAl, as a shape memory alloy, has a minimum energy difference between cubic structure and tetragonal one. In addition, using the virtual crystal approximation and the supercell approximation, we study the electronic structure and magnetic properties for Ni doping Ti2Co1-xNix;In (x=0.25,0.50,0.75), the results show the half-metallic characters of system are still retained and spin mag-netic moments meet the Slater-Pauling rule, and Ti2Co0.75Ni0.25In has the maximum value of density of states, which indicates it may be a suitable material for spin injection; the Sn dope-d Ti2Co0.75Ni0.25In1-ySny(y=0.00,0.25,0.50,0.75,1.00) are investigated based on the virtual crystal approximation. The results show that the doped system not only keep half-metallic char-acters, but their magnetic moments also comply with the Slater-Paul ing rule. The system has the most stable half-metallic nature when In is substituted for Sn up to75%. Finally, the thermody-namic properties of Ti2CoIn and Ti2NiIn compounds are calculated by using the quasiharmonic Dcbye model.The thesis is organized as follows. In Section1, the preface is offered. In Section2, the theoretical background of density functional theory is given. In Section3, the used crystal structure, stability, electronic structure, magnetic properties and Curie temperature, and the influence of main group are illustrated. In Section4, we discuss the trend of phases stability and related electronic and magnetism. In Section5, the changes of lattice constant, electronic structure and magnetic properties with doped concentration of Ni in Ti2Co1-xNixIn (x=0.25,0.50,0.75) are studied. In Section6, the thermodynamic properties of Ti2CoIn and Ti2NiIn compounds are analyzed. Finally in Section7, we give our summary and outlook.