Study On Surface / Interface Band Regulation And Phase Structure Regulation Of Heusler Ferromagnetic Alloy With High Curie Temperature By Density Functional Theory

Density functional theory（DFT）, as the palmary quantum mechanical methods of processing multi-electron system, has been the most powerful tool for studing materials’ s properties such as ground state structure, et al. It could not only investigate the microscopic mechanism of materials, but also predict the novel properties of unknown materials and provide theoretical guidance for experiments. In the present paper, we investigated the surface/interface, band structure modification and phase control of half-metallic Heusler alloy with high Curie temperature based on the DFT calculation.Spintronics making use of the spin degree of freedom of the electron has attracted extensive attention due to much less energy consumption, more powerful data storage capacity and faster information processing capability. Spintronics devices such as magnetic tunnel junctions（MTJs） based on tunneling magnetoresistance（TMR） effect, spin valve based on giant magnetoresistance（GMR） effect and so on play a crucial role in ultra-speedread-out head of hard disk drivers（HDD）, magnetic random access memories（MRAM） and magnetic sensors. High TMR value is extremely essential for high-performance MTJs, and it severely depends on the spin polarization of electronic materials. The reaserches based on the DFT calculation indicate that a lot of Heusler alloys have 100% spin polarization, and owing to high Curie temperature and their structural similarity to the generally used semiconductors such as Ga As and MgO, Heusler alloys have great potential in realistic applications. However, the perfect spin polarization predicted by the DFT is difficult to measure in some experiments for Heusler alloys. Although relatively high TMR values have been achieved in works on MTJs employed Heusler alloy as electrode material, they are still below expectations. In addition, tunnel magnetoresistance decreases rapidly with increasing temperature, showing severe temperature dependence. In order to reveal the reasons for reduced spin polarization in Heusler alloy and decreased TMR values in MTJs, and put forward a feasible solution so as to promote the development of spintronics, by using DFT calculation, we studied the surface/interface band structure modification and phase control of half-metallic Heusler alloy with high curie temperature. Firstly, we investigated the influence of atomic disorder and off-stoichiometric on the electronic and magnetic properties of Heusler bulks. Moreover, we further discussed the spin polarization, atomic relaxation and magnetism of Heusler surface. Based on the above works, we constracted heterojunctions constituted by the Heusler alloy and semiconductor, and studied interface porperties including interfacial atom interaction, magnetism, spin polarization and interface atom disorders.In details, the main results of our studies are summarized as follows:1. Based on the DFT calculation, we studied bulk properties of Heusler alloys. Firstly, the electronic structure, magntism and half metallicity of LiMgPbSn-type Heusler alloy CoFeMnSi are studied. Our calculation indicated that CoFeMnSi has space group of m F 34, the calculated lattice constant and magnetic moment are 5.601? and 4μB respectively. In addition, the half-metallic gap comes from d-orbital interhybridization among Co, Fe and Mn atoms, the magnetic moment satisfied with Slater-Pauling rule, and it maintains half-metallic nature with the increase of lattice constant from 5.3 ? to 5.65?. Owing to the fact that the spin polarization of Heusler alloy is strongly dependent on the atomic arrangement, we further examined the effect of atom diorder on CoFeMnSi. Among 12 kinds of antisite and 6 kinds of swap disorders, Co（Fe）-, Mn（Fe）-, Si（Mn）-antisite and Co-Fe swap disorders are most favorable owing to their lowest formation energies. The investigations on electronic structure show that the energy gap of disordered structures suffers contraction owing to the disorder effect. Besides, the half-metallic character is maintained in all structures with antisite and swap disorders except for those with Co（Mn）-, Co（Si）-antisite and Co-Mn, Co–Si swap disorders. These results indicate that the half-metallicity of CoFeMnSi is quite robust against the atom disorder, and hence it is predicted to be an excellent candidate for the applications of spintronics.Owing to the fact that increaseing Mn contant could obviously improve the TMR value of the MTJs based on Co₂ Mn Si, and inspired by this, we investigated the influence of Mn content on spin polarization of off-stoichiometric Cu2 MnAl-type Heusler alloy Co₂ MnAl by ultilizing DFT calculation. Our analysis indicated that Co₂ MnAl has space group of m Fm3, and the Mn-poor structure most likely results from Co Mn antisite or Al Mn antisite due to their lower formation energy than the Mn-void structure. Besides, the Al Mn antisite has harmless affect on half-metallicity while the drop of spin polarization is caused by Co Mn antisite, and impurity Co atom should take the primary responsibility. The total magnetic moments of Mn Co-type Mn-rich structure with different Mn content are in good agreement with the SP rule. With increasing of Mn content, the spin polarization of Mn-rich structure increases from 75% to 100%. When Mn content rises up to α =1.875, the corresponding compound Co1.125Mn¹.875 Al owns the perfect spin polarization and stable half-metallicity. Hence, a large TMR value could be obtained by using Mn-rich Co₂ MnAl electrode.2 Based on the DFT calculation,the（1 0 0） surface of Hg₂ CuTi-type Heusler alloy Ti₂CoAl is emphasized. Our calculation indicates that Ti₂CoAl has space grouop of m F 34, and surface atoms undergo different atomic relaxation in different atomic terminations. The magnetic moments of the most surface atoms are enhanced due to the reason that periodic crystal field is destroyed at the surface. Moreover, our calculation revealed that some peaks mainly characterized by the d-orbital emerge in the spin down gap of surface atoms and completely destroyed the half-metallicty. Such peaks decline in the subsurface and even disappear in the next subsurface. It can be deduced that these polarized peaks are derived from surface states. Therefore, the appearance of surface states is the reason for decreased surface polarization.3. Based on the studies above, we constructed the heterojunctions constituted by LiMgPbSn-type Heusler alloy CoFeMnSi and semiconductor Ga As. Four top-type and four bridge-type structures were built by connecting termination of CoFeMnSi layers to the top of an As-terminated Ga As layer and a bridge site between interface As atoms, respectively. It can be seen from phase diagram that the interface containing pure Si atom is stable in both top-type and bridge-type structure, the Co Fe-terminated interface is more stable in the bridge-type structure than that in the top-type structure, and a favored Mn Mn- or MnSi^-terminated interface will appear in the top-typestructure under Fe-rich conditions. Further analysison DOS and PDOS indicates that owing to interface effect, the half-metallicity of the Co Fe-, MnSi^-, and Si Si-terminated interfaces arecompletely destroyed. However, the Mn Mn-terminated interface in the top-type structure preserves perfect 100% spin polarization, indicating that the CoFeMnSi-Mn Mn/Ga As/CoFeMnSi-Mn Mn-type MTJs could obtain high TMR values.More recently, Hg₂ CuTi-type Heusler alloy Mn₂CoAl has been demonstrated to be spin gapless semiconductor（SGS）, and not only the electrons but also the holes can be completely spin polarized. We therefore studied the heterojunction composed by Mn₂CoAl and Ga As, and simulated six top-type and six bridge-type structures. Among various interfaces, top-type interface containing pure Mn atom is more stable and easier to be formed. According to analysis on interface atom relaxation, interface Mn and As atoms could form strong Mn-As bond, while interface Co atom inclines to shrink inward and easy bonding with subinterface atom. Different with CoFeMnSi/Ga As and Co₂ Mn Si/Ga As, pure Mn interface in Mn₂CoAl/Ga As did not improve the interface electronic structure. However, top-type interface containing Mn and Al atoms preserves the highest interface spin polarization of 80%, and it deserves further exploration in the future.In addition, the emergence of atomic disorder is unavoidable during the growth of an alloy by employing the present thin-film fabrication technique. We further discussed the influence of interface atom disorder on atom interaction, magnetism and electronic structure of Mn₂CoAl/Ga As heterostructure. Among thirteen interface swap disorders and sixteen interface antisite disorders, our calculation revealed that the interface Mn has a higher probability of being replaced by an Al atom, besides, an As atom from a Ga As slab easily diffuses into a Mn₂CoAl slab and occupies the position of the interface Mn. Mn¹-Al³, Mn₂-Co₂ swap and Al³ï¼ˆMn） antisite disorders elevate the ISP to 100%, and ISPof Mn₂（Co） antisite disorder also reach up to 94%, indicating that these four disorders have positive influence on interface electronic structure. While Mn¹ï¼ˆAl） and Mn¹ï¼ˆAs） antisite disorders together with interface state may be main causes of the low TMR ratio of Mn₂CoAl/GaAs heterostructures.