Density Functional Theory And Non - Equilibrium Green 's Function Method For The Study Of Magnetoresistive Interface Characteristics And Electron Spin Polarization Transport Of Co - Based Heusler Alloy

On the basis of Density Functional Theory (DFT) and owing to its advantages and characteristics, first-principles calculation holds higher reliability and computational efficiency as well as high precision, which makes it to be a powerful tool used for studying movement law of the microscopic particle and predicting macroscopic physical properties. With the help of this mathematical tool it can not only reduce the research costs but also help us understanding some microcosmic mechanism. Non-Equilibrium Green’s Function (NEGF), which describes the propagation behavior of electron, was regarded as an important method in dealing with the electron scattering and transporting problems under non-equilibrium circumstances, since NEGF can connect electron scattering with propagation and therefore the transport property can be directly handled without solving wave function. By combining the two mathematical tools (i.e. DFT and NEGF), we can carry out some theoretical research on the giant magneto resistance phenomenon in Physics.Spintronics devices, which utilize the spin property of electron, such as, a variety of giant magneto resistance and tunneling magneto resistance sensors, giant magneto resistance isolator, giant magneto resistance and tunneling magneto resistance hard disk reader, magneto resistance random access memory and spin transistor, have attracted a lot of attention due to their advantages of lower power consumption and more powerful storage capabilities by comparison with traditional microelectronics devices which use the nature of electric charge of electrons. But the electronic spin polarized transport properties of these devices strongly rely on the spin polarization of the electrode material and the hetero-interfacial characteristics. As an important member of the half-metal family and possessing many advantages such as high spin polarization and high Curie temperature, Co-based Heusler alloys are regarded as highly promising electrode materials in the field of spintronics. However, the actual performance is not always satisfying, for which is far less than ideal one. To make clear the situation that why a gap exists between the actual performance and expected one, we have carried out a detailed investigation on the interface characteristics and spin polarized transport of Co-based Heusler alloy magneto resistance junctions by means of Density Functional Theory and non-equilibrium Green’s function method, to find out the source of the problem and provide a reliable solution for developing high performance magneto resistance materials. To be specific, the thesis is divided into several parts as follows:Firstly, as an important part of magneto resistance device, the electrode material should be structural and thermodynamic stable. Especially in a high annealing environment, whether keeping the highly ordered structure or not, it is crucial in performing the quality of half-metallicity. In this thesis, we have chosen four Co-based Heusler alloys C02YZ (Y=Sc, Cr and Z=Al, Ga) (whose representation of space group is Fm-3m) as a representative of the compounds. First-principles calculations based on the Density Functional Theory and quasi harmonic Debye model are performed to study the structural, elastic, electronic and thermodynamic properties of the Co2YZ Heusler alloys. It is found that the spin polarization of Co2CrZ at Fermi level raises considerably, and Co2ScZ keeps the non-magnetic feature when the external pressure increases. Although outstanding performance of half-metallicity appears to Co2CrZ, the dynamical stability of Co2ScZ is better than Co2CrZ resulting from the phonon dispersion analysis. The results show that neither Co2ScZ nor Co2CrZ is suitable for the use of electrode materials.Secondly, on the basis of confirming both phase stability and typical half-metallicity, we choose Co2MnAl (CMA) as a new electrode material to explore the potential of the corresponding device (in which Ag as the spacer material). Because the formation energy of B2 system is smaller than L21 system and the spin polarization of the former is also larger than the latter, the thesis focuses on the situation of B2 disordered system. Using non-equilibrium Green’s function method, we have analyzed the magneto resistance (MR) ratio of the two different systems and demonstrated that B2-based CMA/Ag/CMA (both representations of CMA and Ag space group are Fm-3m) magnetic trilayers evidently obtain a better spin polarized transport property and are promising giant magneto resistance junctions with high performance.Thirdly, based on Density Functional Theory and non-equilibrium Green’s function method, we focus on another alloy Co2MnSi (CMS) and its sandwiched device. With a 100% electron spin polarized characteristic, it was expected to achieve as high as 100% MR ratio in CMS based CPP-GMR devices. However, the actual value always falls far short of that. We built a model of CMS/Ag/CMS (both representations of CMS and Ag space group are Fm-3m) ultrathin trilayer to simulate the device and found that the most likely interface is formed by connecting MnSi-termination to the bridge site between two Ag atoms. As annealed at high temperature, the formation of interface DO3 disorder is most energetically favorable (i.e., Mn atoms of LI swap with Co atoms of L2). The spin polarization is reduced by both the interface itself and interface disorder due to the interface state occurs in the minority-spin gap. As a result, the MR ratio has a sharp drop based on the estimation of a simplified modeling.Lastly, using Density Functional Theory calculations and relying on the non-equilibrium Green’s function method, we have summed up the relationship between band structure matching and the ballistic transport property of full-Heusler alloys based Co2YZ/Al/Co2YZ trilayers (Y=Sc, Ti, V, Cr, Mn, Fe and Z=Al, Si, Ge) (both representations of Co2YZ and Al space group are Fm-3m). It is found that the transport efficiency was determined primarily by three factors related to the band structures, i.e., the shape of the energy band crossing the Fermi level (Ef) and the distance d=｜P0-Qi｜of the two intersection points of Co2YZ alloy and Al at Ef as well as the absolute maximum of the energy lying in the Ef-crossing energy band,｜Emax｜...