Theoretical Study Of Spin Polarized Transport Properties Of Several Magnetic Molecules

With the fast development of information techniques and the lasting miniaturization of electronic devices, now experimental and theoretical researchers try to discover the novel techniques and promising candidates for the new-generation of electronic devices. During the past years, many significant progresses have been achieved in molecular electronics, and various molecules have been successfully used to design the molecular wire, switch, rectifier, and transistor. Recently, molecular spintronics (the natural combination of spintronics and molecular electronics), a blossoming research area aiming at exploring how the spin transport through molecules, has attracted much research attention since it holds great promise for the next-generation of functional electronic devices with improved performance and enhanced functionality, especially in quantum computing and high-density information storage. In general, theoretical simulations are very useful and helpful to fully explain and understand the transport experimental measurements, even to design functional molecular devices. Due to the progress in enhancement of computational ability and computational methods, density functional theory (DFT) has become very important and powerful tool in molecular electronics and molecular spintronics. In this dissertation for Ph. D degree, we explore the spin-polarized transport properties of several magnetic molecules by performing extensively DFT calculations combined with the non-equilibrium Green’s Function (NEGF) technique.In chapter1, we firstly introduce the basic ideas of density functional theory (DFT) and review its various functionals. Then at the end of this chaper, we briefly introduce several DFT softwares used in this dissertation.In Chapter2, we give an overview of molecular electronics by introducing the relevant experimental methods (i.e. MCBJ and STM), various functional devices and promsing candidates. Then we introduce two theoretical methods used to analyze the transport properties of molecular junctions including scattering matrix and the DFT+NEGF methods. The latter one is the popular theoretical method which have already been implemented in several softwares, and explain the research motivation of my theisis, which are briefly introduced at the end of this chapter.In chapter3, we investigate the magnetic and spin-polarized transport properties of Eu-COT organometallic sandwich molecules. Amony various candidates for molecular spintronics devices, one-dimensional organometallic sandwich molecular wires have attracted many experimental and theoretical investigations. Previous reports have suggested that these sandwiched molecular nanowires can be used to design molecular spin filters, but their spin filter efficiency strongly depends on the interface configuration. In chapter4, we focus on the electronic structures, magnetic, and transport properties of Eu-COT sandwich molecules. The one-dimensional Eu-COT sandwich molecular wire is predicted to be a ferromagnetic semiconductor. The calculated average magnetic moment per Eu atom is about7μB.The total magnetic moment of Eu-COT sandwich molecules increases linearly with the number of Eu atoms, and the linear relation TMM=7n-2holds, which agrees nicely with experimental observations. The spin transport calculations indicate that Eu-COT clusters coupled to gold electrodes are nearly perfect spin filters. The spin filter efficiency (SFE) through Eu-COT molecule is predicted to be about98.5%. In addition, this large transmission spin polarization is very robust regardless of the contact configuration of the examined molecular junctions, which is very important for the future molecular spintronics.The electronic structures and spin transport properties of the magnetic C28and Fe@C28fullerenes are presented in chapter4. The spin-resolved transmission spectra of C28molecular junctions exhibit robust transport spin polarization (TSP) characteristics, which depends on the contact configuration. At the small bias voltage, the conductance of C28is mainly determined by the spin-down electrons. The TSP behavior can be effectively tuned by the gate. According to the simulated inelastic tunneling spectra (IETS) results, we find that the IETS features can be used to explore the contact configurations of three examined C28molecular junctions. The embedded off-center Fe atom in C28cage can enhance the molecular stablility and localize the molecular magnetic moments. To simulate the STM investigating of transport properties of Fe@C28molecule, we firstly identify the most stable adsorption configuration of Fe@C28on Au(111) surface, then STM tip is modeled by single Au atom adsorbed on the hollow site of the Au(111) surface. When the STM tip does not bond with Fe@C28cage, the conductance of Fe@C28molecular junction dominated by the spin-down electrons shows obvious TST with a large SFE under the small bias voltage. When the tip moves push forward Fe@C28molecule and bonds with the cage, we find that the transmission coefficients of spin-up and spin-down channels increase simultaneously, while the corresponding SFE decreases.In chapter5, we summazied several unfinished works which contained the spin transport properties of Fe6Bz2cluster, molecular switches and spin filters which based on porphyrin molecule and preliminary study of quantum interference in molecular devices.