| With the increasing demand in high-density storage,new-type electronic devices,and quantum computing,spintronic devices,which have better performance than con-ventional semiconductor devices,attract more and more attention and research.Molec-ular spintronic devices based on organic magnetic molecules have huge potential of applications for their advantages of long spin relaxation times,high stability,high ef-ficiency,and small size.On one hand,how to effectively tune the magnetic properties of organic molecules is one of the key issues in molecular spintronics.On the other hand,how to improve the spin polarization of electrons on the surface of magnetic elec-trode materials is also one of the key premise for the successful design of spintronic devices.This dissertation is aimed to conduct research on the above two aspects under the framework of first-principles calculation method based on density functional theory.In this dissertation the atomic and molecular adsorption on organometallic phthalo-cyanine MnPc and magnetic pole materials Fe3O4(100)surface were studied for its effect on the geometry,electronic structure and magnetic structure.The reasons of surface magnetic changes of MnPc molecules and Fe3O4(100)were analyzed and well explained.An effective way of reversibly controlling the spin states in organic MnPc molecules in a variety of environments was proposed,and high spin polarization on Fe3O4(100)surface was obtained through surface modification of molecular adsorp-tion.The main content of the full dissertation is as follows:The first chapter reviews the development of spintronics.The application values in spintronic devices were illustrated for organic magnetic molecules and Fe3O4 mate-rials with semi-metallic properties.Following by the overview of research advances in recent years on organometallic phthalocyanines and porphyrins molecules,and surface modification effects on FeeO4 films in geometry,electronic structures,and magnetic structures through both experimental and theoretical,the research purpose,main con-tent and significance of this dissertation is proposed.The second chapter introduced a brief introduction to the first principles calcula-tions,mainly covering the establishment of Density Functional Theory,the exchange related functionals and the DFT+X method for handling of van der Waals force and electronic correlations.At the end,the software which is commonly used in the first principles calculations is introduced.The third chapter studied the adsorption of NO on a MnPc molecule through five different functionals.Previous literature had reported that the adsorption of NO molecules can quench the magnetic moments of MnPc molecules.When the generalized gradient functional(GGA)was used to calculate,the same conclusions with the literature were obtained.However,by further comparing and verifying using GGA+U and three hy-brid functionals,the results were found that the GGA calculation not only caused the underestimation of the energy gap of the system and the reduction of the Mn-N bond length in the adsorption configuration,but also brought about the wrong occupation of the molecular orbitals and the misalignment of orbital energy levels,which led to erro-neous predictions of adsorption-induced molecular magnetic moment quenching.The results corrected the erroneous conclusions in the literature,and further emphasized and confirmed that the electronic correlation effect plays an important role in Pc molecular related system.The fourth chapter studied the controlling of molecular magnetic moment of MnPc molecules in three typical environments,namely free single MnPc molecule,3-layersMnPc film with weak coupling between layers,and MnPc on ferromagnetic substrate Fe(100)surface with strong adsorbate-substrate interaction.It was found that the ad-sorption of N atoms in all the three cases pulls the Mn atoms in the MnPc molecules away from the Pc plane significantly.The strong hybridization between the N-Mn atoms leads to the opening of the energy gap and the quenching,of the magnetic moment in the MnPc molecules.Notably,this quenching is independent of the functionals in the calcu-lations.Further analysis of the density of states and differential charge density revealed that the quenching of the magnetic moments of MnPc molecules is due to the process of electron donation from the Mn atom to the adsorbed N atom and the process of elec-tron redistribution between different spin configurations.The research in this chapter proposed an effective method for reversible switching the spin state of organic MnPc molecules that is suitable for various environments and independent of the substrate.The fifth chapter studied the effect of NO molecular modification on the surface energy band and electron spin polarization of magnetic electrode material Fe3O4(100).NO molecule was found to be tilted adsorbed on Fe(B)site through its N terminal.Adsorption of NO molecules on the one hand leads to the spin-up surface state energy band moving slightly to the deep level;on the other hand,part of the spin-down electrons are filled into the 2?*orbital of the NO molecule,leading to an increasing in the density of states and magnetic moments of O atoms near Fermi level.The spin polarization of Fe3O4 surface is greatly increased due to the adsorption of NO molecules.The research in this chapter provided an effective way to enhance the surface spin polarization of spintronic devices based on Fe304 materials,which was beneficial to the improvement of spin polarization injection efficiency.The sixth chapter summarized the whole dissertation.The innovation of this thesis mainly included three aspects:(1)Through the comparative calculation and analysis of five functionals,the erro-neous conclusions of the literature on NO-induced magnetic quenching of MnPc were corrected,and the important role of electron-correlation effect in the Pc molecular re-lated system was further emphasized and confirmed.(2)Through the calculation of the atomic N adsorption on MnPc molecules in three typical environments(i.e.near free state,weakly coupled substrates,and strongly coupled substrates)and analysis of the associated mechanism,an effective way for re-versible switching the spin state of MnPc molecules which is suitable for various envi-ronments and independent of the substrate was proposed.(3)By analysing the surface geometry,energy band structure,magnetic structure and corresponding microscopic mechanism of magnetic electrode material Fe3O4 100)through NO molecules adsorption,an effective way to enhance the surface spin polar-ization of Fe3O4(100)was proposed which is easily realized in experiment. |
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