Investigation On The Spin Transport Properties Of Organic Nanoparticles And Spin Valve Devices

In2004, the first ferromagnet/organic semiconductor/ferromagnet spin valves have been successfully fabricated. They detected up to40%magnetoresistance, at11K. This indicates that spin can be efficiently injected from the ferromagnetic into organic semiconductor. As a result, people pay more attention to the research on organic spintronics. Light elements (such as C, H, N and O) are the main elements in organic semiconductors, which leads to the weak spin-orbital coupling and hyperfine interaction. Spin scattering, stems from weak spin-orbital coupling and hyperfine interaction, is weak in organic semiconductors, which results in relatively long spin diffusion length. This special property is good for researching the spin transport property in FM/organic systems. Besides, organic materials have other intrinsic advantages, such as, cheap, low-weight, chemically interactive, mechanically flexible, and devices are relatively easy to fabricate. All these let organic spintronics to be widly usefull.Although, spin can be effiecienty injected into organic semiconductors, we couldn’t give a certain mechanism for spin injection. The spin polarization in organic semiconductors at the FM/organic interface, allows the magnetoresistance to be300%. The emergence of the polar layer at the FM/organic interface, changes the sign of the magnetoresistance. All these indicate that the interface of FM/organic plays an important role for spin injection. We mainly focuse on the research of spin transport properties, in FM/molecule nanoparticles and FM/organic semiconductor/FM spin valve structures, to reveal the mechanism of spin injection.The work presented in this dissertation consists of the following three parts:I. Large low-field magnetoresistance in Fe3O4/molecule nanoparticles at room temperatureAcetic acid molecule-coated Fe3O4nanoparticles,450-650nm in size, have been synthesized using a chemical solvothermal reduction method. Fourier transform infrared spectroscopy measurements confirm one monolayer acetic acid molecules chemically bond to the Fe3O4nanoparticles. The low-field magnetoresistance (LFMR) of more than-10%at room temperature and-23%at140K is achieved with saturation field of less than2kOe. In comparison, the resistivity of cold-pressed bare Fe3O4nanoparticles is six orders of magnitudes smaller than that of Fe3O4/Vmolecule nanoparticles, and the LFMR ratio is one order of magnitude smaller. Our results indicate that the large LFMR in Fe3O4/molecule nanoparticles is associated with spin-polarized electrons tunnelling through molecules instead of direct nanoparticle contacts. These results suggest that magnetic oxide-molecule hybrid materials are an alternative type of materials to develop spin-based devices by a simple low-cost approach.Ⅱ. Effects of ferromagnet-molecule chemical bonding on spin injection in an Fe3O4-molecule granular systemSpin injection at the interface of Fe3O4/Vstearic acid molecule is investigated in a comparative study of Fe3O4nanoparticles chemically bonded with molecules (ChemNPs) and Fe3O4nanoparticles with physically absorbed molecules (PhyNPs). The resistance of PhyNPs is two orders of magnitude larger than that of ChemNPs, indicating that the resistance of PhyNPs is dominated by the energy barrier at the Fe3O4-molecule interface. A magnetoresistance of-12%under a field of5.8kOe at room temperature is observed in ChemNPs, in sharp contrast to the zero magnetoresistance in PhyNPs, reflecting that the chemical bonding is crucial for spin injection. We attribute this result to the induced magnetic moment in molecules by proximity effect, which is likely the origin of the spin-dependent tunnelling through molecules. In addition, the estimated relatively large spin polarization of ChemNPs suggests electronic hybridization at the interface and improved oxygen stoichiometry of the Fe3O4surface.III. Reduced spin injection efficiency in organic spin valves with the interface layer of CuPcWe have carried out a systematic study in devices with or without a thin layer of copper phthalocyanine (CuPc) at the interface of organic spin-valve devices of Lao.67Sro.33Mn03/Alq3/Co. Therefore, hole-injection efficiency can be promoted by introduction a5-nm-thick CuPc between the Lao67Sro.33Mn03/Alq3interfaces. Indeed, the resistance of devices with CuPc is much smaller than that of devices without CuPc. However, the magnetoresistance (MR) of devices with CuPc is less than0.5%at50K, which is one order of magnitude smaller than MR of devices without CuPc. These results clearly show that the emergence of CuPc can lower the interfacial barrier height and decrease the spin injection efficiency. The interface barrier has important impact on the spin injection into organic semiconductors.