Theoretical Study On Spin Injection And Spin Transport In Ferromagnetic/organic Semiconductor/ferromagnetic System

Spintronics have been recognized as a new interdisciplinary field of science since 1994 and attracted much attention in industry, it also have been illustrated a good prospect of application and extension. The emergence of spintronics is one of the top ten major events in the field of physics in 1999 and become the hot topic among the filed of condensed matter physics, scientific information and so on. Particular important is that the spintroncis will be the basis in the information industrial and plays an important role for the development of the electronics. The basic elements of the information industry as semiconductor devices, which based on the electronic (or hole) charge characteristics to transmit information. Specially speaking, owing to the confusion of spin magnetic moment caused by the collision between the electrons during the process of the transport,only to consider its charge for the macroscopical characters in the transport. An emerging technology dubbed spintronics, not only use the charge of electrons, but also aims to harness the spin degree of freedom of carriers in lieu of charge to realize these core information processing and storage functionalities. Along with the rapid development of VLSI (Very Large Scale Integrated Circuit) and processing technology, the electronic devices are geared to minimization, in nanometer scales, the property of the spin has a better choice than the charge, such as the multipurpose integrating, consume energy low and high stability. So the spintronics will gradually replace the microelectronics industry to become the mainstream. Generally speaking, spintronics is a nascent form of electronics which uses the spin of electrons to encode and process data, rather than using electric charge, based on the spin polarization and transport properties, then design and develop new spin devices which considered being a rising interdisciplinary field. It is chiefly studies the electron polarization, spin dependent scattering spin relaxation, other related properties and applications.Most of spintronics research is focused on use of inorganic semiconductor. Compared to the inorganic semiconductor, organic semiconductor has its own character; soft organic semiconductors (OSCs) have an opportunity to form a good interface with ferromagnetic metal (FM) or half-metal contacts, so reduce the interface barrier. Owing to the weak hyperfine interaction and extremely weak spin-orbit interaction, the electrion spin diffusion lengths especially long in the organic semiconductor. In addition, the OSCs have strong electron-lattice interactions; injected electrons can induce the disortion of the lattice and result in some charged self-trapped states called polaron or bipolaron excitations. Moreover, they have unique spin-charge relation which helpful to obtain the complex spin injection and transport properties. It has important guide to the spintronics and living system in the function and application. Up to now, the electric properties of quasi-one dimensional conducting polymers have been well understood. However, there is a lack of full understanding about the spin property of OSCs, since it is a new field called organic spintronics.The current study of OSCs is focused on organic semiconductor material, due to the effect of temperature, pressure, electric and magnetic fields, the carriers in the organic semiconductor have different property compared to the traditional semiconductor, so we called it as interdisciplinary field between spintronics and organic semiconductor. In addition, OSCs have abundant electronic, magnetic and optics characteristic and are much more easily synthesized at low temperature with little toxic waste. Based on the electronic and optical properties of organic semiconductors, light-emitting diodes for flat-screen TVs, cell phone displays, billboards and computer display screens have been fabricated. Such as metal atoms, a class of organic compounds containing the nutrition oxition radical–CN, the charge transfer complex crystals and so on, have been used in many spintronic fields. It provides advantaged base of the magnetic organic semiconductor to research its spin property. OSCs is a new and promising research field where organic materials are applied to mediate or control a spin-polarized signal, and attracted by more and more people to focus on it. The research directions include: organic spin-valve device, Organic Luminescent Emitting Diodes device (OLED), organic magnetresistance (OMAR), Discussing the application of organic material in the spintronics apparently has significant value of basic research and potential foreground of applications. Therefore, it is an aspect that a lot of international research groups are interested in.OSCs as a new branch of the field have attracted enough attention. In 2002, Dediu's group firstly report the spin injection and transport in organic material, it has been found that spin injected in the La_0.7Sr_0.3MnO₃/T₆/ La_0.7Sr_0.3MnO₃ (LSMO/T₆/LSMO)system, and the current is spin polarized. In 2004, Xiong et al have also observed spin injection and transport in a LSMO/Alq₃/Co organic spin valve device. The measured magnetoresistance can be as high as 40% at low temperature and no magnetoresistance at the room temperature. In 2006, Majumdar et al have observed the phenomena in the LSMO/polymer/Co which the ferromagnet electrode is LSMO; the influence of the interface barrier is also discussed with emphasis.Quantum theory used by Xie et al. and spin diffusion theory used by Smith, and Z.G.Yu et al. have been demonstrated successfully for studying the spin polarized injection and transport in OSCs. The quantum theory can describe the microcosmic mechanism of spin transport, while the macroscopical theory can obtain some results comparing to the experiment. In recent years, although much effort has been devoted to the study of organic spintronics, many questions are still under debate or indistinct. For example, effect of the electric and magnetic fields for the ferromagnet/organic semiconductor system on the spin injection and transport, the system of ferromagnet/organic semiconductor/ferromagnet is also contained; the polaron ratio on which factors; effect of interface resistance and so on. In this paper, based on the classic spin drift-diffusion theory, we will investigate the above questions.The detailed research contents include:1. Effect of electic field on the spin polarized injection and transport in the ferromagnet/organic semiconductor system.Dediu et al. went into the subject of spin injection in the organic material and observed the spin diffusion length at room temperature is 200nm. The spin diffusion length is related not only with the carrier concentration, but also closely with the temperature, pressure and external electric field. Here we only discuss the effect of the electric field on the length of spin diffusion. The spin diffusion length will change a lot under the influence of an electric field; we redefined the spin diffusion length based on the theory calculation, then obtained the expression of the current spin polarization which has meaning to our further research on the spin injection to the organic semiconductor. Based on the spin drift-diffusion theory and Ohm's law, we theoretically study the effect for electric field on the current spin polarization for the ferromagnet/organic semiconductor system. Considering the effect of the special carriers and electric field, we obtained the current spin polarization on the interface. The result shows that the current spin polarization can be improved a lot by the high electric field. And at the same time, the present study provides further insights into the transition between polarons and bipolarons, spin-related interface resistance. 2. Effect of electric and magnetic fields on the spin polarized injection and transportWe find that the electric and magnetic fields paly an important role in the spin polarization and transport considering the effect of the spin driffusion length. Based on the spin drift-diffusion theory and Ohm's law, we theoretically study the effect of electric and magnetic fields on the current spin polarization for the ferromagnet/organic semiconductor system, the special carriers are also considered. The results indicated that the current spin polarization can be accelerated by change the electrid and magnetic fields. At the same time, we can also obtain the apparent current spin polarization by particularly regard to the factor of the interface resistance and special carriers.3. Effect of electric and magnetic fields on the magnetoresistanceWe have described the effect of the electric and magnetic fields on the current spin polarization for the two layers system. The structure of the sandwich is the best model for the device of spin related. Furthermore, the system is the best model for the experiment. Based on the spin drift-diffusion theory and Ohm's law, we theoretically study the effect of electric and magnetic fields on the magnetresistance and current spin polarization for the ferromagnet/organic semiconductor/ferromagnet system. A calculation method and calculation results show that the the current spin polarization can be improved orders of magnitude by electric and magnetic fields with changed field intensity on the premise of the special carriers in the organic semiconductor. Moreover, the effect of the electric and magnetic fields on the magnetoresistance is also concluded.