Spin Dependent Transport Properties In Organic Magnetic Devices

Organic materials were for long time only associated with electronical insulators. In the last century, however, the idea of organic electronics arose. Organic semiconductor and molecular electronics are two main research realms which attract many interests in organic electronic field. Where organic semiconductors includeπ-conjugated polymer and small molecule, which have abundant properties in electronics, optics and magnetism, and have been extensively applied to organic light emitting diodes (OLED), organic field effect transistors (OFET) and organic photovoltaic cells (OPVC). The development of molecular electronics make people can construct better electronic device with organic materials in molecule level. By far, the experimental and theoretical researches on molecular electronics have been developed widely, and find the molecular device may serve as the functions which are similar with traditional electronics, such as conductance switch, rectifier or even memorizer.As we known, electron has two intrinsic properties, charge and spin. Traditional microelectronics technical only use the charge to process information, while the spin freedom does not play a role in practical application of electronic technology. As early as 1920s, the spin property of electron was found, while until the discovery of giant magnetic resistance (GMR) in 1980s and explanation with two spin dependent scattering and two current model, people began to realize the application value of spin, so the spintronics becomes a hotspot. The spins of electron in spintronics serve as information carrier either independently or combining with the charges. The spin state of electron has longer relaxation time and is not easy to be destroyed by the scattering of impurities and defects, also the spin is easier to control by modulating external magnetic field. People are looking forward to devising the future electronic device with higher speed, lower energy cost, multiple functionalities and higher integrations. The size of the device will enter nanometer region and become the important part of mesoscopic physics.Especially, the combination of organic electronics and spintronics gives birth to some new phenomena and new effects and form a new branch of learning "Organic electronics". Organic electronics is a subject which mainly studies some physical mechanisms or phenomena such as generation, annihilation, transfer or storage of spin in the organic material or device. It is an interdisciplinary subject, including two regions: organic material in chemistry and spintronics in physics. Discussing the application of organic material in 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. Due to weak spin-orbit coupling and hyperfine interaction, organic materials have long spin relaxation, so it is a good candidate for spin injection and transport. Different from the carriers in traditional semiconductors, the carriers in organic materials are soliton, polaron and bipolaron, which have complicate charge-spin relation, and make organic electronics have abundant properties. At present, there are three kinds of organic electronic devices in experiment and theory. The first kind is FM/OSE/FM heterojunction device. Half metal CMR material LSMO and ferromagnetic material Co is mostly adopted as the electrodes and the interlayer uses organic conjugated oligomer sexithienyl (T6) and small molecule Alq3 The focus on this kind of device is how to realize effective spin injection and transport. The second kind of device is composed of common metal electrodes and nonmagnetic materials. The experiment show organic magnetoresistance (OMAR) in weak magnetic field (about mT magnitude). The third kind of device adopts magnetic molecule as interlayer to realize spin dependent functions. Organic magnet is the combination of organic material and magnet, which has been investigated in the past decade. Up to now, several organic magnets have been synthesized, such as organic ferromagnet poly-BIPO, which substitute part of H atoms in polyethylene with magnetic side radicals to achieve magnetism. In the following, some theorist did research on the origin of magnetism in organic ferromagnet poly-BIPO. They got the property of spin density wave (SDW) as well as the effect of electron-electron interaction and boundary condition on SDW. Using organic ferromagnetic material to assemble device and doing research on its transport properties have attracted much interests of scientists and many interesting phenomena such as spin filter and spin rectification have been observed.In all, organic electronics is at the beginning, many concrete problems are in short of deep research. Our paper chooses organic magnetic device as the subject, combining the extend SSH (Su-Schrieffer-Heeger)+Heisenberg model to describe organic ferromagnet and Landauer-Buttiker formula to calculate transport, adopting Green's function and transfer matrix method to get the transmission coefficient, and investigate the spin dependent transport of the system. Fist, the effect of spin excitation on basic properties of organic magnetic material and the spin polarized transport properties of its device were studied systematically, and then the cases of different spin excited states were discussed in details. Based on the previous two currents model, we develop four currents model to investigate the effect of spin flipping scattering on electron transport properties of organic magnetic device. The detailed research and main results are given below:1. Effect of spin excited states on electron transport through organic ferromagnetic deviceBy far, the investigations on organic ferromagnet are mainly limited to ground state, in reality, many external factors such as light, magnetic field and temperature etc., may make magnetic molecule deviate from ground state. There have been some experiments reporting the transport properties of single molecule magnet in spin excited state and find some interesting phenomena, such as negative differential conductance and complete current suppression etc., which may have relation with the effect of spin excitation. This paper chooses organic ferromagnet poly-BIPO as object, and investigates the effect of spin excited states on electron transport of organic ferromagnetic device.1.1 When there were spin excitations in organic molecule, the positive negative alternative spin density wave (SDW) would be destroyed, and local SDW defect appeared around spin excitation. Study on the spin excitation energy of the system found that coupled spin excitation consumed lower energy, when the number of spin excitation get to certain number, the spin excitation energy would no longer change, and the exited region formed a steady domain and the energy existed in domain wall.1.2 At fixed voltage, the total current of the system decreased with the number of spin excitations. The spin polarized current changed a little in low spin excited states, but decreased to zero rapidly in high spin excited states.1.3 Because the size of the molecule is small in device, boundary condition should be considered. We investigated transport properties of the device when the spin excitations located in different positions of the organic magnetic molecule and found the spin polarized current had no obvious change.1.4 Temperature may make the organic magnetic molecule generate collective excitation. We used uniform random distribution to describe the spin excitation in different temperature and found low temperature had little effect on the spin polarized transport, but when the temperature got to a certain value, the spin polarization decreased rapidly.2. Effect of spin flipping scattering on electron transport through organic ferromagnetic device.2.1 The investigations on basic propterties of organic ferromagnetic molecule show that when the spin flipping effect was considered, different spin sublevel will mix together. The electron no longer lies in the spin eigenstate, but lies in a spin-mixing state. Bandwidth and gap will both have some changes with the spin flipping parameter tsf When the spin flipping parameter gets to certain value, the lattice dimerization of organic magnetic molecule will disappear.2.2 The turn on voltage of the device will become small when the spin flipping toπ-electron is considered, the system is easy to conductive. The spin polarization of the device will decrease and can not realize nearly 100% spin filter, but can keep high spin polarization within a large voltage range and the device still has spin filter function.2.3 The system with stronger Heisenberg interation can keep higher spin polarization easily when the spin flipping scattering happens. In the meanwile, the electron-phonon coupling of interlayer organic magnetic molecule will affect the spin polarized state of electron during spin flipping transport. The stronger electron-phonon coupling can make the electron keep higher spin polarization easily during the transport, which indicates that organic ferromagnetic material is an exellent candidate to realize spin filter function.