Spin Polarization In Ferromagnetic/organic System At A Schottky Contact

In the past few decades, spintronics as a very potential research area of condensed physics has attracted a lot of interests. Differenting from the classical electronics, spintronics involves both the electronic and spin characters of an electron. Fundamental studies of spintronics include investigations of spin injection, transport and detection in electronic materials, as well as spin manipulation. Its goal is to understand the interaction between the spin of electrons and crystalline environments and to make useful devices. Spintronics device includes the conformations of magnetic metals or magnetic semiconductors with insulator, semiconductor, conductor, or superconductor, and the mixture of them, also includes molecule or nanometer device. Several model devices have been schemed out. For example, Datta and Das designed the first spin transistor. However, the low efficiency of spin injection into semiconductor blocks the practical applications of such devices. Therefore, more attentions should be paid to the influence of interfacial effects on the spin injection and spin evolution during the transport process.Compared with conventional semiconductors, soft organic semiconductors (OSCs) have an opportunity to form a good interface with ferromagnetic metal (FM) or half-metal contacts, reducing the probability of spin scattering at the interface. The spin relaxation time is much longer than that of conventional semiconductors due to the weak spin-orbital and hyperfine interactions. Different from the traditional semiconductors, in which the carriers are electrons or holes, the carriers in OSCs are some"quasi-particles"such as polarons, bipolarons, and solitons. They have more complex spin-charge relation, which will result in more abundant properties of an organic spin device. The electric, magnetic and optical properties are unique in both the organic molecules and conjugated polymers. 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.Organic spintronics is a subject that mainly studies some physics mechanisms or phenomenon such as the creation, annihilate, transfer or storage of spin in the organic materials or device. It is an interdisciplinary subject, including two regions: organic materials in chemistry and spintronics in physics. 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.In 2002, Dediu's group firstly report the spin injection and transport in organic material, they use semi-metal material LaxSr1-xMnO3 (LSMO) as the source of electrons and sexithenyl (T6) as the organic layer. It has been found that spin was injected into the organic layer, and the current is spin polarized. In recent years, a lot of experiments have confirmed the spin injection and transport in organic materials, for example, in 2004, Xiong et al. have also observed spin injection and transport in a LSMO/Alq3/Co organic spin valve. The measured magnetoresistance can be as high as 40% at low temperature.In 2008, Dediu's group report the spin injection and transport in LSMO/Alq3/Al2O3/Co and report on efficient spin polarized injection and transport in Alq3 organic semiconductor. They employ vertical spin valve devices with a direct interface between the bottom manganite electrode and Alq3, while the top-electrode geometry consists of an insulating tunnel barrier placed between the"soft"organic semiconductor and the top Co electrode. In 2009, Zhang et al. reported the spin injection and transport in 8-hydroxyquinoline-aluminum (Alq3)/Al2O3/Co and discuss the effect of Al2O3 buffer layer.Quantum theory used by Xie et al. and spin diffusion theory used by Smith, and 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 experiments. These existing theories do not address critical issues of the real structures used in experiments which typically consist of a Schottky contact with band bending in a depletion region. 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 transition between polarons and bipolarons on the spin injection and transport; effect of spin-related interfacial resistances and effect of Schottky barrier on spin injection into organic semiconductors, and so on. In this paper, based on the classic drift-diffusion theory, we will investigate the above questions. The detailed research contents include: 1. Effect of polaron and bipolaron on the spin polarized injection and transportDue to strong electron-lattice interaction in OSCs, injected electrons can induce the distortion of the lattice and result in charged self-trapped states called spin polarons or spinless bipolarons. A polaron has 1/2 spin, while a bipolaron has no spin.It should be pointed out that, due to the effect of temperature, pressure or external electric field, bipolarons and polarons in OSCs are not necessarily independent excitations. Two spin polarons may annihilate into one spinless bipolaron, while a bipolaron may also dissociate into two spin polarons. Neglecting that transition, people have understood the preliminary effect of polarons and bipolarons. However, effect of the transition between polarons and bipolarons on the spin injection and transport is indistinct. we assume that the variational polaron proportion decreases exponentially into the organic semiconductor in the structure of ferromagnetic/organic semiconductor. Effects of the decreasing rate of polarons on the current spin polarization are discussed. It is also found that the conductivity matching is an important factor for efficient spin polarized injection into organic semiconductors.2. Effect of spin-related interfacial resistances on the spin polarized injection and transportCurrent spin polarization in ferromagnetic/organic system is studied theoretically from the spin diffusion theory and Ohm's law, and the effects of the spin-related interfacial resistances on the spin-polarized injection are discussed. We show that the current spin polarization can be higher than the spin polarization of the ferromagnetic contact at the interface by modulating the spin-related interfacial resistances. The proposed amplification scheme provides an efficient way to generate a highly spin-polarized current in organic semiconductors because of the easily fabrication of ordered spin-related tunnel barriers at contact structure.3. Effect of Schottky barrier on spin injection into organic semiconductorsTheoretically we study the current spin polarization at ferromagnetic/organic semiconductor Schottky contact. The effects of potential barrier height, the special carriers in organic semiconductor layer and the carrier's mobility, doping concentration near the interface were discussed. From the calculation, it was found that the high mobility of the carriers in organic semiconductors will be helpful for the spin injection. We also found that a significant depletion region at Schottky contact is highly undesiable for spin injection. For efficient spin injection, the depletion region near the interface should be heavily doped and the effective barrier height should be restricted on certain scope.