有机磁性器件自旋相关输运性质研究
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摘要
有机材料在很长一段时间内都被认为是绝缘体,直到上个世纪有机电子学逐渐兴起,发现掺杂的有机材料具有较好的导电能力,这种传统的认识才被打破。有机半导体和分子电子学是有机电子学中两个比较热门的领域,其中有机半导体材料包括高分子聚合物和小分子,它们具有丰富的电学、磁学、光学特性,已经在有机发光二极管(OLED),有机场效应管(OFET),有机光伏电池(OPVC)等方面得到了广泛应用。分子电子学的发展使人们可以在分子层次上使用有机材料构建功能性更好的电子器件。目前,分子电子学的实验和理论研究已经广泛展开,发现有机分子器件可以实现多种与传统电子学类似的功能,如分子开关、分子整流器、分子存储器等逻辑功能器件。
我们知道,电子具有两个重要的内禀属性,即电荷和自旋。传统的微电子技术只利用电荷流进行信息处理,而自旋这一自由度没有在实际应用的电子学技术中发挥应有的作用。实际上,人们早在20世纪20年代就发现了电子的自旋特性,但直到20世纪80年代发现巨磁电阻效应并用自旋相关散射和二流体模型解释之后,人们才开始认识电子自旋的应用价值,于是对电子自旋的研究成为一个热点课题。自旋电子学中电子的自旋或者是独立地或者是跟电子的电荷相特性结合地作为信息储存和传输的载体。电子的自旋态具有较长的弛豫时间,不容易被杂质或缺陷的散射破坏,而且自旋较容易通过调节外部的磁场来进行控制。人们正期待着利用电子自旋特性来设计运行速度更高、能量消耗更低、功能多、高集成的下一代微电子器件,这种器件的尺寸将进一步大大减小,进入纳米尺度范围,成为介观物理相关研究的重要组成部分。
特别地,近几年来将有机电子学和自旋电子学相结合,得到了一些令人振奋的新现象和新效应,形成了一个新的学科分支-有机自旋电子学。有机自旋电子学是研究有机功能材料及其相关器件中的自旋产生、消灭、转移与存储等物理现象和物理机制的学科,它包含化学有机材料和物理两个领域。将二者结合,探讨有机材料在自旋电子学领域的应用具有重要的基础研究价值和潜在的应用背景,这也是当前国际上许多课题组密切关注的一个研究方向。由于有机材料具有较弱的自旋-轨道耦合和超精细相互作用,载流子的自旋弛豫时间比较长,因而是实现自旋极化输运的理想候选材料。不同于传统半导体中的载流子,有机材料中载流子是孤子、极化子、双极化子等准粒子,它们具有更复杂的电荷自旋关系,使有机自旋电子学器件具有更丰富的特性。
目前,实验上和理论上研究的有机自旋电子学器件主要有三类。第一类是磁性电极／非磁性有机材料异质结型器件。实验上多采用半金属的CMR材料LSMO和铁磁性的Co作为电极,中间层通常采用共轭低聚物六噻吩(T6)和小分子Alq3等。对这类器件的关注点是如何实现有效的自旋极化注入和输运问题。第二类器件是由普通的金属电极和非磁性有机材料构成。实验上在很弱的磁场下(10mT)测得了有机磁电阻(OMAR)现象,对这种现象的解释是目前有机自旋电子学领域一个新的难题。第三类器件使用有机磁性分子作为载体来实现自旋相关功能性。有机磁性分子集有机材料和磁性材料的优点于一身,近几年来已经引起人们的广泛关注。这种材料可以实现电导和磁性的共存,并且具有轻质、低消耗的优点,是有机自旋相关器件的优质候选材料。目前,对有机磁性分子器件的研究工作已经初步展开,实验上已经制备出一些有机磁体,典型的如poly-BIPO等,它通过使用磁性侧基取代氢原子的方式实现铁磁性。紧接着,一些理论工作者对有机铁磁体的磁性起源进行了研究,得到了这种材料的自旋密度波性质,同时也研究了电子-电子关联和边界效应的影响。用这种材料组装器件,并对其输运性质进行研究的工作也逐渐开展,得到自旋过滤和自旋整流等有趣的物理现象。
综上所述,有机自旋电子学领域刚刚起步,对很多具体的问题还缺乏深入的研究。本论文以有机铁磁器件为研究对象,结合了描述有机材料的一维紧束缚SSH(Su-Schrieffer-Heeger)模型以及计算介观输运问题的Landauer-Buttiker公式,采用格林函数和传递矩阵方法计算透射率,研究了体系的自旋相关输运性质。论文首先系统地研究了自旋激发对有机磁性分子的基本性质及其器件的自旋极化输运性质的影响,并对不同激发态的情况进行了详细的讨论。同时,在原来二通道模型的基础上发展了四通道模型,研究了自旋翻转对有机铁磁器件自旋相关输运性质的影响。本论文的研究内容和结果如下：
1.自旋激发态对有机铁磁器件电子输运性质的影响
目前,对有机铁磁材料的研究主要局限在基态体系,实际上,很多外界因素的影响如光、磁场、温度等都有可能使磁性分子偏离基态。已经有一些相关实验报道了单分子磁体在自旋激发态下的输运特性,并且发现了很多有趣的物理现象,如负微分电导和完全的电流压制等,这些有可能与自旋激发的影响有关。本文的第三章以有机铁磁分子poly-BIPO为对象,系统地研究了自旋激发态对有机铁磁器件自旋相关输运性质的影响。
1.1有机铁磁分子中存在自旋激发时,基态情况下正负交替的自旋密度将被破坏,自旋激发附近将出现局域的自旋密度波缺陷。对体系自旋激发能的研究发现,耦合激发所消耗的能量最低,当体系的自旋激发达到一定数量时,自旋激发能不再发生变化,被激发的区域形成一个稳定的畴,激发能存在于畴壁中。
1.2在固定偏压下,随着自旋激发数目的增加,体系的总电流将迅速降低,自旋极化电流在低自旋激发时的变化不大,但在高自旋激发态时将迅速降低为0.
1.3由于器件中分子尺寸较短,有可能出现边界效应,因此考虑了相同数目的自旋激发处于不同位置时对器件输运性质的影响,发现自旋激发位置变化不会影响电流的自旋极化。
1.4温度效应会使有机磁性分子产生集体的激发,取不同温度下,侧基自旋取向的方形随机分布,发现低温引起的自旋激发对器件的自旋极化输运性质影响不大,当温度达到一定值时,自旋极化迅速降低。
2.自旋翻转散射对有机铁磁器件电子输运性质的影响
随着有机自旋电子学的发展,有机磁性材料越来越受到人们的关注,因为它结合了有机材料的和磁性材料的优点。已经有相关研究表明,用有机铁磁材料构建分子器件可以实现自旋过滤和自旋整流的功能。然而,以往的大多数理论研究认为有机材料中的自旋弛豫时间较长,电子在输运过程中不发生自旋翻转,因此在计算输运问题时通常都采用二通道模型。在实际过程中,许多外界条件和内部因素都有可能使电子自旋在输运过程中发生翻转。论文的第四章设计了金属／有机铁磁体／金属三明治结构,考虑输运过程中的自旋翻转,用传递矩阵的方法对器件的自旋相关输运性质进行研究。
2.1对有机铁磁材料的基本性质进行研究发现,存在自旋翻转散射时,有机铁磁分子不同的自旋轨道发生混合,电子不再处于自旋本征态上,而是处于一种自旋混合态；能带宽度和带隙会随着自旋翻转参数的增加发生一定的变化,半导体有机磁性分子将向有利于导电的趋势变化：当自旋翻转参数达到一定值时,中间有机分子的二聚化消失,晶格均匀排列。
2.2考虑π电子的自旋翻转效应时,器件的开启偏压变小,更有利于导通,并且通过器件的电流自旋极化率减少,不能实现将近100%的自旋过滤,但在很大的偏压范围内可以保持较高的值,器件仍然具有自旋过滤的功能。
2.3海森堡相互作用较强的体系在发生自旋翻转散射时比较容易保持较高的自旋极化；同时,中间层有机分子电-声耦合作用的强弱会影响电子在自旋翻转输运过程中的极化状态,较强电-声耦合更容易使电子在输运过程中保持较高的自旋极化,这说明有机铁磁材料是实现自旋过滤功能良好的候选材料。
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 device
By 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.
我们知道,电子具有两个重要的内禀属性,即电荷和自旋。传统的微电子技术只利用电荷流进行信息处理,而自旋这一自由度没有在实际应用的电子学技术中发挥应有的作用。实际上,人们早在20世纪20年代就发现了电子的自旋特性,但直到20世纪80年代发现巨磁电阻效应并用自旋相关散射和二流体模型解释之后,人们才开始认识电子自旋的应用价值,于是对电子自旋的研究成为一个热点课题。自旋电子学中电子的自旋或者是独立地或者是跟电子的电荷相特性结合地作为信息储存和传输的载体。电子的自旋态具有较长的弛豫时间,不容易被杂质或缺陷的散射破坏,而且自旋较容易通过调节外部的磁场来进行控制。人们正期待着利用电子自旋特性来设计运行速度更高、能量消耗更低、功能多、高集成的下一代微电子器件,这种器件的尺寸将进一步大大减小,进入纳米尺度范围,成为介观物理相关研究的重要组成部分。
特别地,近几年来将有机电子学和自旋电子学相结合,得到了一些令人振奋的新现象和新效应,形成了一个新的学科分支-有机自旋电子学。有机自旋电子学是研究有机功能材料及其相关器件中的自旋产生、消灭、转移与存储等物理现象和物理机制的学科,它包含化学有机材料和物理两个领域。将二者结合,探讨有机材料在自旋电子学领域的应用具有重要的基础研究价值和潜在的应用背景,这也是当前国际上许多课题组密切关注的一个研究方向。由于有机材料具有较弱的自旋-轨道耦合和超精细相互作用,载流子的自旋弛豫时间比较长,因而是实现自旋极化输运的理想候选材料。不同于传统半导体中的载流子,有机材料中载流子是孤子、极化子、双极化子等准粒子,它们具有更复杂的电荷自旋关系,使有机自旋电子学器件具有更丰富的特性。
目前,实验上和理论上研究的有机自旋电子学器件主要有三类。第一类是磁性电极／非磁性有机材料异质结型器件。实验上多采用半金属的CMR材料LSMO和铁磁性的Co作为电极,中间层通常采用共轭低聚物六噻吩(T6)和小分子Alq3等。对这类器件的关注点是如何实现有效的自旋极化注入和输运问题。第二类器件是由普通的金属电极和非磁性有机材料构成。实验上在很弱的磁场下(10mT)测得了有机磁电阻(OMAR)现象,对这种现象的解释是目前有机自旋电子学领域一个新的难题。第三类器件使用有机磁性分子作为载体来实现自旋相关功能性。有机磁性分子集有机材料和磁性材料的优点于一身,近几年来已经引起人们的广泛关注。这种材料可以实现电导和磁性的共存,并且具有轻质、低消耗的优点,是有机自旋相关器件的优质候选材料。目前,对有机磁性分子器件的研究工作已经初步展开,实验上已经制备出一些有机磁体,典型的如poly-BIPO等,它通过使用磁性侧基取代氢原子的方式实现铁磁性。紧接着,一些理论工作者对有机铁磁体的磁性起源进行了研究,得到了这种材料的自旋密度波性质,同时也研究了电子-电子关联和边界效应的影响。用这种材料组装器件,并对其输运性质进行研究的工作也逐渐开展,得到自旋过滤和自旋整流等有趣的物理现象。
综上所述,有机自旋电子学领域刚刚起步,对很多具体的问题还缺乏深入的研究。本论文以有机铁磁器件为研究对象,结合了描述有机材料的一维紧束缚SSH(Su-Schrieffer-Heeger)模型以及计算介观输运问题的Landauer-Buttiker公式,采用格林函数和传递矩阵方法计算透射率,研究了体系的自旋相关输运性质。论文首先系统地研究了自旋激发对有机磁性分子的基本性质及其器件的自旋极化输运性质的影响,并对不同激发态的情况进行了详细的讨论。同时,在原来二通道模型的基础上发展了四通道模型,研究了自旋翻转对有机铁磁器件自旋相关输运性质的影响。本论文的研究内容和结果如下：
1.自旋激发态对有机铁磁器件电子输运性质的影响
目前,对有机铁磁材料的研究主要局限在基态体系,实际上,很多外界因素的影响如光、磁场、温度等都有可能使磁性分子偏离基态。已经有一些相关实验报道了单分子磁体在自旋激发态下的输运特性,并且发现了很多有趣的物理现象,如负微分电导和完全的电流压制等,这些有可能与自旋激发的影响有关。本文的第三章以有机铁磁分子poly-BIPO为对象,系统地研究了自旋激发态对有机铁磁器件自旋相关输运性质的影响。
1.1有机铁磁分子中存在自旋激发时,基态情况下正负交替的自旋密度将被破坏,自旋激发附近将出现局域的自旋密度波缺陷。对体系自旋激发能的研究发现,耦合激发所消耗的能量最低,当体系的自旋激发达到一定数量时,自旋激发能不再发生变化,被激发的区域形成一个稳定的畴,激发能存在于畴壁中。
1.2在固定偏压下,随着自旋激发数目的增加,体系的总电流将迅速降低,自旋极化电流在低自旋激发时的变化不大,但在高自旋激发态时将迅速降低为0.
1.3由于器件中分子尺寸较短,有可能出现边界效应,因此考虑了相同数目的自旋激发处于不同位置时对器件输运性质的影响,发现自旋激发位置变化不会影响电流的自旋极化。
1.4温度效应会使有机磁性分子产生集体的激发,取不同温度下,侧基自旋取向的方形随机分布,发现低温引起的自旋激发对器件的自旋极化输运性质影响不大,当温度达到一定值时,自旋极化迅速降低。
2.自旋翻转散射对有机铁磁器件电子输运性质的影响
随着有机自旋电子学的发展,有机磁性材料越来越受到人们的关注,因为它结合了有机材料的和磁性材料的优点。已经有相关研究表明,用有机铁磁材料构建分子器件可以实现自旋过滤和自旋整流的功能。然而,以往的大多数理论研究认为有机材料中的自旋弛豫时间较长,电子在输运过程中不发生自旋翻转,因此在计算输运问题时通常都采用二通道模型。在实际过程中,许多外界条件和内部因素都有可能使电子自旋在输运过程中发生翻转。论文的第四章设计了金属／有机铁磁体／金属三明治结构,考虑输运过程中的自旋翻转,用传递矩阵的方法对器件的自旋相关输运性质进行研究。
2.1对有机铁磁材料的基本性质进行研究发现,存在自旋翻转散射时,有机铁磁分子不同的自旋轨道发生混合,电子不再处于自旋本征态上,而是处于一种自旋混合态；能带宽度和带隙会随着自旋翻转参数的增加发生一定的变化,半导体有机磁性分子将向有利于导电的趋势变化：当自旋翻转参数达到一定值时,中间有机分子的二聚化消失,晶格均匀排列。
2.2考虑π电子的自旋翻转效应时,器件的开启偏压变小,更有利于导通,并且通过器件的电流自旋极化率减少,不能实现将近100%的自旋过滤,但在很大的偏压范围内可以保持较高的值,器件仍然具有自旋过滤的功能。
2.3海森堡相互作用较强的体系在发生自旋翻转散射时比较容易保持较高的自旋极化；同时,中间层有机分子电-声耦合作用的强弱会影响电子在自旋翻转输运过程中的极化状态,较强电-声耦合更容易使电子在输运过程中保持较高的自旋极化,这说明有机铁磁材料是实现自旋过滤功能良好的候选材料。
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 device
By 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.
引文
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