【作者】 蒋亮；

【导师】 王伯根；

【作者基本信息】 南京大学 ， 理论物理， 2014， 博士

【摘要】 单分子磁体是一种具有强的内禀磁矩的分子材料。这种分子材料自发现至今天,已经有大约20年左右。单分子磁体由于内禀磁性带来的能级双稳态结构和区别于传统量子点的输运性质受到了实验和理论物理学家的广泛关注。与传统的有机分子材料相比,单分子磁体为分子基底的自旋电子学器件设计提供了合适的特性。由于单分子磁体对于自旋的选择性,使得其适合作为自旋过滤器、自旋阀等自旋输运相关器件的备选材料。在第一章中,简要的介绍本文有关理论背景,和基础的物理概念等。单分子磁体具有双稳态能级结构；低温下,由于分子强内禀磁矩以及自发对称性破缺,磁分子会落在双稳态的一个势阱中,体现出强的自发磁化,导致高的自旋翻转势垒。在磁分子的基础上已进行大量的实验和理论研究,丰富的物理现象被预言观测。如磁性单电子晶体管(SET),自旋Seeback效应,利用电控或温差产生纯自旋流,高的近藤(Kondo)温度,纯自旋流的产生,极化流驱动的自旋翻转等等。随着纳米技术的发展,自旋极化扫描隧道显微镜(SP-STM)和非弹性电子隧穿谱(IETS)技术的发展,单个磁性原子或单分子磁体的自旋方向可以通过自旋转移力矩(STT)效应来控制。基于其可操控性,这种材料为自旋电子学和信息处理器件设计提供了良好的候选方案。第二章,主要介绍了研究中使用的主方程方法及一些基本概念。第三章,研究了串联接在两个普通金属电极之间双磁分子组成的隧道结的输运性质。偏压较低时,隧道结在初始状态磁矩平行／反平行时呈现与低／高阻的状态,这与平行／反平行铁磁电极隧道结的结果类似。强的库伦排斥作用抑制了参数条件下反平行状况下电流。高偏压时,非极化的普通金属电极使得磁分子趋向于非极化状态,类似于普通量子点行为。隧道结为实现记忆存储单元的提供了可能性。第四章,我们研究了串联在正常非极化金属电极之间的两具有不同自旋翻转势垒单分子磁体组成的隧道结输运性质。隧道结中,磁性分子的自旋反转源于自旋转移力矩效应,通过磁性分子的自旋态之间的一系列跃迁实现,必须满足的基本选择规则△m=1/2和△n=1(即隧穿导致的电子数改变为1,总磁矩的变化为0.5)。在低偏压区,较易翻转的磁性分子(反转势垒较小)的磁矩关于隧道结偏压(Sfree VS Vbias)曲线呈现出磁滞回线与双稳态磁性状态,而且磁矩Sfree反转的发生对应于电流的产生变化位置。通过调节偏压的,两磁分子磁结构可以在平行/反平行的磁矩状态进行变化,与此同时隧穿电流也可以实现开/关效应。因而,理论计算结果表明,磁分子隧道结的自旋态可以由偏压来加以控制,使得两个磁分子的磁化方向平行或反平行。此种操作是通过电子库(bath)和磁性分子(SMM)之间的自旋相关的电子输送驱动,并且不需要外部磁场和磁性电极的结构,能够以全电控的方式实现。同样的物理也反映在最近发现的两个稀土单分子磁体自旋阀的现象。由偏压控制双稳态磁状态的单分子磁体可以作为信息存储单元在自旋电子器件中实现。第五章,做了一个简单的总结和展望。更多还原

【Abstract】 Single molecular magnet(SMM) is a kind of single-molecule material with strong in-trinsic magnetism. This kind of molecular material has been found for about twenty years. It attracts many physicists for its special bistable state energy structure and inter-esting transport properties, which are different from traditional quantum dots. Compar-ing to the conventional organic molecular materials, the single molecular magnet (S-MM) has been shown to be a suitable component for future molecule-based spintronic devices. Due to its spin selective properties, the SMM seems to be a very appropriate candidate for designing spin filters or spin valves.In chapter1we give an introduction to investigational background and some ba-sic conceptions. The single molecular magnets have the structure of bistable energy states. At low temperature, the SMM is trapped in one of the potential wells for spon-taneous symmetry breaking, presenting a large intrinsic magnetic momentum which results in a high spin reversal barrier. A lot of theoretical and experimental research-es have been done based on this kind of molecule device and a lot of phenomena are predicted or detected, such as Single electron transistor(SET),spin Seebeck effec-t, high Kondo temperature, pure spin current generator, magnetization reversal driven by spin-polarized current. With the development of nano technology such as the spin-polarized scanning tunneling microscope(SP-STM) and the inelastic electron tunneling spectroscopy(IETS), the spin orientation of a single magnetic atom or a single molec-ular magnet can be controlled via the spin-transfer torque (STT) effect. According to its controllable properties, it can be good candidate for the spintronic and information processing device. In chapter2we give an introduction for the related theoretical methods:master equation approach.In chapter3, we studied the transport through two same single molecular mag-nets, connected to two normal metal leads in series arrangement. Applying low bias voltage, the junction presents low/high resistant state with the SMMs’initial states parallel/antiparallel. This phenomenon is similar with the transport in ferromagnetic parallel/antiparallel junctions. Strong Coulomb repulse suppresses the current in an-tiparallel situation to nearly vanish. At high bias voltage, the middle system containing two SMMs tends to be non-polarized, and acts like ordinary quantum dots. The junc-tion supplies a probability for the memory device.In chapter4, we theoretically explore the spin transport in nano-structures consist-ing of two single-molecular magnets(SMM) sandwiched between a couple of nonmag-netic electrodes. The reversal of the SMM’s spin arises from the spin-transfer torque effect, and it is realized via a set of transitions between the SMM’s spin states, which must satisfy the basic selection rules,△m=1/2and△n=1. In low bias regime, the magnetism of SMM with smaller spin flipping barrier exhibits a hysteresis loop with bistable magnetic states. And the sign reversal of magnetization corresponds to a certain electron current arisen through the junction. By adjusting bias voltage,the magnetic structure of two SMM can be set in parallel/anti-parallel configurations,and the tunneling currents can also be turns on/off by the different magnetic structures. It is shown that the spin states of the SMM in the tunnel junction can be manipulated by the bias voltage, to be parallel or antiparallel to the magnetization of each other. Such a manipulation is driven by a spin-related electron transport between electrons and mag-netic molecules, and needs neither external magnetic field nor magnetic electrodes in the structure.The same physics may also be responsible for the spin-valve phenomena discovered recently in two rare-earth SMMs. And the SMMs with bistable magnet-ic states controlled by bias voltage are expected to be information units in the future spintronic device.The last chapter presents a summary of this dissertation, and then gives some outlook for the investigation.更多还原