单分子磁体自旋电子器件的界面效应研究

Compounds of the single-molecule magnet (SMM) class consist of an inner magnetic core with a surrounding shell of organic ligands. They are now considered as promising candidates for molecular spintronic devices, such as molecular spin-walve, molecular spin transistors and molecular multidot devices, since their structures can be easily tailored through playing with the anchoring groups and grafted to the surface of the metal electrodes. The mechanisms underlying the spin injection and spin transport process across the metal/organic interface are still to be unraveled and remain one of the most important challenges in this new uprising field. We foucus on the interface effects in single-molecule magnets based spintronic devices，and expect to reveal the role of the interface for spin injection into SMMs of the devices,by using the multi-scale methods,the density functional theory and nonequilibrium Green's function techniques,as well as the tight binding models.We aim to point out the dominating mechanisms of how the electron spin polarization transforms as the SMMs connect to the metal electrode by chemical adsorption,to disclose how the complex interaction between the organic magnetic molecules and the metal electrodes spin-dependently broaden and move the molecule energy levels, to understand how the adsorptions affect the magnetic properties of the SMMs,to describe the interfacial spin-dependent metal/molecule hybridization on the enhancement and sign reversal of the injected spin polarization, as well as how the rich physics behind the magnetic behavior of SMMs produce interesting transport properties in the devices such as negative differential conductance and complete current suppression. We expect this project to lead towards the molecular-level engineering of metal /organic interface to tailor the spin injection and bring new electrical and spin functionalities to molecular spintronics devices.

单分子磁体由磁性中心离子和有机配体组成，通过对配体基团进行修饰，易将单分子磁体嫁接到材料表面或电极上，探索其作为自旋阀、自旋晶体管、多量子点器件的研究。器件中金属电极和有机分子基团接触界面的构形和特性对电子自旋的注入和输运起至关重要的作用，是目前分子自旋电子器件的研究重点。本项目将应用量子力学的多尺度方法，将非平衡格林函数方法与第一性原理、紧束缚模型等结合起来，深入系统地研究基于单分子磁体的自旋电子器件中的界面效应。重点研究单分子磁体化学吸附情况下自旋极化率的改变机制：包括界面处金属电极与有机磁性材料复杂相互作用对分子能级自旋相关的展宽和移动，对电极表面磁性的改变；金属/有机分子界面形成自旋相关杂化态对注入自旋极化率大小和方向及输运的调制；分子自身磁性因素的调控及对器件自旋输运的影响。寻找分子器件自旋输运增强的途径，探索新的功能性器件界面结构设计，实现自旋器件特定性能在分子尺度上有效调控。

本项目将非平衡格林函数方法与第一性原理、紧束缚模型等结合起来，深入系统地研究基于单分子磁体的自旋电子器件中的界面效应。包括界面处金属电极与有机磁性材料复杂相互作用对分子能级自旋相关的展宽和移动，对电极表面磁性的改变；金属/有机分子界面形成自旋相关杂化态对注入自旋极化率大小和方向及输运的调制；分子自身磁性因素的调控及对器件自旋输运的影响。项目执行期间，针对四种单分子磁体材料开展了研究。一维磁性分子线[Fem(C5H5)n] 和钴电极组成不同接触界面对体系巨磁电阻（GMR）效应的影响，通过对分子隧道结的自旋极化输运计算发现了电压控制的GMR效应，其中最大的GMR比率达到 2.0 × 104%；随着电压的变化，也发现GMR值由正到负的变化。重点研究了苯环-卟啉-苯环基分子（BPB）和不同金电极构型组成的分子器件的电输运过程，探讨了分子与电极间的耦合方式，中心配位金属改变，功能性基团替代等因素对器件输运性质的影响。金字塔型的分子器件在低压区显示出的较强的NDR效应，这一效应在普通的平面电极从未观察到。钴卟啉-苯环分子结中接触构型相关的宽电压范围的电自旋输运计算，显示多重NDR效应和基团增强的极化输运特性。搭建铁苯环分子Fe(Bz)2通过硫原子分别与金纳米线电极结合，形成T型和H型分子器件，研究该器件的高低两个自旋态下的自旋相关的输运特性，以及自旋热电效应。还研究了DTB分子与金纳米线接触构型相关的双重负微分电阻与整流效应。除此以外，还延伸性研究了封装进过渡磁性金属原子的单壁碳纳米管的电磁输运性质。