第一原理研究苝四甲酸二酐與磁性物質耦合對自旋幫浦和自旋注入的影響

Abstract

有機分子與磁性物質的交互作用提供了新的控制電子自旋的傳輸與注入的方向。依據分子和磁性物質的結構以及耦合的強度不同，分子的磁性結構會有巨大的改變。另外波函數的性質也會極大地影響自旋傳輸，因此需要一個可以包含這些資訊的模擬方法去指引實驗的方向。利用密度泛函理論以及非平衡格林函數，我們研究了苝四甲酸二酐與不同磁性物質的交互作用下，其電子與磁性結構與傳輸的影響。 第一部分為鐵與苝四甲酸二酐的自組結構研究。我們預測此物質一維與二維結構的基態都具有半金屬的特性。一維結構中，分子上的磁矩震盪分布可以保證其基態的穩定性，同時電子傳輸特性是由分子與鐵的耦合態決定。因為耦合態的對稱性質不同導致了負微分電阻的產生。而二維型態也是半金屬，並有更高的熱穩定性與鐵磁穩定性。 第二部分，我們調查了苝四甲酸二酐在鈷六方緊密堆積結構(0001)表面可能的吸附位置、方向和相關的吸附能。發現分子的電子結構跟吸附方向有巨大的關連，且電子結構的不同會改變磁阻的正負號。

The interaction between organic and ferromagnetic (FM) materials provides a new direction for controlling the spin-injection and spin-transport. Depending on the structure and the coupling of the hybridization of organic/FM materials, the magnetic structure of organic materials alters drastically. Hence a simulation technique is needed to guide the experiments. Here, combining the density functional theory (DFT) and the nonequilibrium Greens function (NEGF), we investigate the electronic/magnetic structures and the transport properties affected by the coupling between perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and different FM materials. Firstly we investigate the Fe(PTCDA) complex. Our calculation predicts that 1D and 2D structure are both half-metallic. For the 1D structure, half-metallic state is ensured by the strong PTCDA-Fe coupling, causing the spin charge redistribution of the molecule. From the band structure and transport calculation, we show that transport behaviors are affected by the d-pz hybridized states. Negative differential resistance (NDR) induced by the band symmetry matching is observed. For the 2D structure, much higher thermal and magnetic stabilities are expected. Secondly we investigate the possibility of adsorbing PTCDA at different sites on the hcp Co(0001) surface by comparing the corresponding adsorption energy (AE). Analysis of the density of states for this system shows that spin polarization at the organic/FM interface is altered according to different adsorption sites.