Effect of contact geometry on the spin transfer calculation in amine-ended single-molecule magnetic junctions

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江冠融(KUAN-RONG CHIANG) 查詢紙本館藏

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物理學系

論文名稱

(Effect of contact geometry on the spin transfer calculation in amine-ended single-molecule magnetic junctions)

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摘要(中)

由於化學分子的多變性與在資料儲存、磁性感測器上應用的潛力，有機分子磁性接面在近期開始受到大量的關注。磁性材料與有機分子介面化學鍵結與交互作用不僅導致不同的自旋介面效應並明顯影響電子自旋傳輸行為，近期的研究也指出「鈷/對苯二胺/鈷」單分子磁性接面在拉伸下擁有極佳的自旋入射比率與異常磁阻效應。
然而，我們無法得知實際的介面鍵結情況，而這也可能在自旋介面效應扮演舉足輕重的角色。在這份研究中，我們利用第一原理計算結合密度泛函理論與非平衡格林函數研究單分子在不同的鍵結位置、介面氫離子脫附情形和不同化學分子與磁性材料組合等所造成的電子特性與傳輸行為變化。由於「鈷/對苯二胺/鈷」接面中有著明顯的π方向自旋極化共振通道，我們在這個基礎上提出了兩個高自旋極化的胺端單分子磁性接面：「鈷/二胺基三聯苯/鈷」與「鈷/普羅黃素/鈷」，在零電壓下分別擁有132%與248%的磁阻值，也預期在低偏壓下會有更好的效果。另一方面，這份研究也指出單分子中胺端的氫離子脫附扮演極為重要的角色，由於氫離子的數量會影響到單分子中胺端的未鍵結電子數，將會大幅度影響在不同接點下與鈷電極自旋介面耦合情形，進而影響到整個系統的自旋極化傳輸通道。這些結果可望在磁性材料與有機化學分子的自旋介面耦合效應與電子自旋傳輸性質上開啟新的領域與應用。

摘要(英)

Organic-based magnetic junctions have attracted intensive attentions due to their diversity of molecular synthesis and potential applications on data storage and magnetic sensor. The so-called spinterface between ferromagnetic and organic molecules provides rich physics to closely correlate interfacial chemical bonding with spin transport properties. Recently, amine-ended single-molecule magnetic junction has been proposed with superior spin injection and anomalous magnetoresistance (MR) effect under stretching process.
Since the exact contact geometries remains unknown but may play quite a crucial role in spinterface coupling, various contact geometries such as contact site, linker hydrogen ion dissociation and different central molecule are all considered in this study through the employment of density functional theory (DFT) with non-equilibrium green’s function (NEGF). On the basis of π-resonance channel in Co/BDA/Co junction, we propose Co/p-Terphenyl, 4,4′′-diamine (TBDA)/Co and Co/Proflavine/Co two highly spin-polarized amine-ended magnetic junctions with 132% and 248% MR ratio respectively under zero bias, which expected to be highly enhanced while applying source-drain bias voltage. On the other hand, amazingly, the dissociation of H ion at N linker results in an overwhelmingly change of spin-polarized channel at central molecule by controlling number of unpaired electron for N ion interacting with Co apex under different contact cites. These intriguing results may open up a new arena to engineer the spinterface between FM metal and organic molecule for desired magnetotransport properties and MR effect via the variety of choices in anchoring groups and contact geometries.

關鍵字(中)

★ 單分子磁性接面
★ 第一原理計算
★ 非平衡格林函數
★ 密度泛函理論
★ 電子自旋傳輸

關鍵字(英)

★ Single-molecule magnetic junction
★ First-principles calculation
★ Non-equilibrium Green′s function
★ Density Functional Theory
★ Spin transport

論文目次

Chapter 1 Introduction 1
Chapter 2 Theory 6
2.1 Density Functional Theory 6
2.1.1 Born-Oppenheimer Approximation 7
2.1.2 The Hohenberg-Kohn Theorem 8
2.1.3 Hartree-Fock Approximation 10
2.1.4 The Kohn-Sham Equation 12
2.1.5 Exchange-Correlation Energy Functionals 15
2.2 Bloch wave 16
2.3 Projector Augmented Wave method 17
2.4 Non-equilibrium Green’s function method 18
2.4.1 DFT+NEGF Calculation 18
2.4.2 Transmission Coefficient Calculation 20
2.5 Spinterface 21
Chapter 3 Computational Details 23
3.1 Structural Geometry 23
3.2 Parameters for Structural Relaxation 25
3.3 Parameters for Spin Transport Properties 25
Chapter 4 Results and Discussion 27
4.1 Spinterface and Various Contact Geometries 27
4.1.1 Coupling Mechanism of Co/BDA(BDT)/Co 28
4.1.2 Effect of Tip Adatom on Co/BDA/Co 32
4.1.3 Effect of Contact Site on Co/BDA/Co 34
4.1.4 Co/BDMA/Co – with Additional CH2 37
4.1.5 Rest of π-resonance Amine-ended Channels 39
4.2 H ion Dissociation Issue 49
4.2.1 Effect of H ion Dissociation on Bridge Site Contact 50
4.2.2 Effect of H ion Dissociation on Top Site Contact 53
4.2.3 Fe/BDT/Fe and Co/BDT/Co 61
4.2.4 Spinterface Viewpoint of H ion Dissociation 64
Chapter 5 Conclusion 65
References 67

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指導教授

唐毓慧

審核日期

2019-8-20

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