The interfacial effect on spin-transfer torque in single molecular magnetic junctions: A first-principles study

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徐郁鈞(Yu-Chun Hsu) 查詢紙本館藏

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物理學系

論文名稱

(The interfacial effect on spin-transfer torque in single molecular magnetic junctions: A first-principles study)

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摘要(中)

近年來隨著自旋電子學的快速發展，人們可以藉由電子的自旋狀態來儲存資料，磁電阻式隨機存取記憶 (magnetoresistive random access memory，MRAM) 具有非揮發性 (nonvolatility)、快速讀寫、低功耗及高元件整合密度等特性，因此如何有效得改變磁矩狀態便成為重要的課題。其中使用電流所產生的自旋轉移力矩(Spin-transfer torque) 改變磁矩狀態的MRAM，我們稱之為 STT-MRAM。

在理論模擬上，我們選用兩種不同官能基的有機化合物，對苯二胺 (1,4-benzenediamine, BDA) 以及對苯二硫醇 (1,4-benzenedithol, BDT)，作為磁性單分子通道的材料，並在分子兩端接上鈷 (111) 的奈米金屬線作為提供自旋電子注入的電極。同時，我們改變兩電極間的距離，使用第一原理計算其最適原子位置，並利用非平衡態的格林函數 (Non-equilibrium Green’s function) 計算自旋電子的穿隧特性。我們發現在有機分子與金屬電極的介面上，以氮和硫作為不同通道的錨定離子時，自旋的交換特性有明顯的差異；藉由翻轉右邊電極的磁矩方向，使系統成為平行與反平行的狀態，接著各別改變其右邊電極的化學位能，使通道左右兩側產生電位差，進而計算出通道的自旋電流 (Spin-polarized current) 。我們發現以氮作為錨定離子的 Co/BDA/Co 單分子通道，可以得到很高的磁阻率 (Magnetoresistance)；藉由自旋轉移力矩通式 (General expression) 的預測，也發現此通道可以產生巨大的自旋轉移力矩。因此以氮作為錨定離子的Co/BDA/Co 單分子磁性通道，有很大的機會能在有機分子的 STT-MRAM 應用領域中扮演重要的角色。

摘要(英)

In recent years, the switching of magnetic states by using current in magnetoresistive random access memory has attracted great attentions due to the newly discovered spin-transfer torque (STT) effect in magnetic tunneling junctions. In order to study the interfacial effects on the STT effect in the single molecular magnetic junction, we choose two kinds of molecule with different anchoring ions, 1,4-benzenediamine (BDA) and 1,4-benzenedithol (BDT), sandwiched by two Co(111) nanowires. To simulate the stretching effect, we first increase the distance between two electrodes step by step, and then optimize the structures of each step by using the first-principles calculations, until the junction is breakdown. The NEGF-DFT+LDA calculation is further employed to obtain the spin-polarized transmission spectra and the projected density of states. Although the similar breakdown behavior can be observed in both junctions, our calculation results suggest the superior spin transfer via amine-ended Co/BDA/Co junction, due to the strong coupling between N-p orbital and Co-d orbitals at the Fermi energy. This is in sharp contrast to the traditional molecular junction, where thiol-ended π-conjugated molecule mediates a better electron charge transfer between non-magnetic electrodes. Such NH-anchoring-induced spin-filtering effect, resulting from the significant and broad spin-up transmission features solely in PC configuration, not only gives rise to the high MR value, which can reach up to about 700% under low bias, but also leads to the giant magnitude of STT, which is about three order larger than that of conventional MgO-based MTJs. Theses interesting findings suggest that amine-ended π-saturated single molecular magnetic junction may open a novel and promising organic-based- STT-MRAM applications.

關鍵字(中)

★ 自旋轉移磁矩
★ 單分子通道
★ 磁阻
★ 自旋傳輸
★ 第一原理計算

關鍵字(英)

★ spin-transfer torque
★ single molecular junctions
★ magnetoresistance
★ spin transport
★ first-principles calculation

論文目次

Chapter 1 Introduction 1

Chapter 2 Theory 5

2.1 Density Function Theory 5

2.1.1 Born-Oppenheimer Approximation 5

2.1.2 Hartree-Fock Approximation 7

2.1.3 The Hohenberg-Kohn Theorem 9

2.1.4 The Kohn-Sham Equation 11

2.1.5 Exchange-Correlation Energy Functionals 14

Local Density Approximation (LDA) 14

Generalized Gradient Approximation (GGA) 15

2.1.6 Pseudopotential Method 15

2.2 Non-Equilibrium Green’s Function Method 17

2.2.1 Self-Consistent in NEGF-DFT Calculation 17

2.2.2 Spin-Transport Property Calculation 21

Total Energy 21

Transmission 22

Density of States 23

Spin-Polarized Current 24

2.3 Spin-Transfer Torque 25

2.3.1 Spin Current Density 25

2.3.2 Toy Model 26

2.3.3 STT of Magnetic Tunneling Junction 29

Chapter 3 Computational Details 32

3.1 Structural Geometry 32

3.2 Parameters for Structural Relaxation 34

3.3 Parameters for Spin Transport Properties 35

Chapter 4 Results and Discussions 37

4.1 Structural Relaxation during Stretching 37

4.2 Effect of Anchoring Groups 41

4.3 Effect of Co Adatom 49

4.4 Spin-Polarized Current and Magnetoresistance 51

4.5 Spin-Transfer Torque Effect 60

Chapter 5 Summary 64

References 65

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指導教授

唐毓慧(Yu-Hui Tang)

審核日期

2015-8-27

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