臺灣博碩士論文加值系統

自旋電流和自旋轉移力矩在自旋電子學之應用中扮演重要的角色。自旋電流夾帶著的自旋角動量傳遞給它所流經的鐵磁金屬中的磁矩，對其產生一力矩的作用，即為自旋轉移力矩。一般來說，將電流導入極化後的鐵磁金屬可產生自旋電流。另一較有效率的方法則是利用在俱有強自旋軌道耦合的材料(如重原子金屬)中所發現的自旋霍爾效應。其能產生方向和外加電流垂直的純自旋電流。另外，近年來蓬勃發展的新穎材料-拓樸絕緣體，由於它們奇特表面態特性，使其俱備更高的電流-自旋電流轉換效率，因此被科學家認為是一種極俱發展性的自旋電流源。
自旋轉移力矩不僅能夠被運用於磁矩的翻轉，也能被用於產生磁矩的震盪。在此論文中，自旋轉移矩所激發的磁矩進動，將藉由自旋轉移矩-鐵磁共振量測技術在鐵磁金屬/重原子金屬、鐵磁金屬/拓撲絕緣體等雙層結構中被詳加探討，並以此估算非鐵磁層中的電流-自旋轉移力矩轉換效率。在這樣的結構中，其自旋轉移矩-鐵磁共振之訊號來源主要為異向性磁阻。另一方面，我們進一步以自旋轉移矩-鐵磁共振研究新穎的重原子金屬/磁性絕緣體雙層結構。然而此系統中，因為大部分電流不再通過磁性層，故異向性磁阻之分析方式不再適用。近年被提出的理論立基於自旋霍爾磁阻效應，被引入自旋轉移矩-鐵磁共振的譜型分析。並由實驗觀察加以修正理論的缺陷。而此理論經實驗的驗證與修正將做為進一步研究拓撲絕緣體/磁性絕緣體此新穎雙層結構之出發點。

Spin current and spin transfer torque play important roles in spintronics application. The spin current carries angular momentum, which can be transferred to the magnetization it passes through, a phenomenon known as spin-transfer torques. Generally, spin current can be generated by simply passing a charge current through a ferromagnetic metal (FM). A more efficient way to generate spin current is to utilize spin Hall effect (SHE) observed in the materials with strong spin orbital coupling (SOC) such as heavy metals (HM), which produce pure spin current transverse to the applied charge current. Recently, the novel material topological insulators (TIs) have been regarded as the even more promising candidates for generating pure spin current with extremely high spin charge conversion efficiency due to its surface state property.
The spin transfer torque can be not only used in magnetization switching, but also driving persistent oscillations of magnetization. In this dissertation, the spin transfer-torque-induced ferromagnetic dynamics were investigated via the spin-transfer-torque ferromagnetic resonance (ST-FMR) technique on several bilayer thin film systems, including HM/FM, TI/FM in which the both layers are conducting and the important material property for the non-ferromagnetic layer, efficiency of charge-torque conversion can be well-evaluated. In these systems, the analysis of ST-FMR spectrum stems from the anisotropic magnetoresistance (AMR)-mediated spin diode effect in the FM layer. On the other hand, a novel bilayer system comprised of HM/ferromagnetic insulator (FI) was also investigated using the ST-FMR. Unlike the conducting bilayers, this system only allows current pass through HM layer. The newly proposed theoretical model based on spin-Hall magnetoresistance (SMR) was examined and then modified in our analysis. The finding provides a more accurate approach for using SMR model on ST-FMR measurement. The study provides a basis for the ST-FMR experiment on the novel bilayer structure combined with TI and FI.

Contents

誌謝 4
Publication List 5
摘要 6
Abstract 7
List of Figures 9
Chapter 1 Introduction 16
1.1 Spintronics 16
1.2 Spin transfer torque 18
1.3 Spin torque-induced ferromagnetic resonance 20
1.4 Topological insulators (TIs) 22
1.5 Spin Hall magnetoresistance (SMR) 24
Chapter 2 Experiment setups and procedures 26
2.1 Thin film growth 26
2.1.1 Molecular beam epitaxy 27
2.2 Scanning tunneling microscope (STM) 29
2.2.1 Principles of electron tunneling of STM 29
2.2.1 STM instrument setup 32
2.3 Device fabrication process 34
2.3.1 Photo lithography 36
2.3.2 Ion beam etching (IBE) 39
2.3.3 Atomic force microscope (AFM) 41
2.4 ST-FMR 43
2.5 Alternating gradient magnetometer (AGM) 44
Chapter 3 TIs surface characterization with scanning tunneling microscope (STM) 46
3.1 Introduction 46
3.2 Decapping process of Te/Bi2Te3 47
3.3 STS measurement on Bi2Se3 50
3.4 Summary 53
Chapter 4 ST-FMR on TI/FM bilayer 54
4.1 Theory of ST-FMR 54
4.2 Alternating gradient magnetometry (AGM) and conventional FMR measurements 59
4.3 ST-FMR spectrum on TI/Py bilayer 61
4.4 Summary 69
Chapter 5 ST-FMR on Pt/YIG bilayer 70
5.1 Introduction 70
5.2 Theoretical Model 72
5.3 Result and discussion 76
5.3.1 Line shape analysis and the modification for theoretical model 76
Table 1 Fitting results of current density Jc and spin Hall angle obtained from 3, 4 and 5 GHz spectrums of sample A and B. 85
5.3.2 Angular dependency of the signal 86
5.3.3 The methodology of the curve analysis 89
5.3.4 Magnetic Proximity effect 91
5.4 Summary 93
Chapter 6 Prospects and conclusion 94
6.1 Bi2Se3/YIG bilayer structure 94
6.2 Conclusion 97
Reference 99
Appendix I The fitting code of SMR theoretical model on ST-FMR measurement 103
Appendix II Magnetization reversal processes of epitaxial Fe3Si films on GaAs(001) 107
Appendix III Epitaxial ferromagnetic Fe3Si on GaAs(111)A with atomically smooth surface and interface 115
Appendix IV Investigation of MBE-grown In0.53Ga0.47As (001) 4x2 surface and in-situ ALD TEMA-Hf dosed surface by STM 126

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