磁性穿隧元件中鈷鐵硼及氧化鎂界面效應之研究

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研究生：	王鼎碩 Wang, Ding-Shuo
論文名稱：	磁性穿隧元件中鈷鐵硼及氧化鎂界面效應之研究 Study of interfacial effects at CoFeB/MgO interface in magnetic tunnel junctions
指導教授：	賴志煌 Lai, Chih-Huang
口試委員:
學位類別：	碩士 Master
系所名稱：	工學院 - 材料科學工程學系 Materials Science and Engineering
論文出版年：	2009
畢業學年度：	97
語文別：	英文
論文頁數：	89
中文關鍵詞：	磁性穿隧元件、鈷鐵硼、氧化鎂、阻尼常數、雙磁子散射
外文關鍵詞：	magnetic tunnel junction, CoFeB, MgO, damping constant, 2-magnon scattering
相關次數：	點閱：3 下載：0
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This work has been focused on the interfacial effects at the interface of sputtered CoFeB and MgO, which is widely used in conventional magnetic tunnel junctions (MTJs). The half-MTJ structures with various CoFeB thicknesses were deposited to identify the interfacial effects. The vibrating sample magnetometer (VSM) and X-ray magnetic circular dichroism (XMCD) measurements were used to analyze the static magnetic properties. To investigate the dynamic magnetic properties and extract the damping constant, the ferromagnetic resonance technique was employed. In addition, the interfacial chemical states and structure were examined by X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and X-ray diffraction, respectively.
Finally, It has been observed that the formation of interfacial cobalt and iron oxides, which can contribute to the magnetic inhomogeneous and extrinsic damping constant through a 2-magnon scattering process.

本論文致力於研究磁性穿隧接合元件中，鈷鐵硼自由層和氧化鎂絕緣層間的界面效應。我們利用超高真空濺鍍法製備鈷鐵硼及氧化鎂的雙層結構，其中藉由調變鈷鐵硼自由層的厚度，並使用一些數值分析的方法，分辨出存在於此系統中的界面效應。在靜態磁性質上，我們使用了樣品震動磁量儀(VSM)以及X光磁圓偏振能譜(XMCD)來做量測及分析。而動態磁性質則由鐵磁共振(FMR)能譜，計算出材料的阻尼常數。此外，我們也使用了X光光電子能譜(XPS)、穿透式電子顯微鏡(TEM)以及X光繞射儀(XRD)來分析系統中介面的化學態和材料結構。
我們發現界面的鈷鐵氧化物是導致鈷鐵硼阻尼常數上升的原因。而藉由動態磁性質的分析，則可以更深入地得知，此上升是藉由界面中雙磁子散射的機制所造成的。而這阻尼常數的上升則對實際應用的元件上，產生了翻轉電流過高的影響，不利於元件的操作。

Chapter 1 Introduction………………………………………………...1

Chapter 2 Background…………………………………………………3
1 Tunneling Magnetoresistance and magnetic tunnel junctions……….3
1.1 Introduction to tunneling magnetoresistance (TMR) effect.......3
1.2 Magnetic tunnel junctions (MTJs) in TMR devices………5
1.2.1 Basic concept of MTJs in real application………………5
1.2.2 Exchange-biased spin-valve structure…………………..6
1.2.3 Pseudo spin-valve structure……………………………..7
2 Current-induced magnetization switching (CIMS)…………………..9
2.1 Spin-transfer torque……………………………………………9
2.1.1 Introduction to spin-transfer torque……………………..9
2.1.2 Landau – Lifshitz – Gilbert (LLG) equation…………..10
2.1.3 Gilbert damping constant and dynamic spin reversal….11
2.2 Current-induced magnetization switching in MTJs…………..13
2.2.1 Critical switching current density……………………...15
2.2.2 Challenges of CIMS in MTJs………………………….17
3 MgO-based magnetic tunnel junctions……………………………...19
3.1 Theoretical prediction of MgO-based MTJs………………….19
3.2 Free layer materials in MgO-based MTJs…………………....21
3.2.1 Fe………………………………………………………21
3.2.2 Co………………………………………………………22
3.2.3 CoFe……………………………………………………23
3.2.4 Amorphous materials-CoFeSiB, NiFeSiB and CoFeB..24
3.2.5 Half-metal – full Heusler alloys……………………….25
3.2.6 Synthetic ferromagnetic free layers……………………26
4 Characteristics of CoFeB alloy……………………………………..27
4.1 CoFeB/MgO/CoFeB MTJs…………………………………..27
4.1.1 Growth and crystallization processes………………….28
4.1.2 Differences between amorphous CoFeB and CoFe electrodes……………………………………………..30
4.1.3 Damages during deposition……………………………31
4.2 Important properties of CoFeB alloy…………………………32
4.2.1 Spin polarization, microstructure and chemical composition…………………………………………..32
4.2.2 Thickness, chemical composition and TMR…………..34
4.2.3 Chemical composition and Gilbert damping constant…36
5 Ferromagnetic resonance……………………………………………37
5.1 Introduction to ferromagnetic resonance……………………..37
5.2 Study of ferromagnetic resonance spectrum………………….38
5.2.1 Resonance field………………………………………...39
5.2.2 Resonance linewidth…………………………………...39
6 Experiment methods for extracting damping constant……………...44
6.1 Frequency dependence of FMR………………………………44
6.2 Angular dependence of out-of-plane FMR…………………...44
6.3 Ultrafast all-optical pump-probe……………………………...47
6.4 Complex susceptibility measurement………………………...49

Chapter 3 Experiment and Analysis Technique……………………...51
1 Experimental flow chart…………………………………………….51
2 Sample preparation………………………………………………….51
2.1 Ultrahigh Vacuum Sputtering System………………………..51
2.2 Post Field-Annealing System………………………………...52
3 Analysis Technique………………………………………………….53
3.1 Atomic Force Microscopy (AFM)……………………………53
3.2 X-Ray Diffraction (XRD)…………………………………….54
3.3 Transmission Electron Microscope (TEM)…………………..55
3.4 Vibrating Sample Magnetometer (VSM)……………………..56
3.5 Electron Paramagnetic Resonance (EPR)…………………….57
3.6 X-ray Magnetic Circular Dichroism (XMCD)……………….59
3.7 X-ray Photoelectron Spectroscopy (XPS)……………………60

Chapter 4 Results and Discussion…………………………………….61
1Half-MTJ structures with various CoFeB thickness………………...61
1.1 Introduction.…………………………………………………..62
1.2 Static magnetic properties…………………………………….63
1.2.1 As-deposited state……………………………………...63
1.2.2 Annealing effects……………………………………... 66
1.3 Dynamic magnetic properties………………………………..70
1.3.1 FMR linewidth and Gilbert damping constant………...70
1.3.2 Thickness dependent FMR linewidth………………….72
1.3.3 Annealing effects……………………………………... 74
1.3.4 Summary……………………………………………….76
2 Examination of interfacial characteristics…………………………..77
2.1 Introduction………………………….………………………..77
2.2 Chemical states at the interface………………………………78
2.3 Magnetic properties of specific elements…………………….80
2.4 Summary……………………………………………………...83

Chapter 5 Conclusions……………………………...............................85

References………………………………………………………………86

                                

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