臺灣博碩士論文加值系統

我們針對磁性和超導物質所結合而成一系列雙穿隧結系統的電子自旋傳輸特性作研究，經由操控系統裡的外加磁場、環境溫度、偏壓電極與閘極偏壓，我們可以藉由量測穿隧電阻來瞭解電子自旋的傳輸特性。在此實驗中，我們發現無法以一般穿隧特性所能解釋的異常穿隧磁阻性質，其中細節，我們必須加以自旋相關態密度、電荷能、庫侖階梯與超導性質等特性一起討論。而其中關鍵的物理性質則是在中央島電極所產生的自旋堆積。歸納以下幾點結果：第一，對於一個雙穿隧結系統中，當自旋耗散長度大於外面兩個磁性電極分隔的距離時，穿隧磁阻的存在暗示自旋堆積在中央島的形成。第二，由於元件結構足夠小，使得電荷能增大而足以主控電子的穿隧行為，此時在中央島上自旋堆積的大小將被電荷能所調變，而藉由偏壓電極與閘極偏壓的調控，我們可以掌控電荷能的大小，在於一個對稱的雙穿隧結中，庫侖階梯將使得自旋堆積具有明顯週期性的行為。第三，我們討論在一個超導性的中央島中，因為自旋堆積所造成超導性的破壞對穿隧磁阻所產生的影響。我們的結論是希望能夠量化自旋堆積的大小，並針對它在磁性-超導-磁性單電子電晶體中的穿隧行為所扮演的角色。

We demonstrate spin dependent transports in a series of double-junction systems consisting of ferromagnet and superconductor. The tunnel magnetoresistance is a feature of results studying tunneling processes by controlling conditions of systems such as the external fields, temperature, bias-voltage and gate-voltage. Nontrivial tunnel magnetoresistance is found that cannot be explained by usual phenomenon physics. The details we consider including spin dependent density of states, charging energy, Coulomb staircase structure and superconductivity that are involving in tunneling processes. The key physical property is spin accumulation which is created in the central electrode (or is called island). We find out first tunnel magnetoresistance exists imply that spin accumulation induced in the island while spin relaxation length is long enough comparing with the separation between the two outer ferromagnetic electrode. Second, due to the small size of devices charging effect dominates the tunneling processes and then influences spin accumulation in the island. Bias-voltage and gate-voltage are used to modulate tunneling processes in different conditions that competing charging energy. For asymmetry double-tunnel junctions the Coulomb staircase emphasizes the behavior with spin accumulation modulation. Third, we discuss is the competing between superconductivity and spin accumulation in the island. In conclusion, we want to do is to determine the quantity of spin accumulation in the island and the role it plays in tunneling processes by using ferromagnet-superconductor single-electron transistors.

口試委員會審定書……………………………………………………………… ….i
Acknowledgment…………………………………………………………………….ii
Chinese abstract………………………………………………………...….…………iii
English abstract…………………………………………………………………….....iv
Content………………………………………………………………………………. v
Figure list………………………………………………………………………..…..viii
Chapter 1 Introduction
1.1 Tunnel magnetoresistance………………………………………………… ...2
1.2 Spin accumulation……………………………………………………………3
1.3 Single-electron transistors……………………………………………………5
Chapter 2 Theoretical overview
2.1 Tunnel magnetoresistance……………………………………………………7
2.2 Spin accumulation……………………………………………………………9
2.3 Single-electron transistors………………………………………………..…12
2.4 Spin accumulation in F/N/F SETs……………………………………….….15
2.5 Suppressed superconductivity………………………………………………18
Chapter 3 Sample fabrication and measurement setup
3.1 Sample fabrication of (Co, Ni, NiFe)-Al-(Co, Ni, NiFe) SETs…………….21
3.2 Sample fabrication of Co-Al-NiFe SETs……………………………………22
3.3 Magnetization of F/N/F SETs……………………………………………….23
3.4 Measurement setup………………………………………………………….24
Chapter 4 Experiment results and discussions
4.1 Current-voltage characteristics in Co-Al-Co and Al-Co-Al double-tunnel junctions
4.1.1 Introduction………………………………………………………….26
4.1.2 Co-Al-Co and A-Co-Al double tunnel junctions…………………….27
4.1.3 Measurement setup and current-voltage characteristics……………..28
4.1.4 Magnetoresistance of Al-Co-Al and Co-Al-Co double-tunnel junctions……………………...………………………………………………………29
4.2 Magnetoresistance study in NiFe-Al-NiFe single-electron tunneling devices
4.2.1 Introduction………………………………………………………….34
4.2.2 NiFe-Al-NiFe single-electron transistors……………………………35
4.2.3 Current-voltage characteristics of NiFe-Al-NiFe SETs……………..36
4.2.4 Temperature dependent magnetoresistance………………………….38
4.2.5 Magnetoconductance near Coulomb blockade regime……………...41
4.3 Spin accumulation and oscillation of magnetoresistance in Co-Al-Co single-electron transistors
4.3.1 Introduction………………………………………………………….45
4.3.2 Co-Al-Co single-electron transistors………………………………..46
4.3.3 Conductance-voltage characteristics of Co-Al-Co SETs……………47
4.3.4 Bias-voltage dependent conductance oscillation…………………….49
4.4 Magnetotransport in NiFe-Al-Co single-electron transistors
4.4.1 Introduction………………………………………………………….51
4.4.2 NiFe-Al-Co single-electron transistors…………………………..…52
4.4.3 Conductance versus bias-voltage and gate-voltage characteristics…53
4.4.4 Phase shift in conductance peaks versus gate-voltage………………56
Chapter 5 Simulation results
5.1 Parameter fit………………………………………………………………..59
5.2 Spin accumulation in a F/S/F SET………………………………………….60
5.3 Conductance-voltage characteristics of a F/S/F SET……………………….62
5.4 Suppressed superconductivity………………………………………………65
5.5 Phase shift of Coulomb staircase and spin accumulation…………………..66
6. Conclusion…………………………………………………………………………69
Reference……………………………………………………………………………..70

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