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研究生： 陳柏全
Chen, Po-Chuan
論文名稱： 鈀基磁性多層膜的自旋軌道矩翻轉現象及其類神經運用
Spin-Orbit Torque switching in Pd-based heterostructure and its neuromorphic applications
指導教授： 賴志煌
Lai, Chih-Huang
口試委員: 林秀豪
Lin, Hsiu-Hau
謝嘉民
Shieh, Jia-Min
林文欽
Lin, Wen-Chin
孫元成
Sun, Yuan-Chen
學位類別： 博士
Doctor
系所名稱： 工學院 - 材料科學工程學系
Materials Science and Engineering
論文出版年： 2022
畢業學年度： 110
語文別： 英文
論文頁數： 127
中文關鍵詞： 磁性隨機存取記憶體 、自旋軌道矩 、自旋電子元件 、類神經運算 、磁區壁動力學
外文關鍵詞： magnetic random-access memory, spin orbit torque, spintronics, neuromorphic computing, domain wall dynamics
相關次數： 點閱：3  下載：0 
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    • 自旋軌道矩(SOT)自從10年前發現、確認並且證實之後，一直被視為可以用來驅動傳統磁穿隧結(MTJ)的重要運作機制之一。而MTJ也是磁性隨機存取記憶體(MRAM)的重要元件，也因此SOT的運作機制可以在非常快速、避免破壞性寫入的情形下運作MRAM。不過SOT的運作機制並不是完美的，它仍然存在許多需要改進的缺點。首先，SOT仍舊需要大量的電流(或是輸入能量)來完成寫入動作，但是大量的電流對於微縮化的製程來說是不被允許的；另外，如果沒有額外的外加磁場幫助翻轉，SOT的運作機制常常無法獨力完成寫入動作，因此需要複雜的結構設計或是引入特定材料才能完全SOT的獨立寫入運作。換言之，倘若不是非SOT不可的應用方向，其實SOT的運作機制本身競爭力是不夠和其他類型的記憶體比擬的。針對這一方面，其實有不少非常特殊、先進的應用元件是SOT-MRAM可以大展身手的，比如因為具有磁多階儲存(MML)特性，SOT可以呈現非常特殊的憶阻器(memristor)特性，使其記錄的內容不再是數位式的1或0，而是可以根據輸入訊號大小儲存的類比式記憶。同時，由於SOT記憶元件的寫入/讀取的訊號取線通常可以呈現非線性(non-linearity)，因此有不少文獻報告可以用作常見的類神經運算，如卷積式類神經網路(CNN)和脈衝時序依賴可塑性(STDP)。
    本篇論文使用一個相當少用在SOT材料結構的金屬材料─鈀(Pd)作為實驗主軸。和其他常見的SOT重金屬材料不同，Pd被預測具有較弱的SOT翻轉效率；但是在本篇論文的第5章，Pd其實可以貢獻一定程度的SOT翻轉效率，並不遜於常見的重金屬材料。另外，Dzyaloshinskii-Moriya效應 (DMI)是在SOT運作機制裡常被用來討論的物理參數，其會大幅影響磁區壁的移動能力、以及整體結構需要施加的輔佐外加磁場強度。Pd/Co介面存在相當弱的DMI，使得磁區壁的移動速度遠低於文獻報告的預期，甚至趨近於停滯。這樣的停滯磁區壁，導致了Pd結構可以產生非常穩定的MML表現，其翻轉現象也不再是快速的磁區壁位移，而是大範圍的生成許多細碎翻轉磁區，並藉由不斷的生成翻轉磁區來完成整體翻轉。而這樣的多階儲存特性除了可以用在類神經計算網路，甚至可以模擬在生物上神經聚落(NP)的特殊現象，使得整體元件具有其特有的運作時脈，而不是一味的追求越快越好。總結藉由引入Pd金屬作為SOT的重要結構材料之一，可以賦予磁性層翻轉許多意料之外的反轉表現行為與更多用途。

    It has been more than 10 years since the first discovery and confirmation of spin-orbit torque (SOT) phenomenon. It could successfully operate conventional magnetic tunneling junction (MTJ) under very fast current pulse with assistance of external field. Not only suitable heavy metal materials but also following device fabrication confirm the advantages and shortcomings of SOT-based magnetic random-access memory (MRAM). For one thing, extremely fast read/write speed and excellent device endurance would cost a huge power consumption which is a serious problem for scaling of transmission line. Another issue comes from the request of external field assistance switching. The possible solution for this defect of switching is the potential ability of magnetic multi-level (MML). In other words, since SOT always needs external assistance for successful operation, multi-functionality becomes the most powerful advantage for SOT mechanism to emphasize on. Meanwhile, resistive behavior originating from MML could be applied to many neuromorphic computation applications such as convolutional neural network or spike-time dependent plasticity.
    In this thesis, Pd is selected as heavy metal material for source of SOT. Unlike conventional heavy metal, Pd was once predicted as a weak SOT source but is later proved to have well performance in chapter 5 of this thesis. Moreover, much weaker Dzyaloshinskii-Moriya interaction (DMI) in the interface of Pd/Co could also induced intriguing phenomenon comparing to common Pt/Co interface. The shortage of DMI could result in poor domain wall (DW) mobility and therefore establishing stable enough MML during SOT switching. When the mobility of DW becomes too slow to even move a little bit, the whole switching process becomes a genuine nucleation dominating switching and the device could therefore mimic the behavior of neuronal population in natural nervous system. In summary, this thesis would cover all SOT switching performance observation on Pd-based magnetic layers, and provide prudent physical explanation for its rare but fascinating performance.

    CHAPTER 1 INTRODUCTION 6 1.1. INTRODUCTION 6 1.2. MOTIVATION 7 1.3. OUTLINE 8 1.3.1. Pd/CoFeB PMA system: another material candidate for SOT switching. 8 1.3.2. Nucleation dominating switching mechanism in Pd/Co/Ta heterostructure: full observation and theoretical approach for it. 8 1.3.3. Neuromorphic application: neuronal population behavior in Pd/Co/Ta 9 CHAPTER 2 BACKGROUND 10 2.1. BASIC KNOWLEDGE: TMR AND MTJ 10 2.1.1. Brief introduction of TMR 10 2.1.2. Modern MTJ and its materials 13 2.1.3. Free layer operations in MTJ 16 2.2. DISCUSSIONS ON SOT SWITCHING 18 2.2.1. First glance at SOT 18 2.2.2. History and discussions on SOT 20 2.2.3. Role of spin Hall angle 23 2.2.4. The contribution of DMI in SOT 27 2.2.5. Influence from other factors: damping and torques 32 2.2.6. SOT switching mechanism: Nucleation 37 2.2.7. SOT switching mechanism: DW motion 40 2.3. QUANTITATIVE ANALYSIS FOR SOT 49 2.3.1. FMR, spin pumping and ISHE 49 2.3.2. Harmonic measurement 53 2.3.3. Loop-shift measurement 55 2.4. NEUROMORPHIC APPLICATION 59 2.4.1. Why “neuromorphic”? 59 2.4.2. Common designs for neural network 65 2.4.3. SOT-MRAM in neuromorphic computing 70 2.5. CONCLUSION AND LOOK FORWARD 74 CHAPTER 3 EXPERIMENTAL TECHNIQUES 75 3.1. THIN FILM DEPOSITION 75 3.2. DEVICE FABRICATION 75 3.3. EXPERIMENTAL MEASUREMENTS AND OBSERVATIONS 76 3.3.1. VSM 76 3.3.2. Electrical measurements 76 3.3.3. Kerr microscope 77 3.3.4. XPS 77 CHAPTER 4 LARGE ENHANCEMENT OF SPIN-ORBIT TORQUES IN PD/COFEB: THE ROLE OF BORON 79 4.1. INTRODUCTION 79 4.2. EXPERIMENTAL METHOD 81 4.3. RESULTS AND DISCUSSIONS 81 4.4. SUMMARY 88 CHAPTER 5 MAGNETIC MULTI-LEVEL THROUGH DOMAIN NUCLEATION 88 5.1. INTRODUCTION 88 5.1.1. Heating effect on switching 89 5.1.2. Geometrical effects 90 5.1.3. Creep behavior of DW 90 5.1.4. Structure design 90 5.1.5. Introduction of special materials 91 5.1.6. Artificial defects 91 5.1.7. Switching through domain nucleation 92 5.2. EXPERIMENTAL METHOD 93 5.3. RESULTS AND DISCUSSION 94 5.3.1. Possible defects 98 5.3.2. DW configuration 99 5.3.3. Existence of WB 100 5.4. SUMMARY 102 CHAPTER 6 SOT DEVICE FOR NEURONAL POPULATION 103 6.1. INTRODUCTION 103 6.2. EXPERIMENTAL METHOD 106 6.3. RESULTS AND DISCUSSION 107 6.4. SUMMARY 111 CHAPTER 7 CONCLUSION 115 CHAPTER 8 REFERENCE 119

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