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研究生： 陳聖淮
Chen, Sheng-Huai
論文名稱： 以反鐵磁達成類神經運算突觸可塑性
Antiferromagnet enables Synaptic Meta-plasticity for Neuromorphic Computing
指導教授： 賴志煌
Lai, Chih-Huang
口試委員: 林秀豪
Lin, Hsiu-Hau
楊朝堯
Yang, Chao-Yao
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： 2023
畢業學年度： 111
語文別： 英文
論文頁數： 105
中文關鍵詞： 反鐵磁 、自旋軌道矩 、突觸可塑性 、類神經運算
外文關鍵詞： Antiferromagnet, Spin-orbit torque, Synaptic meta-plasticity, Neuromorphic computation
相關次數： 點閱：30  下載：3 
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    • 由於反鐵磁具有零外露場、兆赫茲的翻轉速度和不易受到外加磁場影響等特性，操控反鐵磁和區分反鐵磁的狀態成為自旋電子學中熱門的研究方向。先前 反鐵磁自旋電子學的研究主要專注在反鐵磁數位以及類比翻轉的行為和讀取反 鐵磁的不同狀態，只有少許的研究強調對比過去元件的優勢。因此這篇論文中 以相對廣泛的方式來研究反鐵磁自旋電子學，從基本的反鐵磁元件翻轉表現， 到在實際上在類神經運算中會使用到的演算法。首先，我們介紹一種新的方式 達成零場翻轉，在這個實驗中，我們排除熱效應所造成面內交換偏壓的建立。 接著，我們發現可以藉由改變反鐵磁的厚度，造成機率性計算和類神經運算轉 換，改變反鐵磁厚度同時也改變手性交互作用和磁區的移動能力，當中我們使 用傳統的 loop-shift 量測方式來獲得 DMI 的強度，還有柯爾顯微鏡來觀察磁區 的穩定度。在最後的研究中，玻璃態多晶反鐵磁與觀察到與時間相依的磁性變 化有關聯，基於在文獻中所使用額外的演算法，當在多工處理時，另一個自由 度:介面上反鐵磁的型態，可以減緩記憶遺失。為了利用我們所觀察到奇特的 反鐵磁特性，這篇論文提供可能的方式來操控反鐵磁，還有連結自旋電子學元 件和實際操作上在軟體工程中用到的功能。

    Exploring ways to manipulate antiferromagnet, differentiate antiferromagnetic states become a flourishing topic because of properties of zero stray field, THz dynamics and immune to external field, etc. Previous antiferromagnet spintronics studies have already investigated antiferromagnet switching behaviors (digital and analog switching) and methods of reading. However, few of works stress their advantages over existing devices. Thus, this thesis offers a broader view of antiferromagnet spintronics from fundamental switching analyses to implementing neuromorphic computing based on improved algorithm. We firstly introduce a new method to accomplish spin-orbit torque zero field switching, examined by the domain evolution. In first experiment, we carefully rule out the possibility of exchange bias established by joule heating. Next, we discover probabilistic and neuromorphic computing can be achieved by varying the thickness of antiferromagnetic layer, modifying both chiral interaction and domain mobility. Conventional loop-shift measurement and Kerr microscope are used to extract DMI field and study domain stability, respectively. In the last work, a glassy property in the polycrystalline antiferromagnet is found to be responsible for the origin of metastable states and time- dependent changes. Based on the paper about augmented algorithm of neuromorphic computing with additional degree of freedom, in this thesis: configuration of interfacial spins, can alleviate memory loss when multitasking. In order to utilize these exotic antiferromagnet behaviors, this thesis provides not only possible ways to control and modify the spin texture of antiferromagnet but also bridges the understanding in research levels and programming.

    1 Introduction.........................................................8 2 Background...........................................................9 2.1 Ferro-, Antiferro- and Ferri-magnet................................9 2.2 Types of antiferromagnets.........................................10 2.2.1 Collinear antiferromagnets .....................................10 2.2.2 Non-collinear antiferromagnets..................................11 2.3 Spin glasses .....................................................12 2.4 Exchange bias.....................................................13 2.5 Spin Hall effect .................................................15 2.6 Anomalous Hall effect.............................................16 2.7 Rashba effect ....................................................17 2.8 Dzyaloshinskii-Moriya interaction.................................18 2.9 Spin-orbit torque.................................................18 2.9.1 Fieldlike torque and damping-like torque .......................20 2.10 Magnetoresistance................................................21 2.10.1 Anisotropic magnetoresistance .................................22 2.10.2 Spin Hall magnetoresistance....................................23 3 Literature review...................................................25 3.1 Types of spin-orbit torque switching .............................25 3.1.1 x, y and z-type switching ......................................25 3.1.2 Field-free switching............................................26 3.2 Writing antiferromagnet states....................................31 3.2.1 Spin-orbit torque switching of antiferromagnet .................31 3.2.2 Field switching of antiferromagnet .............................36 3.2.3 Strain-induced switching of antiferromagnet ....................38 3.2.4 Laser writing of antiferromagnet ...............................39 3.3 Read-out antiferromagnet states ..................................40 3.3.1 Electrical read-out ............................................41 3.3.2 Optical read-out ...............................................43 3.4 Exchange bias switching...........................................45 3.4.1 Thermally-assisted exchange bias switching .....................45 3.4.2 Current-induced exchange bias switching.........................47 3.5 Neuromorphic computing ...........................................48 3.5.1 Multilevel switching............................................48 3.5.2 Short-term plasticity (forgetting curve)........................49 3.5.3 Meta-plasticity of artificial neuron............................50 3.5.4 Algorithm in neural network ....................................50 4 Methods.............................................................52 4.1 Sample preparation ...............................................52 4.1.1 Magnetron sputtering deposition ................................52 4.1.2 Atomic force microscope ........................................52 4.1.3 Lithography.....................................................52 4.1.4 Ion beam etching system ........................................53 4.2 Magnetic properties measurement ..................................53 4.2.1 Vibrating sample magnetometer ..................................53 4.3 Electrical transport..............................................53 4.3.1 Four-point probe station .......................................53 4.3.2 Physical property measurement system............................54 4.4 Optical measurement...............................................54 4.4.1 Kerr microscopy.................................................54 4.4.2 X-ray absorption spectroscopy ..................................54 4.5 Simulation........................................................55 4.5.1 Atomistic simulation of magnetic materials (VAMPIRE) ...........55 5 Experimental results and discussion ................................56 5.1 Domain wall dynamics of field-free switching in Pt/Co/IrMn .......56 5.1.1 Introduction....................................................56 5.1.2 Magnetic properties of Pt/Co/IrMn ..............................56 5.1.3 SOT behaviors in Pt/Co/IrMn ....................................58 5.1.4 Analyses of domains during SOT .................................60 5.2 Dzyaloshinskii-Moriya interaction modulated by IrMn bulk order....63 5.2.1 Introduction....................................................63 5.2.2 SOT switching in different IrMn thicknesses.....................63 5.2.3 DMI field and its correlation with SOT switching................64 5.2.4 Stability of multilevel and domain observation .................66 5.3 Glassy properties of exchange bias and its applications in neuromorphic computing ...............................................69 5.3.1 Introduction....................................................69 5.3.2 Exchange bias reduction and aging...............................71 5.3.3 Spin orientation at IrMn/Co interface ..........................75 5.3.4 Field treatment in aging .......................................79 5.3.5 Training and probing............................................81 5.3.6 Frequency and strength of stimuli...............................82 5.3.7 Potential application in binarized neural network ..............84 6 Summary and Perspective.............................................87 7 Appendix ...........................................................89 7.1 Angular-dependent magnetoresistance ..............................89 7.2 Pt/IrMn/Co XMCD...................................................90 7.3 Magnetoresistance during aging....................................90 7.4 Examination of exchange bias training effect .....................91 7.4 Joule heating.....................................................92 7.5 Crystal field splitting fitting...................................93 7.5 IrMn XMLD under -Hx and -Hy field ................................94 8 Reference ..........................................................95 9 Acknowledgement ...................................................105

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