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標題: | 熱激發純自旋電流和自旋極化電流 Thermal Induced Magnonic Spin Current and Spin-Polarized Current |
作者: | Yi-Jia Chen 陳奕嘉 |
指導教授: | 黃斯衍(Ssu-Yen Huang) |
關鍵字: | 自旋電流,自旋極化電流,自旋賽貝克效應,異常能斯特效應,熱霍爾效應,賽貝克系數,自旋賽貝克系數, pure spin current,spin-polarized current,spin Seebeck effect,anomalous Nernst effect,thermal Hall effect,Seebeck coefficient,spin Seebeck coefficient, |
出版年 : | 2020 |
學位: | 博士 |
摘要: | 自旋電子學中的異常能斯特效應和自旋塞貝克效應,是兩個透過熱激發操縱自旋極化電流和純自旋電流最重要的機制。本論文透過異常能斯特效應來研究在鐵磁金屬薄膜上的自旋極化電流,和透過自旋塞貝克效應來研究一般金屬/釔鐵石榴石雙層膜結構上的純自旋電流。更進一步,本論文也研究電流熱激發和光熱激發的自旋電流。
雖然鐵磁金屬中的異常能斯特效應和磁絕緣體中的自旋塞貝克已經被廣泛的研究了,但是最近有文獻指出,來自熱霍爾效應的信號具有與異常能斯特效應和自旋塞貝克一樣的場變化,其電訊號甚至可能超過原有的自旋訊號。因此研究熱霍爾效應在異常能斯特效應和自旋塞貝克效應中的貢獻至關重要。在本論文中,我改變了電極和接觸線之間的材料以改變他們的塞貝克係數,系統地研究了鐵磁金屬,坡莫合金和磁絕緣體,釔鐵石榴石中的熱霍爾效應。我們結果表明,即使考慮塞貝克係數的厚度依賴性,坡莫合金和釔鐵石榴石中熱霍爾效應的貢獻是可以忽略不計。據此,自旋極化電流在異常能斯特效應中和純自旋電流在自旋塞貝克效應中仍然是不可或缺的。 有許多方法可以產生熱來激發自旋電流,其中包括電流加熱、微波加熱和雷射加熱。其中,光有許多的波長,可以讓加熱方式更加豐富。透過光加熱,與通過電加熱激發的自旋電流不同,光加熱激發的自旋電流可以隨著不同的波長被增強、減弱,甚至反轉。這些結果是因為不同頻率的光有不同的穿透特性。最重要的是,通過翻轉入射光的方向,我們能夠將界面溫度梯度和塊材溫度梯度所產生的自旋貢獻分離。值得注意的是,儘管整體自旋電信號受到光頻率強烈的影響,但界面自旋塞貝克係數和體自旋塞貝克係數卻與頻率無關,這說明自旋塞貝克係數是材料本質的特性。此外,利用光的熱激發,我們觀察到異常能斯特效應和自旋塞貝克效應是瞬時響應。其中,自旋軌道耦合在超快自旋電子學中扮演不可或缺的角色。 The anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE) in spin caloritronics are two of the most important mechanisms to manipulate the spin-polarized current (JSP) and pure spin current (JS) by thermal excitation. In this dissertation, I investigate JS on the normal metal (NM)/yttrium iron garnet (YIG) heterostructures by the SSE and JSP on the ferromagnetic metal (FM) by the ANE. Furthermore, the spin current thermally excited by electrical heating and light heating methods are both investigated. While the ANE in FMs and the SSE in magnetic insulators (MIs) have been extensively studied, a recent theoretical work suggests the thermal Hall effect (THE) have field dependences indistinguishable from and may even overwhelm those of the ANE and the SSE. Therefore, it is vital to investigate the contribution of the THE in the ANE and the SSE measurements. In this dissertation, I systematically study the THE in a FM, Permalloy (Py), and MI YIG, by using different Seebeck coefficients between electrodes and contact wires. My results demonstrate that the contribution of the THE in Py and YIG is negligibly small, if one includes the thickness dependence of the Seebeck coefficient. Thus, the JSP in the ANE and the JS in the SSE remain indispensable for exploring spin caloritronics phenomena. There are serval methods to thermally generate spin current, including electric heating, on-chip current heating, microwave, and laser heating. Among them, light spanning many decays of wavelength, is particularly versatile, which shows that, unlike heating by electrical means, the spin current can be enhanced, reduced, and even reversed, due to the transmittance of the light. Most importantly, by reversing the direction of the incident light, I can separate the contributions of the transverse spin accumulation from the interfacial and bulk temperature gradients. Notably, although the overall spin signals are strongly influenced by the frequency of light, the interfacial and bulk spin Seebeck coefficients are intrinsic and frequency independent. Furthermore, with light excitation, a nearly instantaneous step-like response for both the SSE and the ANE are observed, highlighting the importance of spin-charge conversion by spin-orbit coupling in ultrafast spin caloritronics. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/74713 |
DOI: | 10.6342/NTU202000008 |
全文授權: | 有償授權 |
顯示於系所單位: | 物理學系 |
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