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標題: | 基於鐵磁熱電堆與鐵銠合金之異常能斯特閥 Anomalous Nernst Valve achieved by Ferromagnetic Thermopile and FeRh Alloy |
作者: | Ting-Wei Weng 翁廷瑋 |
指導教授: | 黃斯衍(Ssu-Yen Huang) |
關鍵字: | 自旋電子,異常能斯特效應,異常能斯特閥,熱電堆,鐵銠,磁阻,反鐵磁, spintronics,anomalous Nernst effect,anomalous Nernst valve,thermopile,FeRh,magnetoresistance,antiferromagnet, |
出版年 : | 2019 |
學位: | 碩士 |
摘要: | 現今被發想出的自旋電子元件(spintronic devices)大多與極化自旋流
(spin-polarized current)的傳輸效應相關,其中,極化自旋流的控制、操縱,與偵測,在新興的自旋電子學(spintronics)領域扮演了重要角色。近期,異常能斯特效應(anomalous Nernst effect)被發現能夠被用來利用微小的熱源來穩定的激發極化自旋流,因而相關的研究受到很大的重視。接下來,我們將展示分別利用鐵磁熱電堆(thermopile)與鐵銠(FeRh)合金實現的異常能斯特閥(valve)。在這份研究當中,我們首先展示了基於鐵磁材料的熱電堆,其橫向連結的結構不僅能夠增益其熱電電壓,也能將本身變為一個具有四種組態的異常能斯特閥。 首先,藉由鐵的異常能斯特效應訊號與鎳鐵相反的行為,這兩種材料可以被串聯為熱電堆。更重要的是,因為鐵的異常能斯特電壓在其厚度變薄時會變號並且劇烈增大,因而使用不同厚度的單一材料鐵,熱電堆的電壓可以更進一步地被提升。我們引入了一個熱電效率表示法並與其他文獻比較,結果顯示我們的元件具有極高的效率值。並且,熱電堆內不同磁化組態產生的異常能斯特效應電壓可以被輕易地分辨出來,亦即實現了一個具有多組態的異常能斯特閥。 其 二 , 我 們 展 示 了 , 具 有 一 階 (first-order) 反 鐵 磁 鐵 磁 相 轉 (phase transition)特性的單晶鐵銠合金,可以成為一個能夠以磁場控制其熱自旋電流無與有的異常能斯特閥。另外,因為發生於室溫附近的鐵銠反鐵磁鐵磁相轉機制的存在,尼爾向量(Néel vector)可以藉由場冷卻(field cooling)的過程來穩定控制。有趣的是,我們發現不同的尼爾向量對應的磁阻(magnetoresistance),其相對差異在低溫時會變號且在量值上劇烈增加。我們也觀察到了巨磁阻(giant magnetoresistance)般的效應存在於單晶與多晶的鐵銠當中。 承上述,我們利用了兩種不同的系統,分別演示了鐵磁與反鐵磁系統中極化自旋流的操縱,對於低功耗非揮發性(non-volatile)自旋電子元件的發展十分重要。 Most of the proposed spintronic devices involve spin-polarized transport. The control, manipulation, and detection of spin polarization play a central role in the emerging field of spintronics. Recently, it has been shown that the anomalous Nernst effect (ANE) can be readily used to excite spin-polarized current by only a small amount of thermal energy even without charge current. Related effects and phenomena has attracted a great deal of attention. Here, we demonstrate anomalous Nernst valve realized in the ferromagnetic thermopile and FeRh alloy. We first show that ferromagnetic-materials-based thermopile device, whose laterally connected structure can not only enhance its own thermoelectric signal but also turn itself into a four-state anomalous Nernst valve. By utilizing the opposite sign of the ANE voltage between iron (Fe) in thicker films and permalloy (Py), a thermopile made of the two materials connected electrically in series is realized. More importantly, the voltage of the ferromagnetic thermopile can be significantly increased even if only one material Fe is used, since the ANE voltage of Fe can be reversed and dramatically increased by decreasing thickness. The relatively high thermoelectric efficiency is introduced and obtained from our device. Furthermore, the ANE voltage in thermopile resulting from different magnetization configurations between two ferromagnetic stripes can be clearly distinguished, that is, an anomalous Nernst valve with multiple states is demonstrated. Second, we demonstrate that, endowed by the first-order antiferromagnetic-ferromagnetic (AFM-FM) phase transition nature, epitaxial growth FeRh can be an anomalous Nernst valve with its thermally excited spin current controllable between zero and one by external magnetic field. Besides, because of the existence of AFM-FM phase transition in FeRh near room temperature, the direction of the Néel vector can be readily manipulated via a moderate field at room temperature during cooling process. Interestingly, the relative difference of magnetoresistance for the resultant Néel vector with different orientations can be significantly enhanced at low temperature with reversed sign. We even observe giant-magnetoresistance-like spin-dependent transport behavior in both epitaxial and polycrystalline FeRh. Our results introduce using two different approaches to manipulate spin polarized current in FM and AFM systems that are important for low power non-volatile spintronic devices. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/74461 |
DOI: | 10.6342/NTU201902872 |
全文授權: | 有償授權 |
顯示於系所單位: | 應用物理研究所 |
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