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標題: | 橫向自旋流在具有自旋軌道耦合的3d磁性薄膜之探討 Exploration of transverse spin current in 3d magnetic films with spin-orbit coupling |
作者: | Tsao-Chi Chuang 莊造奇 |
指導教授: | 黃斯衍(Ssu-Yen Huang) |
關鍵字: | 自旋電子學,熱自旋電子學,自旋電流,異常能斯特效應,自旋霍爾效應,自旋塞貝克效應,自旋軌道轉矩,垂直異向性,鐵磁, spintronics,spin caloritronics,spin current,anomalous Nernst effect,spin Hall effect,spin Seebeck effect,spin-orbit torque,perpendicular magnetic anisotropy,ferromagnet, |
出版年 : | 2020 |
學位: | 博士 |
摘要: | 自旋電流(spin current)是一種攜帶自旋角動量的電子流,由於伴隨極小的電荷電流(charge current),在自旋電子元件和熱自旋電子元件的應用中被廣泛地研究。自旋電流不僅可以被電壓梯度激發,也可以由溫度梯度來產生。然而,熱激發之自旋電流的機制仍然不清楚。另外,根據自旋霍爾效應(spin Hall effect),自旋電流的自旋方向(spin index)只能固定在某一個特定方向,限制了在零磁場下由自旋電流翻轉磁矩的可能性。 在本論文中,我們透過異常能斯特效應(anomalous Nernst effect),在不同厚度的鐵磁金屬中,觀察因熱所激發之自旋極化電流(spin-polarized current)。研究中,除了發現異常能斯特訊號會隨著鐵磁金屬薄膜厚度減小而顯著地增大,還觀察到異常能斯特訊號的厚度相依行為具有特別的訊號反轉現象,這個現象可以歸因於透過本質(intrinsic)及側跳(side-jump)機制導致的自旋軌道耦合(spin-orbit coupling)。此發現對於熱自旋電子學(spin caloritronics)中提高自旋極化電流,及開發具有高熱電轉換效率的熱電堆(thermopile),有相當重要的影響。 自旋電子學(spintronics)的重要應用之一是利用電流誘發的自旋軌道轉矩(spin-orbit torque)來翻轉垂直磁矩;然而,自旋軌道轉矩的磁矩翻轉時常需要伴隨額外的外加磁場,這是因為由自旋霍爾效應產生的自旋電流,與電荷電流、自旋方向之間存在嚴格的右手定則關係。在本論文中,我們研究如何在零磁場下達到自旋軌道轉矩之磁矩翻轉,並在實驗中發現了明確零場翻轉的現象,我們將其機制歸因於均勻薄膜裏的斜柱狀微結構。斜柱狀結構的傾斜方向決定了垂直磁矩的上下指向,進而實現了可控指向的零場自旋軌道轉矩之磁矩翻轉。重要的是,我們進一步發現了自旋霍爾效應中右手定則的限制可以被磁矩相依自旋霍爾效應(magnetization-dependnet spin Hall effect)打破。透過磁矩相依自旋霍爾效應,電流可以在磁性薄膜表面形成非常規的自旋累積,其中自旋方向還能夠透過磁矩方向來操控。藉由自旋塞貝克效應(spin Seebeck effect)的量測,我們在陶鐵磁絕緣體釔鐵石榴石(yttrium iron garnet)產生純自旋電流(pure spin current),並注入到具有垂直磁矩的自旋偵測器中,清楚明確地觀察到磁矩相依自旋霍爾效應。由於此結構並非全由金屬層構成,避免了額外的電流訊號,因此我們可以明確定量及分析磁矩相依自旋霍爾效應;並與自旋霍爾效應的訊號大小相比較,能進一步估算其自旋與電荷的轉換效率。 因此,透過電子自旋,我們除了觀察到熱激發之自旋極化電流的增強現象,還實現了電流誘發的零場下之磁矩翻轉,更建立可任意操控自旋方向之自旋電流的效應。本研究在自旋電子學與熱自旋電子學中所發現的特殊自旋電流效應,對於發展能源採集的熱電元件和低功率消耗的自旋電子元件,提供了重要的貢獻。 Spin current, a flow of electron carrying spin angular momentum with minimal charge current, has been extensively studied for the applications of spintronic and spin caloritronic devices. Spin current can be excited by not only voltage gradient but also temperature gradient. However, the mechanism of thermal-induced spin current remains unclear. Also, based on the spin Hall effect (SHE), the spin orientation of spin current can only be fixed at a certain direction, limiting the development of field-free magnetization switching for the magnetic random access memory. In this dissertation, I studied the thermally generated spin-polarized current in ferromagnetic (FM) metals with different thicknesses by the anomalous Nernst effect (ANE). The ANE signal was found significantly enhanced by reducing thickness of FM film. I also showed that the thickness-dependent behavior of ANE had a nontrivial sign reversal behavior, which can be attributed to the spin-orbit coupling (SOC) through intrinsic and side-jump mechanisms. This discovery could be essential for enhancing spin-polarized current and for developing thermopile with high thermoelectric conversion efficiency in spin caloritronics. One of the important applications in the spintronics is the current-induced spin-orbit torque (SOT) switching of perpendicular magnetization (PM); however, it often accompanies an undesirable external magnetic field due to the strict right-hand rule among charge current, spin current, and spin index in the SHE. In this dissertation, I demonstrated field-free SOT switching. The deterministic field-free switching was attributed to the oblique columnar microstructure for the otherwise uniform thin films. The direction of oblique columnar structure dictated the up and down of PM, resulting in the polarity-controlled field-free SOT switching. Importantly, I further demonstrated that the restriction of the right-hand rule in the SHE is lift by the magnetization-dependent spin Hall effect (MDSHE). Through the MDSHE, a current can induce unconventional spin accumulation at the magnetic film surface, where the spin index is additionally manipulated by the magnetization directions. By utilizing the spin Seebeck effect in ferrimagnetic insulator yttrium iron garnet (YIG), I generated and injected pure spin current into a spin detector with PM, and unambiguously demonstrated the MDSHE. Without the complications in all-metallic structures, I explicitly identified the MDSHE, compared its size with the SHE, and further estimated its spin-to-charge conversion efficiency. By utilizing the electron spin, I demonstrate the enhancement of thermal spin-polarized current, achieve the current-induced field-free magnetization switching, and establish the manipulation of spin current with arbitrary spin orientation. My discoveries of the novel spin current phenomenon in both spin caloritronics and spintronics offer significant advantages in developing energy-harvesting thermoelectric devices and low-power-consumption spintronic devices. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/73415 |
DOI: | 10.6342/NTU202004399 |
全文授權: | 有償授權 |
顯示於系所單位: | 物理學系 |
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