去磁場與自旋漲落效應對於自旋電流傳輸之影響

Effect of Demagnetization Factors and Spin Fluctuation on Spin Current Transport

吳柏勳(Po-Hsun Wu)

指導教授 ： 黃斯衍

國立臺灣大學/理學院/物理學研究所/博士(2020年)

https://doi.org/10.6342/NTU202004400

摘要

藉由施加溫度梯度，純粹自旋電流(pure spin current)可於磁性絕緣體中透過自旋塞貝克(spin Seebeck effect, SSE)效應被激發。其中，釔鐵石瑠石(Y3Fe5O12, YIG)為一種磁性絕緣體材料，因為擁有諸多優勢如高居禮溫度、低自旋傳輸阻尼等，在近年來被廣泛用於以絕緣體為對象之自旋電子學研究。值得注意的是，縱向自旋塞貝克效應(longitudinal SSE)、自旋霍爾磁阻(spin Hall magnetoresistance)以及非局域自旋傳輸(non-local measurement)等效應，都是首先於塊材材料而非薄膜系統中被發現。在進行實驗量測時，通常會將塊材製作成長方體或是圓柱體等非橢圓體(non-ellipsoid)的形狀。在外加磁場下，當材料為非橢圓體的磁性塊材，在材料周邊會形成非均勻的去磁場(demagnetizing field)分布，此外，去磁場的空間分布行為和塊材的形狀以及尺寸有著高度相關，並於表面發散而出，因此這個效應會對自旋傳輸訊號有很大的影響。然而，在過去對自旋電流的研究中，儘管磁性塊材被廣泛的使用，卻從未有人探討去磁場這個經典物理現象和自旋傳輸的關聯性。
於本實驗中，透過使用各種不同尺寸的釔鐵石瑠石塊材於磁性以及自旋傳輸量測，我們發現去磁場對於磁異向能以及自旋傳輸量測有很大的影響。我們證實存在於自旋塞貝克效應以及自旋霍爾磁阻中的異常平台行為是由於釔鐵石瑠石的特殊表面磁矩結構所導致，並且此異常行為會隨著釔鐵石瑠石尺寸的改變呈現系統性的變化，且與形狀異向性有著緊密的關聯。我們進一步研究表面磁疇(magnetic domain)隨外加磁場的演化，並於臨界磁場發現劇烈的90度磁疇翻轉。我們也透過使用變角度的面內霍爾效應(planar Hall effect)量測證實此磁疇翻轉現象的存在。
另一方面，為了能夠更有效率地將電子的自旋特性應用於新一代的裝置及電子元件，如何更有效地操控自旋電流近幾年成為自旋電子學(spintronics)以及自旋熱激發電子學(spin Caloritronics)領域中一個相當重要的議題。最近的研究指出，利用反鐵磁中於相轉變時發生的自旋漲落(spin fluctuation)，可以大幅提升自旋霍爾電壓二至三倍。值得注意的是，發生於相轉變期間的自旋漲落不僅存在於反鐵磁材料中，亦存在於鐵磁性(ferromagnetic, FM)材料中，然而，因為鐵磁材料普遍具有高居禮溫度而鮮少被研究。在本研究中，我們使用熱激發自旋電流探測鐵磁性的鎳銅合金(NixCu1-x)中的自旋漲落現象。藉由調控鎳銅合金中鎳的濃度，我們可以系統地調控居禮溫度的大小。我們進一步發現，無論是什麼濃度的鎳銅合金，在變溫量測中，反自旋霍爾電壓(inverse spin Hall voltage)對溫度曲線中有一峰值，並且峰值所對應的溫度和居禮溫度一致。在鐵磁性材料中，當外加溫度到達居禮溫度會發生磁性相轉變，因此磁矩將由原先的有序排列變為無序排列並且伴隨很強的自旋漲落，而我們的結果表明自旋漲落能夠很大程度地提升自旋霍爾電壓。另一方面，在外加溫度梯度下，我們發現異常能斯特效應及反自旋霍爾效應同時存在於FM/YIG系統中，並且此兩效應之間和自旋極化率(spin polarization)有著密切的關聯。當鎳與銅的比例為八比二時，其居禮溫度相當接近於室溫。更重要的是，由於自旋漲落的影響，這個比例的鎳銅合金之自旋霍爾角(spin Hall angle)可以在室溫下被大幅的提升，甚至為重金屬鉑金(Pt)的四倍之多。最後，我們研究自旋玻璃(spin glass)的銅錳合金中自旋漲落對於自旋霍爾電壓所造成的影響。和具有長程磁矩間交互作用的反鐵磁或者鐵磁材料不同，自旋玻璃是具有短程磁矩間交互作用的系統，並且其於臨界溫度附近亦有很強的自旋漲落行為。有趣的是，我們發現自旋凍結溫度和反自旋霍爾電壓隨溫度曲線的峰值所對應的溫度，隨著銅錳合金的濃度變化呈現相同的趨勢。然而兩者的數值卻總是有兩倍左右的差距，這是由於複雜的自旋凍結(spin freezing)過程所造成。我們的結果展示出自旋電流不但可以被增強，也可以作為一種高度靈敏的工具用於探測複雜的自旋凍結過程。

關鍵字

去磁場效應 ； 磁異向性 ； 磁疇翻轉 ； 自旋塞貝克效應 ； 異常能斯特效應 ； 反自旋霍爾效應 ； 反鐵磁 ； 鐵磁 ； 自旋電流 ； 自旋極化率 ； 自旋霍爾角 ； 自旋漲落 ； 自旋凍結 ； 自旋玻璃

並列摘要

Pure spin current (magnonic spin current) in magnetic insulators can be thermally-excited in magnetic insulators. Yttrium iron garnet (Y3Fe5O12, YIG) is one of the most important insulators in the study of pure spin current phenomena due to its advantages of high Curie temperature, low spin transport damping, and without parasitic contribution from charge current. Interestingly, many spin-dependent phenomena such as spin Hall magnetoresistance (SMR) and non-local measurement, longitudinal spin Seebeck effect (SSE) were first reported in bulk specimens but not in thin films. Generally, bulk specimens are in non-ellipsoid shapes. Under external magnetic field, it will induce a non-uniform distribution of demagnetizing field, which is associated with shape of specimens and the surface divergence of magnetization. Therefore, the demagnetizing field can have considerable effects on spin current transport, but which has not been taken into account in previous studies on pure spin current. In my work, I utilized different sizes of YIG slab to investigate the influence of demagnetization on spin transport measurements. I first showed an unusual plateau behavior is in the thermal SSE and electrical SMR measurement. I further demonstrated it is induced by surface magnetization of YIG, and can be systematically changed with the sizes of YIG, closely connected to the shape anisotropy. In addition, I investigated the evolution of surface magnetic domain with an abrupt 90-degree magnetic rotation, and corroborated it by the angular dependent planar Hall effect.
To utilize spin current, it is crucial to understand how to efficiently manipulate the spin current. It was reported that the spin signal can be largely enhanced by spin fluctuation (SF) during magnetic phase transition in antiferromagnets. Interestingly, the spin fluctuation can also occur in ferromagnets near critical temperature, but has not been clearly studied experimentally due to large Curie temperature (Tc). In this work, I used the thermally-excited spin current to study the spin fluctuation in ferromagnetic (FM) NixCu1-x alloys. The Tc of NixCu1-x can be systematically reduced by decreasing the concentration of Ni. I utilized spin current excited from YIG to explore SF in NixCu1-x. I found the temperature-dependent inverse spin Hall (ISHE) voltage with maximal value at Tp due to SF. This result reveals the significant enhancement of spin current transport can be achieved by effect of spin fluctuation. Crucially, I found the coexistence of ANE and ISHE in a FM/YIG system. The competition relation between two effects is closely related to the spin polarization of FM. In addition, the spin Hall angle in paramagnetic Ni80Cu20 can be enhanced by spin fluctuation, which is four times larger than that of Pt at room temperature. I also investigated the spin glass CuMn alloy with short-range interaction of spin fluctuation. I found close relation between the spin current transports and spin fluctuation. By comparing the composition dependent Tp of spin signal and spin freezing temperature (Tf), I found they present a similar behavior but Tp is always larger than Tf. This result can be attributed to the complex spin susceptibility and spin freezing process in spin glass systems. Therefore, the spin current not only can be enhanced by SF but also a sensitive tool to probe the complex spin freezing process.

並列關鍵字

spin current ； spin Seebeck effect ； spin Hall magnetoresistance ； magneto-optic Kerr effect ； planar Hall effect ； anomalous Nernst effect ； spin fluctuation ； spin polarization

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