自旋漲落對於自旋塞貝克效應之影響

Abstract

藉由施加溫度梯度以及外加磁場，純粹自旋電流（pure spin current）可由亞鐵磁性材料釔鐵石榴石（Y3Fe5O12）中被激發，這個效應稱為自旋塞貝克效應（spin Seebeck effect）。由於純粹自旋電流是一種不伴隨電流的自旋角動量流（spin angular momentum flow），並且釔鐵石榴石為絕緣體，因此自旋電流無法經由電性量測的方式被偵測。最常使用的方法是在釔鐵石榴石上鍍一層非磁性重金屬（例如鉑（Pt）、鎢（W）、鉭（Ta）...等等），當自旋電流注入重金屬時，由於反自旋霍爾效應（inverse spin Hall effect），純粹自旋電流會被轉換成電流（charge current）。因此，我們能夠經由量測反自旋霍爾電壓（inverse spin Hall voltage）間接地探測純自旋電流。 為了使電子的自旋性質能更有效地應用在電子元件上，如何提高自旋電流訊號一直是被強烈關注的重要議題。最近研究發現，將反鐵磁絕緣體（例如NiO或CoO）插入鉑與釔鐵石榴石之間時，可以提升反自旋霍爾電壓二至三倍。值得注意的是，此增強現象只有在NiO的厚度為1~2奈米且在室溫時才會發生。當NiO的厚度持續變大時，由於自旋擴散長度的限制，反自旋霍爾電壓將指數遞減。受到有限尺寸效應（finite size effect）的影響，1~2奈米NiO的尼爾溫度（Neel temperature）應低於室溫，而先前所觀測到的訊號增強現象都是在室溫下進行量測，因此訊號增強的現象被認為並非是由反鐵磁的磁矩結構所造成。研究進一步發現，在變溫量測中，可以觀察到反自旋霍爾電壓對溫度曲線中存在一個峰值，而峰值所對應的溫度非常接近反鐵磁絕緣層的尼爾溫度。對於反鐵磁材料而言，當其處於高於尼爾溫度的環境時，磁矩之間的長程交互作用會被破壞，然而磁矩之間仍可能存在短程交互作用，因此訊號增強被認為與自旋漲落（spin fluctuation）有關係。 於本實驗中，我們使用銅錳合金（CuMn）來研究自旋漲落對於自旋塞貝克效應所造成的影響。根據研究，銅錳合金的磁性會隨著錳濃度由低至高，依序呈現自旋玻璃態（spin glass state）、磁團簇態（magnetic cluster state）、反鐵磁態(antiferromagnetic state)。不僅如此，銅錳合金的相變溫度也隨著銅錳合金的濃度改變而改變。我們的結果顯示，即使在磁性轉化溫度(blocking temperature)以上，具有不同比例的銅錳合金仍然強烈受到短程交互作用所引起之自旋漲落影響。我們更進一步地發現，在幾何受挫磁系統（spin frustration system）中，自旋凍結抑制了反自旋霍爾電壓。在自旋塞貝克效應量測中，我們發現反自旋霍爾溫度的大小以及正負號隨著錳的濃度改變而改變，這個現象與d軌域電子數有強烈的關係。 我們也研究無序系統非晶釔鐵石榴石（amorphous YIG）的自旋傳輸能力以及在接近相變溫度時所造成自旋漲落的影響，本實驗將非晶釔鐵石榴石插入鉑(Pt)與多晶釔鐵石榴石基板(Polycrystalline YIG slab)之間，發現非晶釔鐵石榴石的自旋擴散長度（spin diffusion length）相對於其他一般絕緣體長，說明非晶釔鐵石榴石擁有良好的自旋傳輸能力。根據我們量測到的溫度相關自旋塞貝克實驗顯示，反自旋霍爾電壓之峰值隨著非晶釔鐵石榴石之厚度變化，證實了自旋漲落確實對於自旋塞貝克效應有很強烈的影響。我們的研究顯示了自旋塞貝克效應是一種可以作為探測具有短程交互作用的工具。

The pure spin current can be thermally induced by applied thermal gradient and magnetic field to ferrimagnetic insulator YIG (Y3Fe5O12), called spin Seebeck effect (SSE). Pure spin current can be electrically detected in the adjacent heavy normal metal layer, such as platinum (Pt), tungsten (W) and tantalum (Ta) via the inverse spin Hall effect (ISHE). Since the spin Hall voltage is usually in the order of microvolt, improve the spin-to-charge conversion efficiency becomes an important issue. Interestingly, while a conventional insulator layer, such as SiO2, or MgO, can easily block the spin current, the ISHE voltage can be enhanced when an antiferromagnetic insulator, such as NiO or CoO, is inserted between Pt and YIG even above the Néel temperature (TN). More interestingly, the temperature dependent SSE measurement exhibits that the maximum of the SSE voltage changes with the thickness and locates at corresponding TN. Since the enhancement can be observed even above the TN, therefore, the antiferromagnetic ordering is not the prevailing role. Possible mechanisms including AF magnons and spin fluctuation are considered to be important. In this work, we utilize Cu1-XMnX alloys over the entire composition range to study the influence of spin fluctuation in SSE. We show that the magnitude of the ISHE voltage and the sign of spin Hall angle (θSH) vary with composition and correspond to the number of 3d electrons. Most importantly, the temperature dependence of the SSE with different composition is strongly influenced by the short-ranged spin fluctuation even above the blocking temperature (Tb). The ISHE voltage (VISHE) is further suppressed by the spin frozen in geometrically frustrated magnet. Our work also studied amorphous YIG (α-YIG) which is also a disordered system. In this experiment, α-YIG was inserted between Pt and YIG slab. The spin diffusion length was found to be longer than other conventional insulators, that is, α-YIG has better spin transmission ability. According to our temperature-dependent SSE measurement, the peak value of the VISHE varies with the thickness of the α-YIG, also implying that the spin fluctuation strong affects the SSE. Our results show that the SSE is a useful tool that can be used to probe complex spin structures with short-range interaction.