非晶質硫族化合物異質結構中自旋軌道矩之研究

Abstract

拓樸絕緣體 (Topological insulators) 在近幾年吸引非常多的注意力，因為在拓樸絕緣體中，發現了非常高的自旋軌道矩轉換效率。而高的轉換效率意味著所消耗的能量較低，因此，拓樸絕緣體被視為下一個世代自旋電子元 件的熱門人選。在被證實具有拓樸絕緣體性質的材料系統中，鉍基硫族化合物是最被大家廣泛研究的材料，例如:鍗化鉍, 硒化鉍。然而，拓樸絕緣體擁有高的轉換效率是來自於其拓墣保護的表面態造成的自旋動量鎖定 (spin momentum locking)。而為了保護其表面態，必須使拓樸材料具有高度的結 晶性。這樣的需求限制了在工業上的應用。因此，我們選擇使用了傳統的濺鍍系統，成長非晶性的鉍基硫族化合物，碲化鉍。並藉由實驗室所開發的廣場磁光科爾效應磁滯曲線偏移量測方法，得到其自旋軌道矩轉換效率。在論文的第一部份, 我們將原本使用電學量測的磁滯曲線偏移量測方法優化成為 利用光學量測。在 Ta/CoFeB/Hf/MgO 異質結構中，我們發現以往用電學量 測的方式會低估材料本身自旋軌道矩轉換效率，其原因是 Hall bar 本身的幾何結構會造成電流在十字區域造成分流，導致電流密度降低，使預估的效率較低。在第二部份, 我們用磁控濺鍍系統去成長非晶質的碲化鉍, 並成長具有垂直異向性的鐵磁層(Pt/Co/Pt)。藉由磁滯曲線偏移量測, 我們得到高達 1.2 的自旋軌道矩轉換效率，這和其他透過分子束磊晶所成長的高結晶性鉍基硫族化合物是可比擬的。同時我們也量測到利用電流誘發碲化鉍中自旋軌道矩而產生的磁矩翻轉，更進一步證明非晶質的碲化鉍異質結構是可以應用在未來的記憶體元件中。

Topological insulators (TIs), for example, epitaxial Bi-based chalcogenides (BixSb1−x, BixSe1−x) have gained more attention in recent years since they possess large spin-orbit (SOT) efficiency due to spin-momentum locking originated from topologically-protected surface states. The large SOT efficiency makes them great candidates for next generation spintronics devices. However, epitaxy of these Bi-based chalcogenides is typically necessary to ensure the topologically-protected surface state, which limits the application of these materials in industry. In the first part of this thesis, we use Ta/CoFeB/Hf/MgO heterostructure to optimize our hysteresis loop-shift measurement by wide-field magneto-optical Kerr effect (MOKE), and find out that there is an under-estimation of spin-orbit torque efficiency when using electrical means due to current shunting of Hall bar geometry. In the second part, we use conventional dc magnetron sputtering to grow non-epitaxial BixTe1−x thin films and subsequently grow perpendicular magnetized layer (Pt/Co/Pt) on top of it. Through hysteresis loop shift measurement, the maximum damping-like SOT efficiency can reach 1.2, which is comparable to other TI works deposited by molecular beam epitaxy. Current-induced switching of thermally-stable BixTe1−x-based heterostructure is also demonstrated to show that non-epitaxial Bi-based chalcogenide can be applied in future memory devices.