利用自旋泵浦研究拓撲絕緣體/磁性絕緣體異質結構的介面與自旋輸運性質

Abstract

擁有螺旋(helical)拓撲表面態的三維拓撲絕緣體(topological insulator)的發現為下一代低功率自旋電子器件提供了巨大的機會。最引人關注的是拓樸表面態所具有的自旋動量鎖定，預計會產生大的自旋軌道轉矩(spin-orbit torque)以操縱磁性元件。儘管對拓樸絕緣體的自旋傳輸(spin transport)已經進行了大量研究，但不同研究團隊報告的實驗結果顯示出顯著的不一致性。材料問題，特別是缺乏理想的拓樸絕緣體—鐵磁界面，被認為是實驗的主要缺陷。在本論文中，我們探討拓樸絕緣體—鐵磁界面條件對使用鐵磁共振自旋泵浦(FMR spin pumping)的自旋注入和磁化動力學產生的巨大影響，並預期本實驗結果可用來解釋文獻不一致性。 我們先對各種拓樸絕緣體—鐵磁金屬異質結構進行鐵磁共振自旋泵浦測量。實驗結果顯示，當鐵磁金屬中存在嚴重的材料相互擴散(interdiffusion)以及微波引起的自旋整流效應時，拓樸絕緣體中的自旋—電荷轉換(spin-to-charge conversion)很小並且難以從寄生訊號(parasitic signal)中分離出來。為了消除相互擴散和寄生效應，我們採用亞鐵磁絕緣體釔鐵石榴石(yttrium iron garnet)薄膜與拓樸絕緣體結合。 釔鐵石榴石的高熱穩定性可防止界面發生嚴重的化學反應。藉著使用高品質的鉍化硒(Bi2Se3)—釔鐵石榴石異質結構作為拓樸絕緣體—鐵磁絕緣體材料系統，我們檢測到高效的自旋泵浦效應。除了測量到鐵磁共振線寬明顯加寬，更重要的是，共振場也有顯著的變化，顯示有界面面內磁異向性(in-plane magnetic anisotropy)的存在。這些觀察結果意味著鉍化硒—釔鐵石榴石的強交換耦合磁化動力學。 深入的鉍化硒厚度依賴性研究以及實驗對照組的結果表明磁化動力學與鉍化硒的拓撲表面態密切相關。這得到最大的界面磁異向性和阻尼增強(damping enhancement)的支持。此外，磁異向性和阻尼增強在6奈米厚的鉍化硒附近觀察到，此厚度是二維極限。釔鐵石榴石的磁化動力學調變反映了最近報導的硫化亞銪(EuS)—鉍化硒和碲化鉍銻((Bi,Sb)2Te3)—銩鐵石榴石(thulium iron garnet)中的高溫磁鄰近效應（magnetic proximity effect）。 為了進一步探測鉍化硒—釔鐵石榴石中的磁鄰近效應，我們進行了溫度依賴性鐵磁共振研究。我們發現高達180 K的居禮溫度，在此溫度周圍觀察有共振線寬和共振場的扭結(kink)特徵。磁鄰近效應進一步誘導了競爭的垂直磁異向性，並減少了在室溫下觀察到的面內磁異向性。此外，一個可能源於自旋泵浦引起自旋累積的交換等效場隨著溫度降低而增強。在交換等效場的幫助下，我們測量到鉍化硒—釔鐵石榴石的零外加場鐵磁共振。釔鐵石榴石的新穎零場鐵磁共振提供了無場自旋電子應用的巨大潛力。 高效的自旋泵浦和調變後的磁化動力學，以及鉍化硒中的弱自旋—電荷轉換，意味著界面交換耦合會抑制自旋動量鎖定。這種拓樸表面態的自旋紋理(spin texture)的改變可以理解為時間反轉對稱性被交換耦合破壞的結果。使用薄膜轉移技術，我們最後演示鉍化硒和釔鐵石榴石之間的金隔絕層可以有效地減少鉍化硒和釔鐵石榴石的直接耦合。同時，與鉍化硒—釔鐵石榴石相比，鉍化硒—金—釔鐵石榴石中的自旋—電荷轉換效率較大。 我們對拓樸絕緣體—鐵磁絕緣體的磁化動力學的深入研究為拓樸絕緣體的自旋電子元件設計提供了有價值的訊息。從基礎科學的角度來看，我們的結果揭示了自旋動量鎖定和磁鄰近效應之間的不兼容性。

The discovery of three-dimensional topological insulators (TIs) hosting helical topological surface states (TSSs) offers great opportunities of next-generation low-power spintronic devices. Of utmost interest is the spin-momentum locking possessed by TSSs, which are expected to generate large spin-orbit torques to manipulate magnetic devices. Although investigations on spin transport of TIs have been feverishly conducted, experimental results reported by different research teams showed notable inconsistency. Material problems, especially the lack of ideal TI/ferromagnet interface, is believed to be the main deficiency of experiments. In this dissertation, we address this issue by showing that TI/ferromagnet interface condition profoundly affects spin injection and magnetization dynamics using ferromagnetic resonance (FMR) spin pumping. We started with FMR spin pumping measurements on various TI/ferromagnetic metal (FMM) heterostructures. We have shown that, in the presence of severe interdiffusion of materials as well as microwave-induced spin rectification effect in FMM, the spin-to-charge conversion in TIs was small and hard to extract from parasitic signals. To eliminate interdiffusion and parasitic effects, ferrimagnetic insulator yttrium iron garnet (YIG) films were adopted to combine with TIs. The high thermal stability of YIG prevented severe chemical reactions at the interface. Using high-quality Bi2Se3/YIG as a model TI/ferromagnetic insulator (FMI) system, we detected efficient spin pumping indicated by pronounced broadening of FMR linewidth (ΔH), and more importantly, large interfacial in-plane magnetic anisotropy (IMA) manifested as shifts of the resonance field H_res. The observations imply strongly exchange-coupled magnetization dynamics of Bi2Se3/YIG. Detailed Bi2Se3 thickness dependence study supplemented by a number of controlled experiments indicated that the magnetization dynamics was closely associated with the TSSs of Bi2Se3. This was supported by largest interfacial IMA and damping enhancement observed near Bi2Se3 thickness of 6 nm, corresponding to the two-dimensional limit of Bi2Se3. The modulation in magnetization dynamics of YIG reflected recently reported high-temperature magnetic proximity effect (MPE) in EuS/Bi2Se3 and (Bi,Sb)2Te3/thulium iron garnet (TmIG). To further probe MPE in Bi2Se3/YIG, temperature (T)-dependent FMR study has been conducted. Curie temperature as high as 180 K was found, around which kinks feature of ΔH and H_res were observed. MPE further induced a competing perpendicular magnetic anisotropy that diminished the IMA observed at room temperature. In addition, an exchange effective field H_eff possibly originating from spin-pumping-induced spin accumulation built up toward low T. With the aid of H_eff, FMR without an external field was observed in Bi2Se3/YIG. The novel zero-field FMR of YIG represents great potential for field-free spintronic application. The efficient spin pumping and modulated magnetization dynamics, but small spin-to-charge conversion in Bi2Se3, implied the suppression of spin-momentum-locking by interfacial exchange coupling. Modification of the spin texture of TSSs can be understood as the consequence of broken time-reversal (TR) symmetry by exchange coupling. Using film transfer techniques, we finally demonstrated that a Au insertion layer between Bi2Se3 and YIG effectively reduced the direct coupling of Bi2Se3 and YIG. In the meantime, spin-to-charge conversion was enhanced in Bi2Se3/Au/YIG as compared with that in Bi2Se3/YIG. Our thorough study on magnetization dynamics of TI/FMI provides valuable information for design of TI-based spintronic devices. Fundamentally, our results reveal incompatibility between spin-momentum locking and MPE.