氧化銅驅動軌道流對自旋軌道矩之增強效應

Abstract

本論文研究透過在鈷鐵硼/鉑的異質結構中加入氧化銅層作為軌道流來源，以提升其自旋軌道矩的效率。研究動機源自近年來對於利用軌道霍爾效應以產生高效率阻尼型(damping-like)轉矩之自旋電子元件的興趣。在本論文中探討了兩種形成氧化銅層的方式：一是具可控氧氣流量的反應性濺鍍，另一是在空氣中自然氧化。透過二次諧波霍爾電壓量測，我們系統性地研究了氧化程度（藉由調整 Q 值與氧化時間）、氧化銅厚度以及鉑厚度對自旋軌道矩效率的影響。 實驗結果顯示使用反應性濺鍍製備的氧化銅可顯著提升阻尼型自旋軌道矩的效率，其最高增幅相較於鈷鐵硼/鉑控制樣品達到 79%。此增強效果對於濺鍍過程中的 Q 值高度敏感，顯示精準控制氧化程度的重要性。自旋軌道矩效率對氧化銅 厚度的相關性進一步指出此增強效應主要源於體相（bulk）的效應，而非源自於界面效應。此外，藉由改變鉑厚度也證實，當鉑厚度約為 4 奈米時，其軌道轉自旋（orbital-to-spin conversion）的效率最佳。相比之下，雖然自然氧化產生的氧化銅同樣能產生約為36%的可觀自旋軌道矩增強，但由於其對氧化厚度、氧化程度與界面品質的控制較弱，因此在可調性與重現性方面可能會有較多的限制。 結論而言，本論文的實驗結果證實由氧化銅所產生的軌道霍爾電流能有效提升重金屬／鐵磁材料系統中的自旋軌道矩。實驗結果同時也顯示反應性濺鍍是一種較為可靠且可調的軌道矩工程方法，有機會為未來新一代自旋電子裝置導入軌道矩機制提供了可能的實驗框架。

This thesis investigates the enhancement of spin-orbit torque in CoFeB/Pt heterostructures by adding a CuOx layer as an orbital current source. The motivation stems from recent studies of utilizing orbital Hall effects to generate efficient damping-like torques in spintronic devices. Two oxidation methods were explored to form the CuOx layer: reactive sputtering with controlled oxygen flow, and natural oxidation of Cu through ambient air exposure. Using second harmonic Hall voltage measurements, we systematically examined the influence of oxidation level (via Q-factor and oxidation time), CuOx thickness, and Pt thickness on the resulting SOT efficiency. The experimental results show that reactively sputtered CuOx significantly enhances the damping-like SOT efficiency, with the strongest enhancement reaching 79% compared to the CoFeB/Pt control sample. This enhancement is highly sensitive to the Q-factor during sputtering, demonstrating the importance of precisely tuning the oxidation level. The dependence of SOT efficiency on CuOx thickness further reveals that the enhancement originates from a bulk orbital current effect instead of an interfacial effect. Moreover, varying the Pt thickness confirms that orbital-to-spin conversion is most efficient when the Pt layer is approximately 4 nm thick. In contrast, naturally oxidized Cu also leads to observable torque enhancement, with a maximum increase of about 36%. However, this method offers limited tunability and potentially less consistent performance due to the lack of ability to control oxide thickness, oxidation level, and interface quality. Overall, these findings confirm that orbital Hall currents generated from CuOx can effectively enhance SOT in heavy metal/ferromagnet systems. The study demonstrates that reactive sputtering is a reliable and tunable approach to orbital torque engineering and provides a potential experimental framework for integrating orbital torque mechanisms into next-generation spintronic devices.