藉由自旋漲落效應來增強NixCu1-x合金中的純自旋電流以及自旋軌道力矩

Enhancement of Pure Spin Current and Spin-Orbit Torque by Spin Fluctuations in NixCu1-x Alloy

林胤澤(Yin-Ze Lin)

指導教授 ： 黃斯衍

國立臺灣大學/理學院/物理學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202201545

摘要

自旋電子學已經從利用鐵磁材料中自旋極化電流(spin-polarized current)和自旋轉移力矩(spin-transfer torque, STT)的現象發展到利用純自旋電流(pure spin current)的現象，其中包括具有強大的自旋軌道耦合效應(spin-orbit coupling, SOC)材料中的自旋霍爾效應(spin Hall effect, SHE)和自旋軌道力矩(spin-orbit torque, SOT)。純自旋電流具有有效傳遞自旋角動量的獨特特性，在金屬中伴隨著極小的電荷載子並且在絕緣體中完全沒有電荷載子。許多材料已經被當作用來探索純自旋電流的材料，例如半導體、過渡金屬、拓撲絕緣體、半金屬、過渡金屬二硫化物以及其他等。對於過渡金屬而言，自旋霍爾角(spin Hall angles, θ_SH)的值是由能帶結構決定的，並且是特定金屬所固有的性質。與大部分屬於非磁性的5d金屬不同，3d金屬經常屬於鐵磁性(ferromagnetic, FM)或是反鐵磁性(antiferromagnetic, AFM)，而純自旋電流與自旋極化電流同時共存於其中。
事實上，3d金屬為探索純自旋電流提供了新的途徑和功能。在臨界溫度下由於自旋漲落效應(spin fluctuation)而增強純自旋電流是最有趣的自旋電流現象之一，雖然這有在反鐵磁材料中得到深入的研究，但在鐵磁材料中的研究較少。在本論文中，我展示了在3d鎳銅合金NixCu1−x中純自旋電流、自旋極化電流和自旋漲落效應之間的交互作用。藉由調整鎳銅合金NixCu1−x的成份比例，我們將純自旋電流與自旋極化電流各自的作用分開。藉由利用適當比例Ni-Cu合金中自旋電流與自旋漲落效應之間的相互作用，我們在室溫下獲得前所未有高的自旋霍爾角46%，大約是Pt的五倍大。為了直接且獨立地驗證室溫下Ni0.8Cu0.2有如此大的電荷自旋轉換(charge-to-spin conversion)，我通過Ni0.8Cu0.2中的自旋軌道力矩進行了電流誘發的確定性磁化方向翻轉實驗。這裡，我使用Pt/Co/Pt以及Ni0.8Cu0.2/Pt/Co/Pt兩個多層膜結構。當薄的鐵磁層Co夾在兩層重金屬層Pt之間可以產生垂直磁異向性(perpendicular magnetic anisotropy, PMA)。我們採用電流誘發的自旋軌道力矩來翻轉Co的垂直磁化方向。最重要的是，由Ni0.8Cu0.2產生的電流誘發自旋軌道力矩進一步證實了具有非常大的自旋轉換，顯示出在室溫下分別具有0.4的類阻尼矩效率(dampinglike torque efficiency, ξ_DL)與0.5的自旋霍爾角。

關鍵字

自旋電子學 ； 鎳銅合金 ； 純自旋電流 ； 自旋漲落效應 ； 自旋霍爾效應 ； 自旋軌道耦合效應 ； 自旋軌道力矩 ； 自旋霍爾角 ； 垂直磁異向性

並列摘要

Spintronics has evolved from exploiting spin-polarized current phenomena and spin-transfer torque in ferromagnetic materials to pure spin current phenomena, including spin Hall effect (SHE) and spin-orbit torque (SOT) in materials with strong spin-orbit coupling (SOC). A pure spin current has the unique attribute of efficiently delivering spin angular momentum with minimum charge carriers in metals and no charge carriers in insulators. Many materials have been explored as pure spin current materials, such as semiconductors, transition metals, topological insulators, semimetals, transition metal dichalcogenides, and more. For the transition metals, the values of spin Hall angles (θ_SH) are dictated by the band structures and inherent to the specific metals. Unlike the 5d metals, which are usually nonmagnetic, the 3d metals are often ferromagnetic (FM) or antiferromagnetic (AFM), where both pure spin current effects and spin-polarized current effects coexist.
Indeed, the 3d metals have provided new avenues and functionalities for pure spin-current explorations. The enhancement of spin current at the critical temperatures due to spin fluctuation is one of the most intriguing spin current phenomena, which has been intensely studied in AFMs, but less so in FMs. In this thesis, I demonstrate the interplay of pure spin current, spin-polarized current, and spin fluctuation in 3d NixCu1−x. By tuning the compositions of the NixCu1−x alloys, we separate the effects from the pure spin current and spin-polarized current. By exploiting the interaction of the spin current with spin fluctuation in suitable Ni-Cu alloys, we obtain an unprecedentedly high spin Hall angle of 46%, about five times larger than that in Pt, at room temperature. To directly and independently verify the large charge-to-spin conversion of Ni0.8Cu0.2 at room temperature, I conduct the experiment on current-induced deterministic magnetization switching through spin-orbit torque (SOT) in Ni0.8Cu0.2. Here, I use Pt/Co/Pt and Ni0.8Cu0.2/Pt/Co/Pt films. The thin FM Co layer, sandwiched between two heavy metal Pt layers, acquires perpendicular magnetic anisotropy (PMA). We then employ current-induced SOT to switch the perpendicular magnetization of the Co layer. Most importantly, the large spin conversion is further corroborated by the current-induced spin-orbit torque generated by Ni0.8Cu0.2, which shows high dampinglike torque efficiency (ξ_DL) of 0.4 and a spin Hall angle of 0.5 at room temperature.

並列關鍵字

spintronics ； NixCu1−x alloys ； pure spin current ； spin fluctuation ； spin Hall effect ； spin-orbit coupling ； spin-orbit torque ； spin Hall angle ； perpendicular magnetic anisotropy

參考文獻

第一章參考文獻

1. Friedel Weinert, “Wrong Theory-Right Experiment: The Significance of the Stern-Gerlach Experiments”, vol. 26, no. 1, pp. 75-86 (1995).

2. Po-Hsun Wu, Effect of Demagnetization Factors and Spin Fluctuation on Spin Current Transport, National Taiwan University, Doctoral Dissertation, 142-171 (2020).

3. W. Lin, K. Chen, S. Zhang, and C. L. Chien, Phys. Rev. Lett. 116, 186601 (2016).

4. Z. Qiu, J. Li, D. Hou, E. Arenholz, A. T. N’ Diaye, A. Tan, K.-i. Uchida, K. Sato, S. Okamoto, Y. Tserkovnyak, Z. Q. Qiu and E. Saitoh, Nature Comm. 7, 12670 (2016).