鈷鐵硼/鎢/鈷鐵硼系統之零外加場自旋軌道矩翻轉機制與表現

Abstract

隨著自旋霍爾效應和拉什巴效應的發現與應用，自旋軌道矩磁性隨機存取記憶體實現非揮發性存取技術、高資料密度及耐用性，在自旋軌道矩磁性隨機存取記憶體中，採用垂直異相性的系統提供了更快的讀寫速度以及更高儲存密度。然而，需要外加水平磁場克服對稱性進而翻轉磁矩一直以來都是個關鍵的挑戰，目前已有很多機制在不施加外加場的情況下實現磁矩翻轉，但考量到製程兼容性與整合可行性，只有其中的一部分機制已被驗證可應用在自旋軌道矩磁性隨機存取記憶體中，有鑑於具備高穿隧磁阻比的鈷鐵硼/氧化鎂結構已被應用在當今系統，一個建立在此結構的解決方案是值得探討的。 在本篇論文中，對同時具備平面異相性及垂直異相性的鈷鐵硼/鎢/鈷鐵硼結構進行了零外加場自旋軌道矩翻轉機制及尺寸微縮的深入探討。首先對材料架構進行解釋並量測系統的平面異相性，接著，分別進行了磁滯曲線平移和電流致磁矩翻轉量測，測定了自旋軌道矩功效及零外加場磁矩翻轉表現，透過在自旋軌道矩功效隨平面磁場的作圖中發現非典型的似磁滯曲線，證實了內置偏置磁場的存在，在角度相關的量測中，餘弦行為驗證平面磁矩與內置偏置磁場之間的平行關係。在排除該結構中其他可能造成零外加場磁矩翻轉的潛在機制後，我們將重點移到尼爾橙皮效應上，透過原子力顯微鏡從表面形貌得到的數據推算粗糙引起的尼爾場大小，與磁滯曲線平移實驗得到的測量值比較，提供了一個實驗與理論預測的橋樑。隨後，試片被製備成柱狀，即便尺寸微縮，零外加場磁矩翻轉的表現依舊維持。此外，我們也展示了其他結構工程像是楔形結構與加場沉積的數據結果。總體來說，本篇論文在粗糙引起零外加場磁矩翻轉的機制與初步尺寸微縮進行了詳細的探討。

With the discovery of spin-orbit torque (SOT), resulting from spin Hall effect (SHE) and/or Rashba effect, SOT-magnetic random access memory (SOT-MRAM) has revolutionized non-volatile memory technology by enabling high data storage density and great endurance. Among SOT-MRAMs, type-z geometry with perpendicular magnetic anisotropy (PMA) especially provides faster read/write switching speeds as well as higher storage density. However, the requirement of an external magnetic field to break the symmetry and facilitate magnetization switching has been a critical challenge. Several mechanisms have been proposed to achieve field-free magnetization switching without the aid of an external magnetic field. In consideration of compatibility with back end of line (BEOL) and its feasibility of integration, only a subset of them have been verified workable in SOT-MRAM system. Given that CoFeB/MgO structures with high tunneling magnetoresistance (TMR) ratio have been employed in existing systems, a field-free solution leveraging it is worth exploring. In this thesis, a comprehensive investigation into field-free SOT switching mechanism and feasibility of scaling down are conducted on CoFeB/W/CoFeB T-type structures with both in-plane and out-of-plane magnetized layers. Initially, the determination of layer structure is explained, followed by the investigation of in-plane magnetic anisotropy. Later on, hysteresis loop shift measurements and current-induced magnetization switching are performed to characterize the SOT efficacy and field-free SOT switching performance, respectively. With the discovery of an unconventional hysteresis-like loop in SOT efficacy as a function of in-plane magnetic field (H_x), a built-in bias field is verified. In the angle-dependent measurements, a cosine function like behavior confirms the parallel relationship between the in-plane magnetization and the built-in bias field. After ruling out the potential mechanisms resulting in field-free SOT switching in this T-type structure, we shed light on the Ne ́el orange-peel effect. Also, the roughness-induced Ne ́el field (H_N) is estimated from surface topography using atomic force microscopy (AFM), which is then compared with the measured value from loop shift measurement. This acts as a bridge between measurement and calculation. Subsequently, devices are fabricated into pillars, in which field-free switching performance persists even after size shrinking. Additionally, other structure engineering like wedge structure and deposition with in-situ magnetic field are also demonstrated. Overall, this thesis provides a deeper insight into roughness-induced field-free mechanism along with the preliminary investigation into feasibility of scaling down.