氧化處理對鎢基結構自旋軌道轉矩效應的影響

Abstract

近年來，為了克服自旋軌道磁性隨機存取記憶在實際應用層面的挑戰，高自旋軌道耦合材料的研究與創新持續推進。自旋軌道磁性隨機存取記憶具備高速讀寫、低功耗及耐久性，使其成為新一代非揮發性記憶體的重要候選技術。 本研究旨在提升自旋軌道轉矩效率，並探討氧化對自旋軌道磁性隨機存取記憶的影響，特別是在 z 型結構中實現零場磁化翻轉。我們透過改變自旋電流源及氧化梯度來調控特性。在自旋電流源方面，我們以鎢基重金屬層進行主動濺射處理，發現適量的氧氣可顯著提升自旋軌道轉矩效率。此外，我們探討氧化鎢層的形成與其對電子結構的影響，發現適度氧化的氧化鎢可作為有效的自旋源，增強自旋霍爾效應。然而，由於鎢基材料對氧氣比例高度敏感，最佳氧化條件的範圍較窄，限制了研究窗口。 為克服此問題，我們進一步利用氧電漿處理整個鎢基試片，透過調控氧氣流量、射頻功率、真空度及施打時間來控制磁性變化。此方法不僅有助於推測樣品在空氣中的變化，還能利用鈷鐵硼在氧電漿作用下磁性減弱的特性，在 z 型架構中實現零場磁化翻轉。具體而言，在霍爾棒產生不對稱性並達成零場翻轉。本研究為提升磁性隨機存取記憶體效能提供新方向，未來將進一步優化氧化條件與材料組合，以實現更高效的自旋軌道扭矩記憶體技術。

In recent years, research and innovation in high spin-orbit coupling materials have advanced to address the challenges of spin-orbit magnetic random access memory (SOT-MRAM) in practical applications. SOT-MRAM features high-speed read/write capabilities, low power consumption, and excellent durability, making it a promising candidate for next-generation non-volatile memory. This study aims to enhance spin-orbit torque (SOT) efficiency and investigate the effects of oxidation on SOT-MRAM, particularly achieving field-free magnetization switching in a z-typed structure. We regulate characteristics by modifying the spin current source and oxidation gradient. In terms of the spin current source, we performed active sputtering on tungsten-based heavy metal layers and found that an appropriate amount of oxygen significantly enhances SOT efficiency. Furthermore, we explored the formation of the tungsten oxide layer and its impact on the electronic structure, revealing that moderately oxidized tungsten oxide serves as an effective spin source, strengthening the spin Hall effect. However, due to the high sensitivity of tungsten-based materials to oxygen content, the optimal oxidation condition has a narrow window, limiting the research scope. To address this issue, we further applied oxygen plasma treatment to the entire tungsten-based sample, controlling magnetic variation by adjusting oxygen flow rate, RF power, vacuum pressure, and exposure time. This method not only helps predict sample changes in the air but also utilizes the magnetic weakening effect of CoFeB under oxygen plasma treatment to achieve field-free magnetization switching in a z-type structure. Specifically, an asymmetry is induced within the Hall bar, enabling field-free switching. This study provides new insights for enhancing MRAM performance, and future research will further optimize oxidation conditions and material combinations to achieve more efficient spin-orbit torque memory technology.