研究多種鎢基自旋電子元件之自旋軌道矩

Abstract

自旋矩的磁阻式隨存取記憶體正快速發展於嵌入式應用，並由於具備非揮發性質、快速運作速度、和低功耗的優點，磁阻式隨存取記憶體具備作為末級快取的潛力。在能夠從電荷電流中生成自旋電流的材料中，5d過渡金屬如鉭 (Ta)、鉑 (Pt)和 鎢 (W)由於其中強大的自旋軌道耦合，經常被選用，可透過自旋霍爾效應產生自旋電流，進而產生自旋軌道轉矩翻轉鐵磁層之磁化方向。在 5d過渡金屬元素中，鉭和鎢成為當代具有高穿隧磁阻比的鈷鐵硼/氧化鎂(CoFeB/MgO)磁穿隧結構中自旋電流的來源。此篇論文主要聚焦於鎢的異質結構。首先，透過檢驗其晶體結構、磁性、電性、以及自旋軌道矩相關等基本性質，建立基礎的瞭解。研究顯示，阿法相的鎢 (α-W) 因其在所有過渡金屬中最高的自旋霍爾電導率，和可實現的電流誘發自旋軌道矩翻轉而成為潛在的高效自旋電流材料。在對基本性質有瞭解後，對標準和零場翻轉的鎢異質結構的自旋軌道矩效率和熱穩定性 進行了在不同溫度和脈衝寬度下的檢驗。值得注意的是，自旋軌道矩效率在攝氏70度下仍保持恆定，而熱穩定性隨著溫度上升而下降。此外，展示了基於磁壁運動的磁記憶性翻轉行為，在不同脈衝寬度下，磁記憶性翻轉的區間大小保持不變，但在較高溫度下減小。最後，研究深入探討了元件的尺寸和幾何形狀對性質的影響。磁壁去釘扎模型被驗證可正確的估算自旋軌道矩效率和臨界翻轉電流密度。此外，由於在霍爾條 (Hall bar)元件與磁穿隧結構中相似的結果，其被驗證為對自旋軌道矩效率和臨界翻轉電流密度進行評估的可靠測試元件。本論文全面深入地探討了以鎢為自旋軌道矩來源的自旋軌道矩元件，為未來記憶體應用提供了全面性的理解。

Spin torque-based magnetic random access memory (MRAM) is rapidly advancing for embedded applications and holds the potential to serve as a last-level cache owing to its nonvolatile nature, fast operation speed, and low power consumption. Among materials proficient in generating spin current from charge current, 5d transition metals like Ta, Pt, and W are frequently selected due to the strong spin-orbit coupling (SOC) therein, which can induce spin currents and spin-orbit torque (SOT) through the spin Hall effect (SHE). In the realm of 5d transition metal, Ta and W emerge as prevalent sources of spin currents for contemporary CoFeB/MgO-based magnetic tunnel junction (MTJ) with high tunneling magnetoresistance ratio (TMR). This thesis primarily focuses on W-based magnetic heterostructures. First, the foundational comprehension is built through examining the basic properties including structural, magnetic, electrical, and SOT-related properties. The investigation reveals that α-W stands out as a potential candidate for efficient generating SOT due to the highest spin Hall conductivity = 3.71x10^5 Ω^-1^m-1 among all transition metal and the achievable current-induced SOT switching. Following the fundamental examinations, SOT efficiency and thermal stability factors in standard and field-free W-based magnetic heterostructures are characterized across various temperatures and pulse-widths. Notably, SOT efficiency remains constant up to 70 °C (343 K), while the thermal stability factor monotonically decreases with rising temperature. Besides, the memrisitive switching behaviors based on domain wall motion are demonstrated, presenting the size of the memristive window remains invariant under different pulse-widths but reduced at elevated temperature. Last, the study delves into the dimensions (ranging from 5-µm Hall bar, micrometer-sized pillar device, and submicrometer-sized pillar device) and geometry (Hall bar, pillar, and three-terminal MTJ) of the devices. The domain depinning model is verified to securitize the SOT efficiency or critical switching current density accurately. Moreover, the Hall bar device, yielding the consistent results with three-terminal device, is identified as a reliable test vehicle for characterizing SOT efficiency and critical switching current density before committing substantial resources to three-terminal SOT device fabrication. In conclusion, this thesis offers comprehensive insights into W-based SOT devices, providing fundamental understanding for leveraging W in future memory applications.