合成反鐵磁中鐵磁層厚度不對稱調控之研究

Abstract

本研究探討在合成反鐵磁（Synthetic Antiferromagnet,SAF）結構中，鐵磁層厚度差（Δt）對自旋軌道轉矩（Spin-Orbit Torque, SOT）效率的影響。採用不同厚度組合的Pt-與W基底SAF系統，利用黃光微影、磁控濺鍍等製程製作出微米級霍爾元件，並透過各種電磁量測方法，包括各向異性磁阻（AMR）位移、單向自旋霍爾磁阻（USMR）、振動樣品磁強計（VSM）與縱向磁光克爾效應（L-MOKE）等，系統性分析了SOT效率與結構參數之關聯。研究結果指出，在對稱（Δt ≈ 0）的SAF結構中，可獲得最大之單位電流有效場（H_eff/I），顯著提升SOT效率，並且有效抑制淨磁化與偶極場。同時，W基SAF系統由於其較大的負自旋霍爾角，展現出更高的USMR與SOT效應。此外，與單層結構比較後，雖然SAF可提供熱穩定性與低功耗操作優勢，但在高電流下的USMR強度略低，顯示結構對稱性與厚度不均將影響自旋電流分佈與界面散射機制。本研究提供具體實驗證據，證明鐵磁層厚度設計在提升SOT-MRAM元件效能中的關鍵角色，並為次世代自旋電子記憶體之結構優化提供實用準則。此研究評估了場型自旋軌道轉矩（field-like spin-orbit torque, FL-SOT）之效率，以更深入解析其對轉矩機制的貢獻。透過 AMR Loop-shift所導出的單位電流有效場分析，可觀察 Pt-與 W-基 SAF 結構在鐵磁層厚度與結構不對稱變化下的 FL-SOT 效率趨勢。其中，對稱結構（Δt = 0）具備最佳 FL-SOT 效率，歸因於其具備平衡之自旋電流吸收能力。此結果凸顯出鐵磁層厚度設計能提升對場型轉矩效率之優化具關鍵意義，對於低功耗自旋電子元件的設計具有重要啟示。

This thesis investigates the impact of ferromagnetic (FM) layer thickness asymmetry (Δt) on spin-orbit torque (SOT) efficiency in synthetic antiferromagnet (SAF) structures. Pt-based and W-based SAF devices with varying FM thicknesses were fabricated using photolithography and magnetron sputtering. Multiple characterization techniques, including anisotropic magnetoresistance (AMR) loop-shift, unidirectional spin Hall magnetoresistance (USMR), vibrating sample magnetometry (VSM), and longitudinal magneto-optical Kerr effect (L-MOKE), were employed to evaluate the relationship between structural design and spin-torque efficiency. The results demonstrate that symmetric SAF configurations (Δt≈0) yield the highest effective field per current (H_eff/I), maximizing SOT efficiency while minimizing net magnetization and dipolar fields. W-based SAFs, with larger negative spin Hall angles, exhibit stronger USMR and SOT responses than their Pt-based counterparts. Compared to single-layer systems, SAF structures provide enhanced thermal stability and lower power operation; however, slightly reduced USMR magnitude at high current suggests that spin-current distribution and interface scattering are affected by layer symmetry and thickness mismatch. Overall, this work provides experimental evidence of the critical role played by FM thickness engineering in optimizing SOT-driven switching performance and offers practical guidelines for future high-efficiency SOT-MRAM device design. The role of field-like spin-orbit torque (FL-SOT) efficiency provides further insight into spin-torque dynamics. By analyzing the current-induced effective field obtained from AMR loop shifts, both Pt-based and W-based SAF structures were found to exhibit distinct trends in FL-SOT efficiency as a function of ferromagnetic layer thickness and structural asymmetry. Notably, symmetric SAF configurations (Δt = 0) achieved the highest FL-SOT efficiency due to balanced spin-current absorption. This highlights the importance of optimizing FM thickness design not only for maximizing damping-like torque but also for enhancing FL-SOT performance in energy-efficient spintronic devices.