磁性異質結構中自旋矩引發單向磁阻之研究

Study of Spin-Torque Induced Unidirectional Magnetoresistance in Magnetic Heterostructures

張庭瑀(Ting-Yu Chang)

指導教授 ： 白奇峰

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202102056

摘要

磁阻效應 (Magnetoresistance) 描述在不同磁化量方向其縱向電阻的變化。至今為止，磁阻效應被廣泛研究並應用在硬碟 (hard-disk drive) 以及次世代磁阻式隨機存取記憶體 (next-generation magnetoresistive random access memory, MRAM) 上，例如巨磁阻 (GMR) 以及穿隧磁阻效應 (TMR)。近年來，人們發現自旋霍爾效應也能產生磁阻。這些磁阻不僅能用來表徵電荷流—自旋流轉化效率，同時也能用來作為磁性元件的讀取機制。這些磁阻分別為自旋霍爾磁阻 (SMR) 和單向磁阻 (UMR)。自旋霍爾磁阻描述透過吸收或反射自旋霍爾效應感應出的自旋流來造成層膜異質結構的電阻變化，而單向磁阻則是描述來自於自旋相關散射和磁振子-電子散射所造成的電阻變化。雖然單向磁阻可由上述兩種效應解釋，但其理論仍有缺陷。在這篇論文中，我通過施加大範圍的電流與外加磁場進行電流依賴及磁場依賴之單向磁阻量測，發現在鎢鈷鐵硼雙層異質結構中具有很大的單向磁阻值，其值大於過去研究中在室溫的金屬磁性異質結構中所量測到的單向磁阻。我也發現在大電流區間下有額外的機制出現並影響單向磁阻，而此機制與自旋霍爾磁阻一樣，與金屬磁性異質結構的阻尼似自旋矩轉移轉換效率 ( ) 有關。接著我進一步利用宏自旋模擬(macrospin simulation)驗證自旋霍爾感應的自旋矩與單向磁阻的關聯性，其說明了額外的單向磁阻是來自於磁化量的偏移。因此對於此額外的單向磁阻，我暫時將其稱之為自旋矩轉移單向磁阻(STT-UMR)。除此之外，藉由模擬及實驗結果，我可以估算阻尼似自旋矩轉移轉換效率。除此之外，我也進行電致磁化翻轉量測，發現阻尼似自旋矩轉移轉換效率的趨勢與藉由模擬和實驗數據估算出來的值相差不大。因此，我的研究成果說明了單向磁阻與自旋矩轉移之間強烈的關聯性，其對於探討單向電阻的來源以及應用可行性是有幫助的。

關鍵字

自旋霍爾效應 ； 自旋矩轉移 ； 磁性異質結構 ； 磁阻 ； 微米尺寸元件

並列摘要

Magnetoresistance (MR) interprets the variation of the longitudinal resistance depending on the orientation of the magnetization. The MR effects, such as giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR), are extensively studied and applied to the hard-disk drive and next-generation magnetoresistive random access memory (MRAM). Recently, spin Hall effect (SHE) is also found to generate some novel MR effects. These MR effects can not only be used to characterize the charge-to-spin conversion efficiencies in different multilayer heterostructures but also act read-out mechanisms for magnetic device. These novel MR effects are the spin Hall MR (SMR) and the unidirectional MR (UMR), respectively. The mechanism of SMR describes the variation of the longitudinal resistance for the multilayer heterostructures caused by the absorption or the reflection of the SHE-induced spin current, while the mechanism of UMR depicts the change of the longitudinal resistance caused by the spin-dependent scattering and the electron-magnon scattering. However, although it can be explained by the two effects mentioned above, an universal theory of UMR is still lacking. In this thesis, I discover a giant UMR ratio UMR/Rxx for the W/CoFeB bilayer heterostructures by performing the current-dependent and field-dependent UMR measurements with using a broad range of the current and external magnetic field. The UMR/Rxx for the W/CoFeB samples is much larger than those for the metallic magnetic heterostructures at the room temperature in previous studies. I observe the existence of an extra UMR modifying the UMR at the high current regime. Moreover, I find this additional UMR is related to the damping-like spin-torque (DL-ST) efficiency of the magnetic heterostructure, which is similar to the SMR case. To confirm the correlation between SHE-induced ST and UMR, I carry out the macrospin simulations, which reveals that the additional UMR originates form the tilting of the FM magnetization. Therefore, for this additinoal UMR, I tentatively call it spin-torque-transfer UMR (STT-UMR). Besides, of the measured samples can also be determined by incorporating the experimental data and the simulation results. Furthermore, I also perform the current-induced magnetization switching measurement to estimate of the measured samples, suggesting the results from the current-induced magnetization switching is fairly consistent with those from the experimental and simulation results. The finding suggests a strong correlation between UMR and STT, which supports to get insight into the origin and the application feasibility of giant UMR.

並列關鍵字

spin Hall effect ； spin-torque transfer ； magnetic heterostructure ； magnetoresistance ； micro-sized device

參考文獻

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