可調變垂直磁異向性亞鐵磁絕緣體銩鐵石榴石薄膜的濺鍍成長及磁化翻轉研究

本論文建立銩鐵石榴石Tm3Fe5O12薄膜的成份檢測，以用來調查薄膜成份以及長晶參數的關聯和最佳化Tm3Fe5O12薄膜。根據一系列的成分分析，我們為第一個團隊報導使用離軸濺鍍的方式成功成長高品質的Tm3Fe5O12薄膜。此Tm3Fe5O12薄膜除了擁有非常平整的表面和優異的結晶性，也展現出我們期待的垂直磁異向性。從鐵磁共振的譜形可以得知我們薄膜的共振半高波寬小於Tm3Fe5O12塊材的數值，我們也使用變頻鐵磁共振第一次報導Tm3Fe5O12薄膜的磁性阻尼係數:在垂直外場下為0.133，在平行膜面外場下為0.146。除此之外，我們探討不同Tm3Fe5O12成分對於薄膜的結構和磁性性質的影響: 我們發現垂直膜面的晶格常數及平行膜面的應變和薄膜中的Tm和Fe比例有線性的關係；我們進一步的展示利用調變薄膜的成分可以控制薄膜的磁性性質，包含飽和磁化強度和矯頑磁場，其中特別的是我們可以靠調變薄膜成分來提升薄膜的應變，使得垂直磁異向場增強為170%。為了有效利用Tm3Fe5O12垂直磁異向性和電性絕緣的特性，我們執行Pt/ Tm3Fe5O12的電流引致的磁化翻轉研究。我們檢測在Pt及Tm3Fe5O12的介面中擁有優異的自旋傳輸性質，而自旋混合電導的實部和虛部被量測為1.1×1015 Ω-1m-2 和1.2×1014 Ω-1m-2。而高效率的力矩效率被決定為 97±0.1 Oe 每1.88×106 A/cm2電流密度。當中最重要的是此磁化翻轉過程只需要使用2.5×106 A/cm2的低電流密度和5 Oe極小的外加平行膜面磁場即可達成，小於其他團隊已報導的數值，而這個原因可能是來自於我們調控薄膜成分來降低Tm3Fe5O12薄膜的矯頑磁場，或是說控制薄膜的垂直磁異向性強度，進而降低翻轉磁化所需的電流密度。我們的工作提供Tm3Fe5O12薄膜濺鍍成長和材料性質關鍵性的研究，並展示Pt/ Tm3Fe5O12高效率的磁化翻轉實驗，更重要的是我們對於Tm3Fe5O12薄膜磁性性質的調變可以用來使磁化翻轉的電流密度進一步的降低，支持我們離軸濺鍍成長的Tm3Fe5O12薄膜成為對於自旋電子學應用的重要材料。

關鍵字

磁性絕緣體；銩鐵石榴石；垂直磁異向性；磁化翻轉；濺鍍

並列摘要

In this work, film composition analysis was established to study the relation of the film composition and growth parameters for optimization of TmIG films. We were the first group reporting to apply off-axis sputtering to successfully grow high-quality TmIG films on GGG(111) substrates. Sputtered TmIG films possessed the flat surface morphology with the roughness of ~0.2 nm and the excellent crystallinity, and exhibited PMA. Narrow linewidth ΔH of 100 Oe of sputtered TmIG film in the FMR spectrum is comparable to the single crystal bulk value of 135 Oe, and the magnetic damping was first reported to be 0.133 and 0.146 under out-of-plane and in-plane fields. We further investigated the influence of film composition on structural and magnetic properties. Out-of-plane lattice constants and in-plane film tensile strains varied linearly with respect to the Tm/ Fe ratio. Remarkably, we demonstrated the tunability of magnetic properties such Ms and Hc by manipulating the film composition in TmIG films and perpendicular anisotropy field was enhanced by increasing the in-plane tensile film strain, verifying that the magnetostriction effect contributes to this PMA. In order to exploit the advantage of PMA and the electrically insulating nature of TmIG films, efficient current-induced magnetization switching in Pt/TmIG via SHE was performed. Excellent spin transport at the interface of Pt and TmIG was characterized with spin mixing conductance real part Gr of 1.1×1015 Ω-1m-2 and imaginary part Gi of 1.2×1014 Ω-1m-2 and high damping-like torque efficiency was determined as 0.97±0.1 Oe per current density jc of 1.88×106 A/cm2. Significantly, the magnetization reversal process by SOT was achieved by a low critical current density jc of 2.5×106 A/cm2 and an exceptionally low in-plane field of 5 Oe, lower than the other reported values. Our low current density can be mainly attributed to the smaller coercive field in the AHE hysteresis loop with a small magnetization in our TmIG films. This work provides the vital investigation about sputtered TmIG film properties and sputtering film growth for the magnetic community. Importantly, our demonstration for manipulation of magnetic properties in TmIG was able to reduce the threshold current density for efficient magnetization switching. It supports that our high-quality sputtered TmIG films with tunable magnetic properties provide a new route to tailoring rare earth iron garnet films for spintronic application.