具垂直異向性鈷鐵硼合成亞鐵磁體薄膜的鐵磁共振研究

Ferromagnetic resonance study on the perpendicular magnetic anisotropic CoFeB synthetic ferrimagnet

陳友生(You-Sheng Chen)

指導教授 ： 吳文方

共同指導教授 ： 林昭吟

國立臺灣大學/工學院/機械工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.00712

摘要

由於其獨特的磁特性，鈷鐵硼合金薄膜近年來引起了很大的注意。其獨特之處在於同時擁有很高的穿隧磁阻效應、垂直磁異向性以及軟鐵磁性。這些特性使得鈷鐵硼有機會應用於各樣的自旋電子元件，例如自旋力矩翻轉式記憶體(Spin transfer torque RAM, STT-RAM)、自旋力矩傳遞式共振器(STT-oscillator)以及高敏感度磁感應器。然而，對於具垂直磁異向性的氧化鎂/鈷鐵硼/非磁性金屬多層結構而言，鈷鐵硼隣近層的厚度、成長順序等都對於其磁特性有強烈的影響。這帶來了兩個問題：其一是這樣的影響力常被認為是由於隣近層在鈷鐵硼上產生的界面非均質性，而一般的靜態量測方法並不能夠量化地描述這樣的非均質性；其二是，這樣高度製程相關的特性變化，如果要合成鐵磁多層膜組成的磁性記憶體，那我們對於其中各層的特性的瞭解將會十分困難。由於一般的磁性量測方法並不能夠分離各鐵磁層的個別貢獻，在自旋電子元件的設計中，仍然只能使用單層鐵磁層結構的特性做為估算多層膜特性的依據。在本研究中，我們使用鐵磁共振的方法，以動態的自旋響應去研究鈷鐵硼的多層結構。企圖能提供更準確的數據以利於元件的設計。鐵磁共振，是一種對磁異向性以及非均質磁性很敏銳的動態量測方法。同時也能分別偵測到多層結構中的各層的特性。
本研究的步驟與結果分為以下五個部份。首先我們研究含單層鐵磁層的鉭/鈷鐵硼/鉭結構，藉由變換磁性層以及覆蓋層厚度的方式去研究鈷鐵硼/鉭界面上造成的元素混合帶來的非均質特性。接者，研究氧化鎂/鈷鐵硼/鉭以及鉭/鈷鐵硼/氧化鎂兩種不同成長順序的結構，結果顯示不同成長順序會同時影響鈷鐵硼的結晶、界面混合程度與垂直異向性。第三，我們研究了不含氧化鎂的合成亞鐵磁結構（鉭/鈷鐵硼/鉭/鈷鐵硼/鉭），鐵磁共振的結果指出中間鉭層的厚度會同時大幅影響其上層與下層的鈷鐵硼的磁化強度。第四是研究含有氧化鎂層並具有垂直異向性的合成亞鐵磁結構，即氧化鎂/鈷鐵硼/鉭/鈷鐵硼/氧化鎂。在這系列的實驗中，我們觀察到了不尋常的正交磁異向性以及下層鈷鐵硼的磁化強度大幅減少。最後，也是第五部份是使用釕替換鉭做為中間層。比較了含釕與鉭的亞鐵磁結構之後，我們發現含釕的結構中其下層鈷鐵硼有較低的磁化強度減少量，以及鈷鐵硼較低的結晶特性造成了其磁異向性的差別。
總結以上實驗成果，我們的鐵磁共振研究證實了界面混合及鈷鐵硼結晶的程度是受制於鈷鐵硼的鄰近金屬層的厚度及鍍膜順序。另一個重要發現是，鐵磁共振法有效率地偵測到合成亞鐵磁體中各層鈷鐵硼的磁異向性與半結構中單層鈷鐵硼特性確實有很顯著的差異，因此用上下兩個半結構的特質去預測完整結構的性質並非正確之舉。我們的研究結果顯示，鐵磁共振法可以應用在未來自旋電子元件設計前的特性測試。

關鍵字

磁性薄膜 ； 鐵磁共振 ； 垂直異向性 ； 鈷鐵硼

並列摘要

CoFeB alloy thin film has attracted a lots of attention because its’ unique magnetic properties. Together with high tunneling magnetoresistance ratio (TMR), perpendicular magnetic anisotropy (PMA) and low magnetic damping, CoFeB (CFB) is a good candidate to be applied in spintronic devices, such as spin-torque-transfer (STT) memory devices, STT oscillator and high sensitive magnetic sensors. However, PMA of this kind of MgO/CFB/normal-metal structure is highly dependent on synthesis process. This problem lead to two issues: First, interfacial inhomogeneity is an important factor, but common used static methods are not sensitive to magnetic inhomogeneity. This makes it hard to discuss the mechanism of anisotropy resulted from different synthesis condition. Secondly, this highly process dependency makes it hard to predict the magnetic properties of each CFB layer in synthetic ferrimagnet (SyF) structure. Up to now, most design of SyF structures is still based on the single layer characterization.
In this work, ferromagnetic resonance (FMR) is utilized to study CFB thin film system dynamically to identify the accurate magnetic anisotropy, the inhomogeneity and the magnetization of each layer in SyF structures. Research steps and related results are composed as follow. First part, single layer CFB with capping and buffer Ta layers are studied, revealing that the interfacial inhomogeneity of CFB/Ta depends on the thickness of CFB. Second part is a study on MgO/CFB/Ta and Ta/CFB/MgO structures with PMA. FMR results show the difference in high order magnetic anisotropy and interface mixture between these two structures with different sequence of stacking. Third part is a study on CFB SyF without MgO. Our results show a strong influence of spacing Ta layer on both top and bottom CFB layer yielding different anisotropy and magnetization. Forth part contains results of MgO/CFB/Ta/CFB/MgO with PMA in various thickness of Ta-layer, showing an orthogonal configuration and a significant difference of magnetization between two CFB layers. Last part is the study on MgO/CFB/Ru/CFB/MgO. Comparison of Ru and Ta demonstrates a different rate of magnetization-suppression and a different degree of crystallization, which are the main reasons for different magnetic anisotropy in two CBF layers.
In conclusion, our study shows the method of FMR is an effective way to identify the interfacial inhomogeneity and crystallization of CFB, influenced by the type and thickness of neighbor metal layer and the order of deposition. Furthermore, FMR detects a significant difference of magnetic anisotropy between half MgO/CFB/Ta structures and full SyF. These results could be applied for the pre-testing of magnetic multilayeres for the future design of spintronic devices.

並列關鍵字

magnetic thin film ； Ferromagnetic resonance ； Perpendicular anisotropy ； CoFeB

參考文獻

1. Baibich, M.N., et al., Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices. Physical Review Letters, 1988. 61(21): p. 2472-2475.

2. Fert, A., Nobel Lecture: Origin, development, and future of spintronics. Reviews of Modern Physics, 2008. 80(4): p. 1517-1530.

3. Dedkov, Y.S., et al., Growth and spin-resolved photoemission spectroscopy of the epitaxial α-Al[sub 2]O[sub 3]/Fe(110) system. Applied Physics Letters, 2002. 81(14): p. 2584.

4. Faure-Vincent, J., et al., High tunnel magnetoresistance in epitaxial Fe/MgO/Fe tunnel junctions. Applied Physics Letters, 2003. 82(25): p. 4507.

5. Yuasa, S., et al., Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions. Nat Mater, 2004. 3(12): p. 868-71.