臺灣博碩士論文加值系統

由於磁紀錄媒體的高密度需求，鐵磁材料的垂直異向性變成一個很重要的研究方向，以往研究例如Co/Pt等多層膜雖有良好的垂直異向性，但是其磁阻率和成本皆不利於元件在市場上的優勢。近來有研究利用MgO-CoFeB-Ta介面產生的影響，使CoFeB的易磁化軸垂直於膜面，本實驗主要利用濺鍍(sputter)成長人工反鐵磁結構MgO(1nm)/CoFeB(1.2nm)/Ta(x nm)/CoFeB(1.2nm)/MgO(1nm)來探討其垂直異向性和磁性耦合的變化，發現隨Ta厚度變化，磁矯頑場有震盪的現象，顯示出CoFeB之間的耦合會隨著Ta夾層的厚度有平行及反平行的變化，據我們了解，這是文獻中第一次在垂直式超薄磁性膜中觀察到所謂RKKY (Ruderman-Kittel-Kasuya-Yosidan) 的耦合現象。這些薄膜系統在變溫的量測更呈現豐富的磁性變化，我們觀察到溫度降低到10K時，Hc從200 Oe增大至2200 Oe，因此變溫時除了耦合能外，異向能的變化並不能被忽略，導致反鐵磁系統在低溫和高溫翻轉的鐵磁層不相同。另外，我們成長MgO(1nm)/CoFeB(1.2nm)/Ru(3nm)/CoFeB(1.2nm)/MgO(1nm) 多層膜，發現在低溫時也具有垂直異向性，表示單層結構Ru-CoFeB-MgO介面也可能具有垂直異向性的，或者有可能是因為耦合作用造成易磁化軸出現在於垂直方向。

The perpendicular magnetic anisotropy has become a very important research field because the applications on high density magnetic recording media and magnetic random access memory. A recent report has revealed that CoFeB-MgO based structure may change the magnetic easy axis from in plane to the perpendicular direction if the thickness reduces less than ~1.5nm. In this thesis, we fabricated a synthetic antiferromagnetic structure, based on the perpendicular CoFeB-MgO structure, and did detailed magnetic characterizations of these novel multilayered structures. The thesis is divided into three parts:

1. Fabrication of MgO(1 nm)/CoFeB(1.2 nm)/Ta(x nm)/CoFeB(1.2 nm)/MgO(1 nm), 0.5< x< 3, and study the magnetic coupling between CoFeB layers at room temperature: We found that these films all show perpendicular magnetic anisotropy due to the stabilization of MgO interface. The magnetic coupling between two CoFeB layer through Ta is also observed. The magnetic coupling between CoFeB shows RKKY type oscillation with a period ~ 1.3 nm. Base on the best our knowledge, this RKKY oscillation of the magnetic coupling in perpendicular ultrathin films is the first time reported.

2. Extend the magnetic measurements as function of temperature within the range 10 K - 350 K: The magnetization hysteresis curves show strong temperature dependence. The coercivity enhances to > 2 kOe at 10 K relative to ~ 20 Oe at room temperature indicating a large change of the anisotropy strength (K). The coupling strength also shows a weaker temperature dependence and the associated exchange bias field (J) is ~ 100 Oe. The most striking observation is that a spin - flip to spin - flop transition has been identified at ~ 200 K at which K/J ~ 1.

3. Fabrication of MgO(1 nm)/CoFeB(1.2 nm)/Ru(x nm)/CoFeB(1.2 nm)/MgO(1 nm) and testing the nonmagnetic spacer effect: As a comparison, we also fabricated and characterized a series of MgO(1 nm)/CoFeB(1.2 nm)/Ru(x = 2.5, 3, and 3.5 nm)/CoFeB(1.2 nm)/MgO(1 nm) structures. We found that either no PMA or very weak PAM was observed at room temperature. However, at low temperature, a transition from superparamagnetic state to ferromagnetic state is observed and the blocking temperature of the structure is ~ 150 K.

第一章 序論 1
第二章 理論背景 3
2.1 垂直異向性(Perpendicular Magnetic Anisotropy, PMA) 3
2.2 反鐵磁耦合與RKKY理論 5
2.3 Stoner Wohlfarth Model 9
2.4 Spin-Flip and Spin-Flop Transitions 13
第三章 實驗儀器與實驗步驟 15
3.1 濺鍍系統 15
3.1.1樣品製作 15
3.1.2濺鍍原理 16
3.1.3注意事項 17
3.2 物理性質量測系統(Physical Properties Measurement System, PPMS) 17
3.3 實驗流程與步驟 19
第四章 實驗結果與討論 20
4.1 MgO/CoFeB/Ta(x)/CoFeB/MgO室溫磁性量測 20
4.2 MgO/CoFeB/Ta(x)/CoFeB/MgO磁性與溫度的關係 26
4.3 MgO/CoFeB/Ru/CoFeB/MgO 磁性測量: 以 Ru 取代 Ta 之比較 34
第三章 結論 40
第五章 參考文獻 42

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