垂直式人工反鐵磁MgO/CoFeB/Ru/CoFeB/MgO與溫度相關的磁特性分析

The temperature dependent magnetic characterization of Perpendicular magnetic synthetic antiferromagnetic MgO/CoFeB/Ru/CoFeB/MgO

謝秉儒(Bing-Ru Xie)

指導教授 ： 陳恭

國立中正大學/理學院/物理學系/碩士(2015年)

摘要

過去眾多的研究報告已經告訴我們，由於Fe-O之間的介面效應，CoFeB/MgO的薄膜結構擁有很強的垂直磁異向性。但是，當Ru作為保護層時垂直磁異向性會消失。在我們所做的這項研究裡，我們製作具有對稱結構的MgO/CoFeB/Ru/CoFeB/MgO薄膜，並發現如果Ru厚度夠薄的話會具有很強的垂直磁異向性。當將樣品放置於低溫下測量之後，我們發現存在一個額外的類似體積晶格異向性的項，以致於整體的異向性是由介面異向性(interface anisotropy)和體積異向性(bulk anisotropy)之間的競爭來決定其大小。我們的論文分成幾個部分逐步加以討論如下。
1.從我們過去所做的Ta/MgO/CoFeB(x)/Ru、Ta/MgO/CoFeB/Ru(x)磁滯曲線實驗裡，探討CoFeB/Ru介面對異向性的影響，發現Ru金屬層厚度對異向性有很大的影響。再將雙層結構CoFeB/Ru(x)/CoFeB 與單層結構做比較，進一步觀察到雙層結構中Ru厚度在0.6~3.7 都保持著垂直異向性，尤其在Ru小於1奈米的時候，垂直異向性比單層高出許多。
2.從過去改變磁性金屬層厚度的論文裡(CoFeB(x)/Ru(0.7)/CoFeB)，可以估算雙層結構的死磁層與磁化強度的結果，將其結果用來計算CoFeB/Ru(x)/CoFeB磁滯曲線的耦合能，並探討水平異向性與垂直異向性耦合能的差異。並了解耦合能隨著Ru厚度不同的震盪現象對異向能數值造成的影響。
3.低溫下的現象無法由單軸的模型表示，因此我們將人工反鐵磁的Stoner-Wohlfarth單軸基本模型再加入體積立方(雙軸)異向能，用來模擬低溫下MgO/CoFeB/Ru(0.7)/CoFeB/MgO磁滯曲線，試著說明降溫對人工反鐵磁能量變化的影響，並解釋其現象。

關鍵字

垂直異向性

並列摘要

Previous study has shown that ultrathin CoFeB/MgO based structure has strong perpendicular magnetic anisotropy due to Fe-O interface effect. However, the PMA disappears when Ru is used for the protection layer. In this study, we fabricated a symmetrical structure of MgO/CoFeB/Ru/CoFeB/MgO and found that PMA recovers if Ru thickness is thin enough.After detailed Ru-thickness and temperature-dependent measurements,we found an extra bulk crystalline anisotropy term and the overall anisotropy is determined by the competition between the interface anisotropy and the bulk anisotropy. In this regard, we are divided the thesis into several sections as below:
1. From our previous study on the hysteresis curves of MgO/CoFeB/Ru(x)/CoFeB/MgO, we found that the thickness of Ru metal layer has a great impact on the anisotropy. Moreover, the Ru thickness in this structure has a wider range (0.6 - 3.7 nm)to maintain the high PMA. Especially, a much higher PMA is observed when the thickness of Ru is lower than 1 nm.
2. From the results of the change of the magnetic layer thickness, we are able to analyze precisely the magnetic dead layer and magnetization of the ferromagnetic layers. These results are used to calculate the coupling strength of the exchange coupling between ferromagnetic layers.
3. The measurements at low temperature (10 – 300 K) shows a magnetic transition of the easy axis occurs. In the temperature range from 200 K – 300 K, the easy axis is along the perpendicular direction, while in the temperature range below 100 K. An intermediate state in the range of 100 K - 200K is also observed. In order to understand this magnetic transition, we perform a model calculation based on single domain macrospin structure. By introducing an extra anisotropy term,i.e. from uniaxial to biaxial, the off perpendicular transition at low temperature can be simulated successfully.

並列關鍵字

MgO/CoFeB/Ru/CoFeB/MgO

參考文獻

[2] N. Wiese, T. Dimopoulos, M. Rührig, J. Wecker, H. Brückl et al. “Antiferromagnetically coupled CoFeB/Ru/CoFeB trilayers” Appl. Phys. Lett. 85, 2020 (2004)

[3] S. S. P. Parkin, N. More,and K. P. Roche ”Oscillations in exchange coupling and magnetoresistance in metallic superlattice structures: Co/Ru,Co/Cr,and Fe/Cr”Phys. Rev. Lett. 64, 2304 (1990).

[4] P. J. H. Bloemen, H. W. van Kesteren, H. J. M. Swagten, and W. J. M. de Jonge, “Oscillatory interlayer exchange coupling in Co/Ru multilayers and bilayers” Phys. Rev. B, 50, 13505 (1994).

[6] S. S. P. Parkin,” Systematic Variation of the Strength and Oscillation Period of Indirect Magnetic Exchange Coupling through the 3d, 4d, and 5d Transition Metals Phys. Rev. Lett. 67 (25), 3598-3601 (1991).

[7] Soo Young Jang, Chun-Yeol You, S. H. Lim, and S. R. Lee,“Annealing effects on the magnetic dead layer and saturation magnetization in unit structures relevant to a synthetic ferrimagnetic free structure” J. Appl. Phys. 109, 013901 (2011).