MgO/CoFeB/Nb(or Ru)/CoFeB/MgO 結構之交換耦合及垂直異向性探討

Study the interlayer exchange coupling in MgO/CoFeB/Nb(or Ru)/CoFeB/MgO structure with perpendicular anisotropy

鄭琮譯(Tsung-I Cheng)

指導教授 ： 陳恭

國立中正大學/理學院/物理學系/碩士(2012年)

摘要

本論文探討利用濺鍍系統(Sputtering System)成長垂直式人工反鐵磁結構，結構主要分成兩種：
1. Ta(10)/MgO(1)/Co20Fe60B20(1.2)/Nb(x1)/Co20Fe60B20(1.2)/MgO(1) /Ta(1)，x1＝0.5~3.5，單位：nm。
2. Ta(10)/MgO(1)/Co20Fe60B20(1.2)/Ru(x2)/Co20Fe60B20(1.2)/MgO(1) /Ta(1)，x2＝1~4，單位：nm。
並且在人工反鐵磁結構退火前、後，探討其磁異向性和磁性耦合的變化。
在MgO(1)/CoFeB(1.2)/Nb(x1)/CoFeB(1.2)/MgO(1)結構中，我們發現樣品於退火後，磁化易軸偏向垂直膜面方向，且隨著Nb厚度增厚，兩個鐵磁層間的耦合會呈現平行與反平行耦合交替震盪的現象，亦即 Ruderman-Kittel-Kasuya-Yasidan (RKKY)的耦合現象。此結構的震盪週期約為1.3nm，最大反鐵磁耦合強度發生在Nb＝1nm，耦和強度J～0.03erg/cm3。此外，為了進一步分析此結構的基本磁性現象，我們將此結構分成上結構Ta(5)/MgO(1)/CoFeB(1.2)/Nb(y1)和下結構Ta(5)/Nb(y2)/CoFeB(1.2) /MgO(1)/Ta(5)，單位：nm。藉由改變頂層(底層)Nb厚度，觀察到在退火後，隨著Nb厚度增厚，上、下結構皆具有垂直磁異向性，上結構之垂直磁異向性有隨著Nb厚度增加而變弱的趨勢，只有在Nb＝1.2~1.8nm之方正性(mr/ms)~1，垂直磁異向性較佳。因此全結構Ta/MgO/CoFeB/Nb/CoFeB/MgO/Ta隨著Nb厚度增厚，磁異向能/交換耦合能的比例逐漸降低。
在MgO(1)/ CoFeB(1.2)/Ru(x2)/ CoFeB (1.2)/MgO(1)結構中，我們發現此結構在退火前為水平磁異向性且兩鐵磁層(CoFeB)間會呈現平行與反平行耦合交替震盪的現象，震盪週期約1.2nm。在熱退火後，中間層Ru＝1.5~3nm的樣品其磁化易軸轉為垂直方向且呈現反鐵磁耦合震盪的現象，震盪週期約為1nm，最大耦和強度發生在Ru~2nm，耦合能J~0.063erg/cm3。

關鍵字

人工反鐵磁 ； 垂直異向性 ； 交換耦合

並列摘要

In this thesis, we fabricate perpendicular synthetic antiferromagnetic structures by sputtering system, basing on the perpendicular CoFeB-MgO structure. The thesis is divided into two parts:
1. Ta(10)/MgO(1)/Co20Fe60B20(1.2)/Nb(x1)/Co20Fe60B20(1.2)/MgO(1) /Ta(1), x1＝0.5~3.5, unit：nm。
2. Ta(10)/MgO(1)/Co20Fe60B20(1.2)/Ru(x2)/Co20Fe60B20(1.2)/MgO(1) /Ta(1), x2＝1~4, unit：nm。
Before and after annealing, we study the variation of the magnetic anisotropy and the interlayer exchange coupling between CoFeB layers in the structures.
We find that these samples all show perpendicular magnetic anisotropy due to the stabilization of MgO interface in MgO(1)/CoFeB(1.2)/Nb(x1)/CoFeB(1.2)/MgO(1) structures after annealing. The interlayer exchange coupling shows Ruderman-Kittel-Kasuya-Yasidan (RKKY) type behavior. The oscillatory coupling period is 1.3nm and the maximum antiferromagnetic coupling strength J~0.03erg/cm3 is observed when the thickness of Nb is 1nm. In order to understand the magnetic characteristic of the full structures, we prepare two half structures: Ta(5)/MgO(1)/CoFeB(1.2)/Nb(y1) (top structure) and Ta(5)/Nb(y2)/CoFeB(1.2)/MgO(1)/Ta(5) (bottom structure). We observed that the perpendicular magnetic anisotropy in top structure showed strong thickness dependence of Nb layer. The magnetic squareness ratio(mr/ms) approachs 1 when the thickness of Nb is in the range of 1.2nm to1.8nm in MgO/CoFeB/Nb top structure. This Ta-layer-thickness dependence may explain the observation of the reduction of magnetic anisotropic energy(Ku)/exchang coupling energy(J) ratio when increasing the thickness of Nb in the structure of MgO/CoFeB/Nb/CoFeB/MgO.
In the structure of MgO(1)/CoFeB(1.2)/Ru(x2)/CoFeB(1.2)/MgO(1) we observe that the magnetic easy axis is in-plane but still with parallel-antiparallel coupling at their as-deposited state. The oscillatory period is 1.2nm between CoFeB layers. The easy axis becomes perpendicular and again show parallel-antiparallel coupling with oscillatory period ~ 1nm in the range of 1.5nm≦Ru≦3nm after annealing. The maximum antiferromagnetic coupling strength J~0.063erg/cm3 which is observed when the thickness of Ru is about 2nm.

並列關鍵字

CoFeB ； antiferromagnetic ； exchange ； perpendicular ； coupling

參考文獻

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