奈米FePt複合材料多層膜之磁性質與顯微結構研究

Study of magnetic properties and microstructures of nanocomposite FePt multilayer films

陳勝吉(S.C.Chen)

指導教授 ： 郭博成

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2004年)

https://doi.org/10.6342/NTU.2004.10294

摘要

摘要
本研究以直流與射頻磁控濺鍍的方式鍍製Ti及MgO底層於康寧玻璃或矽基板上，並採用高磁晶異向性之FePt薄膜為磁性記錄層，以形成Ti/FePt、MgO/FePt/MgO、MgO/FePt/Ag及MgO / (FePt/Ag)5多層膜結構，探討不同底層及製程參數對奈米FePt複合材料多層膜之顯微結構、磁性質及其易磁化軸[001]方向之影響。
從X光繞射及磁性量測的分析顯示，Ti底層會促進FePt磁性層產生(111)之從優取向，且FePt薄膜的晶粒大小、有序化程度及頑磁力Hc均會隨著Ti底層厚度的增加而變大。不同Ti底層厚度之Ti/FePt合金薄膜同樣以600℃之基板溫度濺鍍300 nm的FePt磁性層時，在Ti底層厚度為100 nm可獲得較佳的平行膜面磁性質，其平行膜面頑磁力Hc//約為7.3 kOe，飽和磁化量Ms為672 emu/cm3，平行膜面角形比S//約為0.8。
關於MgO底層的製作，我們發現MgO底層(200)平面之X光繞射峰的強度可藉由濺鍍時引入N2氣體而提高，在N2流量比為N2 : Ar = 2 : 5時有最高的MgO(200)繞射峰強度，此時MgO底層會有較佳的結晶性，而其晶格常數約為4.255 Å。若再經600℃的基板溫度加熱後，由於應力的釋放，MgO底層之晶格常數可降為4.198 Å，如此有利於降低MgO底層(200)與FePt磁性層(001)在介面上的不匹配度。
研究結果顯示，較薄之MgO底層(≦5nm)能促使(111)從優取向之FePt磁性層結晶方位發生改變，而過厚的MgO底層則無此效果。此外由於高溫下Ag原子可擴散至FePt的晶界，使晶界之應變能大幅提高，因此以Ag為保護層之MgO/FePt雙層薄膜系統除可提升FePt磁性層之垂直異向性外，也有助於FePt頑磁力的提高，因而MgO 5nm/FePt 20nm/Ag 5nm多層膜在400℃的低基板溫度下退火30 min，其Hc//值可達3923 Oe。
另外，我們發現將(FePt 4nm/Ag 2nm)5多層膜成長於5 nm之MgO底層，經600℃基板溫度退火30 min後，可獲得(001)從優取向之FePt薄膜，此時垂直膜面頑磁力Hc⊥為2462 Oe。引入Ag緩衝層(≦20nm) 於(FePt 4nm/Ag 2nm)5多層膜與MgO底層之間，可進一步提高(FePt 4nm/Ag 2nm)5多層膜之垂直異向性及頑磁力。從高解析之橫截面晶格影像發現，在沒有Ag緩衝層的情況下，FePt磁性層與MgO底層間會形成較低應變能的半整合性介面，而當引入Ag緩衝層時，FePt磁性層與Ag緩衝層則會形成較高應變能的整合性介面，因此引入Ag緩衝層會促使fcc

關鍵字

磁性質 ； 鐵鉑多層膜

並列摘要

Abstract
Ti and MgO underlayer are deposited on corning glass and Si substrate by dc and rf magnetron sputtering, then deposited high magnetocrystalline anisotropy FePt films as magnetic layer on the underlayer. The effects of various underlayers and process parameters on the microstructures, magnetic properties, and easy axis of nanocomposite Ti/FePt, MgO/FePt/MgO, MgO/FePt/Ag, and MgO/(FePt/Ag)5 multilayer films are investigated.
X-ray diffraction and Magnetic properties measurements indicate that the Ti underlayer can promote (111) preferred orientation of FePt magnetic layer. The average grain size, degree of order, and in-plane coercivity (Hc∥) of FePt layer are increased with increasing the Ti underlayer thickness. When the 300 nm thick FePt magnetic layer is deposited on corning glass with substrate temperature of 600℃, the optimum in-plane magnetic properties of the Ti/FePt film are obtained as introducing 100 nm thick Ti underlayer. This Ti/FePt film has Hc∥of 7.3 kOe, saturation magnetization (Ms) of 672 emu/cm3, and its in-plane squareness (S∥) is about 0.8.
Intensity of the (200) peak in the X-ray diffraction pattern of MgO underlayer increases as introduction of N2 gas during deposition of MgO. The maximum intensity of the MgO (200) peak is achieved at N2 flow rate ratio of N2 : Ar = 2 : 5. This MgO underlayer has better crystallinity and its lattice parameter is 4.255 Å. After annealing at substrate temperature of 600℃, the lattice parameter of MgO underlayer will decrease to about 4.198 Å due to stress relief. This is beneficial for reducing interfacial misfit between (200) of MgO underlayer and (001) of FePt magnetic layer.
The (111) preferred orientation of FePt magnetic layer can be changed by introducing a thinner MgO underlayer (≦5nm). Furthermore, Ag atoms can diffuse into FePt grain boundary at higher temperature, which will increase greatly grain boundary energy. Therefore, both the perpendicular anisotropy and coercivity of FePt layer increase by using a Ag capped layer on the MgO/FePt double layer films. A in-plane coercivity of 3923 Oe can be obtained from MgO 5nm/FePt 20nm/Ag 5nm films even heated at a lower substrate temperature of 400℃ for 30 min.
The FePt films with (001) preferred orientation and out-plane coercivity (Hc⊥) of about 2462 Oe can be achieved by stacking structure of (FePt 4nm/Ag 2nm)5 multilayer films deposited on the 5 nm thick MgO underlayer after annealing at substrate temperature of 600℃for 30 min. It is found that both the perpendicular anisotropy and coercivity of (FePt 4nm/Ag 2nm)5 multilayer films are enhanced by introducing a Ag buffer layer (≦20nm) between the (FePt 4nm/Ag 2nm)5 films and MgO underlayer. The high-resolution cross-sectional lattice image shows that the interface between FePt magnetic layer and MgO underlayer is a semi-coherent interface, which has a lower strain energy. However, as introducing Ag buffer layer between FePt magnetic layer and MgO underlayer, a coherent interface associated with higher strain energy is obtained at interface between FePt magnetic layer and Ag buffer layer. Therefore, introduction of Ag buffer layer between FePt magnetic layer and MgO underlayer will promote the formation of hard magnetic fct

並列關鍵字

FePt multilayer films ； Magnetic properties

參考文獻

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