鈷鉑/銥錳雙層膜系統垂直交換偏壓之研究

Study of perpendicular exchange bias in CoPt/IrMn bilayer system

蔡朝伊(Chau-Yi Tsai)

指導教授 ： 林金福

共同指導教授 ： 許仁華(Jen-Hua Hsu)

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.03162

摘要

本論文主要著重於研究將具垂直異向性(perpendicular magnetic anisotropy, PMA)之單層鈷鉑(Co49Pt51)合金薄膜應用於垂直交換偏壓(perpendicular exchange bias, PEB)系統之上。本論文中主要探討之鈷鉑合金薄膜以及鈷鉑/銥錳雙層膜系統皆在全室溫下鍍製，亦即在整個薄膜系統的製備過程中未曾加熱使製程溫度高於室溫。
在本論文的第一部分，藉由研究鉭/鎳鐵/鐵錳(Ta/NiFe/FeMn)水平交換偏壓系統，我們發現引入旋轉式濺鍍(rotational sputtering)技術有利於反鐵磁層之結晶性，並進而提升交換偏壓之性質。在此部分的實驗中，我們不僅成功觀察到明顯之室溫交換偏壓性質，經由進一步最佳化之後，室溫交換偏壓值可高達350 Oe。
在第二部分中，藉由順序引入鉭/鉑兩種底層，我們成功地在全室溫的製程條件下製備出具高垂直異向性之鈷鉑合金薄膜(RT-CoPt)。此外，我們亦以使用L11序化相製程條件之高溫製備鈷鉑合金薄膜HT-CoPt(已被證明具有高垂直異向性)做為對照組，將以上兩者的性質做一比較。兩者的x光繞射圖譜類似，皆可觀察到一明顯之鉑(111)之繞射峰，表示兩者皆具有高度結晶及織構的鉑層。亦證明引入鉭底層可在不做任何加溫及後退火的情況下使鉑層產生不遜於後退火效果之高度結晶與織構。此外，兩者皆由於鈷鉑層厚度太薄而僅能觀察到一接近鉑(111)繞射峰之鈷鉑(222)肩峰。而在HT-CoPt之繞射圖譜中，鉑(111)繞射峰及鈷鉑(222)肩峰皆朝向40度有一偏移，此現象可能肇因於高溫製程引發之交互擴散。透過表面形貌的觀察我們發現RT-CoPt具有非常平坦的表面，均方根表面粗糙度Rrms僅為0.5 nm；反觀HT-CoPt則呈現凹凸不平之表面形貌，均方根表面粗糙度Rrms高達 2.4 nm。磁性質方面，兩者皆呈現高垂直異向性之特性，且矯形比與磁晶異向能常數亦相當接近，亦即兩者在垂直異向性性能在伯仲之間。但在矯頑磁力與翻轉模式上則有較大差異，RT-CoPt應較適用於自旋電子學元件；而HT-CoPt則較適用於磁紀錄媒體。經由進一步磁性質與磁力顯微鏡分析與觀察，我們發現鉭及鉑在全室溫製備鈷鉑之織構與垂直異向性上皆扮演重要角色，缺一不可。
在第三部分中，我們成功地將上述全室溫製備鈷鉑合金薄膜應用在鈷鉑/銥錳雙層膜垂直交換偏壓系統中並成功地在室溫下觀察到垂直交換偏壓現象。值得注意的是，我們亦觀察到幾個不同於傳統交換偏壓系統之現象:一、此垂直交換偏壓是在未經引入任何磁製程之情況下自發產生；二、我們觀察到的是單一偏移磁滯曲線而非如文獻中曾提及在特定自發交換偏壓系統中出現之雙偏移磁滯曲線；三、伴隨交換偏壓出現的是矯頑磁力的下降而非傳統上觀察到之上升。此系統之training effect研究亦指出存在系統中之垂直交換偏壓現象相當穩定。然而再分別將此系統在鍍製時外加一磁場或施以磁後退火後，我們發現垂直交換偏壓值皆有明顯下降。在厚度效應的研究中我們觀察到最高之室溫垂直交換偏壓數值達533 Oe，此時之鈷鉑層厚度為3 nm而銥錳層厚度為10 nm。
而在第四部分中，承襲上一部份，我們發現若在磁製程中施加相反方向之磁場，在鈷鉑/銥錳雙層膜系統之交換偏壓數值上會產生一對稱破壞之現象。亦即在將磁製程之外加場反向後，不只磁滯曲線偏移之方向相反，交換偏壓之數值亦改變。此現象明白顯示出在鈷鉑/銥錳雙層膜系統中存在的一個本質上的對稱破壞現象。在此部分中，我們以晶體結構與自旋結構的角度出發嘗試探討此機制。
最後，為進一步探索此自發性交換偏壓背後之機制，我們引入鐵錳取代銥錳與室溫製備之單層鈷鉑合金薄膜耦合並與鈷鉑/銥錳系統做一比較。鈷鉑/鐵錳系統亦使用與鈷鉑/銥錳系統相同之製程參數並在全室溫且未引入入合磁製程的情況下鍍製。雖然在鈷鉑/鐵錳系統中亦觀察到與鈷鉑/銥錳系統類似之自發性垂直交換偏壓現象，但兩者之磁滯曲線形狀與翻轉模式有明顯不同。前者磁滯曲線傾斜、矯形比低且垂直交換偏壓數值較小而後者磁滯曲線方正、矯形比高且具有交高之垂直交換偏壓數值。此外，兩者交換偏壓之厚度效應行為亦完全不同。所以兩者產生自發性交換偏壓之機制亦很有可能不同。而交換偏壓之機制不同主要可能來自於鐵磁/反鐵磁界面結晶結構抑或自旋結構的不同。藉由穿透式電子顯微鏡及掃描穿透式電子顯微鏡之為結構觀察及分析，我們發現兩者在鐵磁/反鐵磁界面結晶結構上幾乎看不出差別，僅在掃描穿透式電子顯微鏡影像中發現鈷鉑/鐵錳系統之介面相對較為不平整。所以，此二系統交換偏壓機制之不同主要應來自於鐵磁/反鐵磁界面之自旋結構的不同而非結晶結構的不同。

關鍵字

薄膜 ； 濺鍍 ； 垂直異向性 ； 垂直交換偏壓 ； 鈷鉑合金 ； 銥錳 ； 鐵錳

並列摘要

This present work focus on applying single-layered Co49Pt51 alloy thin films with perpendicular magnetic anisotropy(PMA) to a perpendicular exchange bias (PEB) system. Co49Pt51 alloy thin films were fabricated at absolutely room temperature (RT),i.e. without heating up substrates during whole fabricating processes. The PEB system was composed of Co49Pt51/IrMn bilayers and also fabricated at absolutely RT.
In the first part, we demonstrate that the rotational sputtering technique benefits crystal structure of antiferromagnetic layer in longitudinal exchange bias system and thereby enhances the EB value by Ta/NiFe/FeMn EB system. And the system is optimized based on this technique with the highest EB value of 350 Oe. In the second part, Co49Pt51 alloy films were fabricated on glass substrates at room temperature (RT) with significant PMA using a combination of Ta and Pt underlayers (RT-CoPt). Their properties were compared with those produced by conventional high temperature route (HT-CoPt). The XRD patterns are similar but slightly peak shifts suggest interdiffusion may exists in HT-CoPt samples. The microstructural studies revealed extremely smooth surface with root mean square roughness (Rrms) of 0.5 nm for the RT-CoPt films; while the HT-CoPt film showed wavy surface with Rrms = 2.4 nm. The out-of-plane and in-plane hysteresis measurements displayed the existence of strong PMA for both the RT-CoPt and HT-CoPt films. The squarness ratio (SQR) and magneto-crystalline anisotropy constant (Ku) are similar between two cases but coercivities and switching behavior are quite different.
In the third part, perpendicular exchange bias (PEB) effect in the as-grown Co49Pt51/IrMn bilayers was demonstrated at room temperature using single-layered Co49Pt51 alloy thin film as ferromagnetic (FM) layer. Several unusual features were observed in this system, viz.,: i) the PEB was spontaneously established without any artificial magnetic treatments; ii) single-shifted loops were obtained rather than doubleshifted ones, and iii) the spontaneous PEB effect was accompanied by a reduction in perpendicular coercivity, Hc. Training effect studies indicate that the PEB effect is stable in this system. Significant reduction in the PEB effect was found for the CoPt/IrMn films either grown under inducing field or subjected to post field-annealing. The thickness dependence of PEB effect on the FM and AFM layers were also investigated and the largest PEB value of 533 Oe was obtained for the sample grown with 3-nm CoPt and 10-nm IrMn layers.
In the fourth part, in continuation with the existence of perpendicular exchange bias (PEB) in Co49Pt51/IrMn bilayers in absence of external inducing field during deposition, the symmetry-breaking of PEB behavior in these films with respect to the inducing field direction was investigated. Not only the loop shift direction altered by reversing treating field, but different PEB values were observed in samples grown and annealed with field with direction toward film growth direction (here we define this direction as positive direction, PF) and the opposite (here we define this direction as negative direction, NF). The dissimilar PEB values obtained with different inducing field directions clearly suggest the existence of symmetry-breaking in the Co49Pt51/IrMn PEB system. The mechanism of manipulating loop shifting directions and symmetry-breaking in the system are investigated and discussed in the viewpoint of crystal and moment structures.
Finally, FeMn was introduced as antiferromagnetic (AFM) layer to couple with single-layered Co49Pt51 alloy thin film and compared with Co49Pt51/IrMn bilayer system in order to further explore the mechanism of spontaneous perpendicular exchange bias (PEB) we recently found in CoPt/IrMn bilayers. Bilayers of CoPt/IrMn and CoPt/FeMn were prepared under same conditions by sputtering at RT without any inducing field. Although PEB phenomenon was observed in as-grown CoPt/FeMn bilayers as well, the loop shape and PEB behavior were obviously different. Comparing to CoPt (5 nm)/IrMn (10 nm) system, CoPt (5 nm)/FeMn (10 nm) bilayer shows a sheared loop similar to double-shifted loop and much lower SQR (0.52) and He (180 Oe). In addition, two systems reveal entirely different dependence of PEB on AFM layer thickness. In CoPt/FeMn case, the dependence is more complicated with an unusual peak when AFM layer is 10 nm. By taking account of the dissimilar loop shapes and distinct dependence of PEB on AFM thickness, the mechanism of spontaneously-established PEB in these two systems are considered to be different. Investigations of interfacial microstructures through cross-sectional transmitting electron microscopy reveal no obvious difference between two systems. Only in scanning transmitting electron microscopy image, the interface of AFM/FM in CoPt/FeMn was observed relatively rougher than in CoPt/IrMn. Therefore, the difference in PEB behavior of these two systems are considered to not be mainly caused by difference in interfacial microstructures, but the interfacial magnetic structures. The results provide a further understanding on the mechanism of spontaneous PEB.

並列關鍵字

thin film ； sputtering ； perpendicular magnetic anisotropy ； perpendicular exchange bias ； CoPt alloy ； IrMn ； FeMn

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