銥錳交換耦合之(鈷鐵硼/氧化鋁/鈷)磁穿隧元件的機械性，電性及磁性研究

Mechanical, electrical and magnetic properties of IrMn exchange-biased CoFeB/AlOx/Co magnetic tunnel junctions

陳元宗(Yuan-Tsung Chen)

指導教授 ： 吳振名 ； 任盛源 ； 姚永德

國立清華大學/工學院/材料科學工程學系/博士(2006年)

摘要

鈷鐵硼(Co60Fe20B20)/氧化鋁(AlOx)/鈷(Co)所組成之磁穿隧元件(Magnetic tunneling junction, MTJ)，因Co60Fe20B20具有高磁化率(Polarization)的特性，引起廣泛之研究。然而，在許多研究中卻忽視了MTJ的機械性質。例如:磁伸磁(Magnetostriction)、應力效應(Straining effect)對於整個磁穿隊元件之影響。本文中主要研究MTJ之晶體微結構、磁性質、機械性質、電性及奈米檢測。藉由磁控濺鍍法(DC-magnetron sputtering)製作鐵磁性、金屬薄膜，和射頻磁控濺鍍法(RF-magnetron sputtering)製作AlOx陶瓷薄膜。首先，先針對單層之鈷和鈷鐵硼薄膜探討，以深入了解各單層膜之全部相關特性。接著，著手製作磁穿隧元件即Co60Fe20B20/AlOx/Co。藉由高解析之電子顯微鏡(High-resolution cross-sectional transmission electron microscopy (HR X-TEM)之能量分析儀(Energy dispersive X-ray)技術了解界面元素組成分布對其磁伸縮之影響。更進一步地探討鐵磁層(Ferromagnet)鈷和反鐵磁層(Antiferromagnet)銥錳間交換耦合(Exchange coupling)之現象，並將之加入整個磁穿隧元件，最終使其成為一個具有交換耦合特性之MTJ元件。除此之外，亦研究外加施力對MTJ之穿隧磁阻(Tunneling magnetoresistance, TMR)之影響；探討在應力作用之下，其磁阻及電阻改變之趨勢，並研究TMR和交換偏壓場(Exchange-biasing field, Hex)大小之變化，來總結本篇論文之研究。
在單層之Co 和CoFeB薄膜中，從歐傑電子能譜儀(Auger electron spectrometer, AES)之縱深成分分析(Depth profiling)發現膜表面具有17.5-25 Å厚的氧化鈷層(CoOx)和(CoFeB)Ox會影響磁性質，並且推導出該CoOx和(CoFeB)Ox之磁性為鐵磁性(Ferromagnet)或弱鐵磁性(Weak ferromagnet)並非習知的順磁(Paramagnet)或反鐵磁性。在Co60Fe20B20/AlOx/Co MTJ之磁伸縮研究中，發現存在於界面O、Al、Fe、Co之含量，對於其淨磁伸縮之傾向各有不同程度之影響。在Co/IrMn之交換耦合中，針對不同鐵磁及反鐵磁薄膜厚度探討交換偏壓場Hex大小。IrMn(111)優選指向(Texturing)對其磁性質影響。最後，把整個元件在外力作用下探討其TMR磁阻變化並發現各種不同特徵，以利於在磁性隨機存取記憶體(Magnetoresistance random access memory, MRAM)或應力感測器(Strain gauge sensor)之應用。
從我們實驗結果可知，最佳系統的MTJ組合為：Si(100)/Ta(30 Å)/CoFeB(75 Å)/AlOx(30 Å)/Co (75 Å)/IrMn(90 Å)/Ta(100 Å)，因為此系統於外加應力 25 × 10-6範圍內，具有高TMR、高Hex和最小磁伸縮，且元件性質對於外加應力變化的反應程度是最少的，適合於磁頭和MRAM實際上應用。

關鍵字

磁伸縮 ； 磁穿隧元件 ； 應力效應 ； 交換耦合 ； 穿隧磁阻 ； 銥錳(111)優選指向

並列摘要

Co60Fe20B20/AlOx/Co is a typical device used as a magnetic tunneling junction (MTJ), because Co60Fe20B20 has a high polarization characterization, which means the high tunneling magnetoresistance (TMR). This kind of MTJ system has been studied extensively. However, many researches downplayed the mechanical properties, such as magnetostriction and straining effect of MTJ. In this study, I concentrated on the studies of the crystal structure, magnetic, mechanical, electrical, and nanoscale properties of this MTJ. DC-magnetron sputtering was employed to fabricate various magnetic layers and RF-magnetron sputtering AlOx tunneling layer. First, the single Co and Co60Fe20B20 layer were investigated respectively in order to understand the properties of each single layer. Next, Co60Fe20B20/AlOx/Co device was fabricated to observe the interfacial effect on magnetostriction of MTJ; nanobeam energy dispersive X-ray (EDX) device was incorporated in high-resolution cross-sectional Transmission Electron Microscopy (HR X-TEM). Furthermore, the exchange-biasing phenomenon of the Co(Ferromagnet)/Ir20Mn80(Antiferromagnet) system was studied. Finally, the TMR of the IrMn exchange-biased MTJs was investigated: including the variations of TMR, resistance (R), and exchange-biasing field (Hex) as a function of AlOx thickness ( ) .
From the Auger-depth profile analysis, it was found that there is one CoOx or (CoFeB)Ox oxide layer, lying on top surface of the Co or CoFeB film, and another CoOx or (CoFeB)Ox oxide layer, lying near the glass interface. Due to the proximity effect, the CoOx or (CoFeB)Ox oxide layer is weak ferromagnet, not paramagnet or antiferromagnet as expressed. In addition, the atomic concentrations of Fe, Al, and O as a function of in the laminated CoFeB/AlOx( )/Co can affect the trend of its net magnetostriction. The Ir20Mn80 texture plays an important role on the magnetic properties in the Co/Ir20Mn80 system. Eventually, all the MTJs could sustain the external stress effect without failing.
Based on my studies, the optimal candidate of this MTJ system is: Si(100)/Ta(30 Å)/CoFeB(75 Å)/AlOx(30 Å)/Co (75 Å)/IrMn(90 Å)/Ta(100 Å), because it has the high TMR, large Hex, and the smallest magnetostriction. Moreover, in the 25 × 10-6 real environment this type of MTJ is stable in the recording head and MRAM application; because it is less sensitive to the external stresses.

並列關鍵字

Magnetostriction ； Magnetic tunneling junction, MTJ ； Straining effect ； Exchange coupling ； Tunneling magnetoresistance, TMR ； IrMn(111)(Texturing)

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