Study of magnetic properties on TiO2/Co/TiO2 and Co/TiO2/Co trilayers

本研究在高真空下利用交流(RF)磁控雙濺鍍法製作ㄧ系列二氧化鈦/鈷/二氧化鈦三層膜，樣品成長在矽基板(100)上。我們嘗試在真空下高溫回火步驟，製造稀磁性半導體結構，並研究其磁性質。研究結果發現在真空高溫退火過程中，雖然鈷粒子會均勻擴散至非晶的二氧化鈦薄膜，其鐵磁性的行為卻因此消失，由實驗結果推斷在高溫回火過程中，鈷可能形成反鐵磁的氧化物的結構。 另外我們也製作ㄧ系列鈷/二氧化鈦/鈷的三層膜結構，研究在三層膜結構下，二氧化鈦對兩鐵磁層之間的影響。磁場下磁化率量測結果顯示在低溫下，不同厚度的二氧化鈦薄膜會造成兩鐵磁層呈鐵磁或反鐵磁的行為。電子自旋共振量測結果也顯示其共振場對角度關係的峰值會隨著不同厚度而偏移90˚，顯示其鈷兩層間交換耦合的異向性隨厚度不同而有改變。在厚度為4~6 nm時，其偏移量最大。我們的實驗證實，鈷/二氧化鈦/鈷的三層膜可能作為穿隧磁阻材料之ㄧ。但其鐵磁層之間透過二氧化鈦的交換機制還需要進一步的探討。In this thesis, a series of TiO2/Co/TiO2 trilayer samples were synthesized by high vacuum based co-sputtering in a RF magnetron system. The samples were grown on the Si(100) substrates. We try to synthesize the Co doped TiO2 dilute magnetic semiconductor structure via thermal treatments under vacuum environment, and study its magnetic property. The results reveal that during thermal treatments in a vacuum environment, the Co grains may form the oxide phase structure, and lose their magnetic properties. In the second part, we synthesize a series of Co/TiO2/Co trilayer films, with varying the TiO2 spacer layer thickness, and fixing the Co layer thickness to study their interlayer coupling effect. As analyzed from M-H results, the thickness of TiO2 spacer layer (X) strongly influences the ferromagnetic layers to be alignment in parallel or in antiparallel at low temperature. The ESR analysis results also reveal that the peaks of Hr versus angle shift from 90˚ under different TiO2 spacer layer thickness. At X=4~6 nm, it shows the largest shift which is very different from the result of single Co layer. Our experimental results suggest that Co/TiO2/Co may be a candidate for the tunneling magnetoresistance system. However the mechanism of interlayer coupling of Co-layer via TiO2 requires a further investigation.Contents Abstract(inChinese )…………………………………………………… i Abstract (in English)…….…………………………………..ii Content……………………………………………………………iii Chapter 1 Introduction……………………………………………………1 Chapter 2 Concepts and Theories………………………………………5 2-1 Basic Magnetic Interaction…………………… ……………….5 2-2 Theoretical Models for Dilute Magnetic Semiconductor……...…8 2-3 Interlayer exchange coupling…….………………...…………11 Chapter 3 Experiments………………………………………………...15 3-1 Experimental Flowchart……………………………………….15 3-2 High Vacuum Sputter System………………………………..16 3-3 Sputtering Techniques………………………………………..17 3-4 Thin Film Fabrication………………………………………..19 3-5 Measurements Techniques……………………………………22 3-5-1 Compositional Analysis………………………………….22 3-5-2 Structure Analysis………………………………………..22 3-5-3 Magnetic Analysis……………………………………….26 Chapter 4 Results and Discussion…………………..……………….32 4-1 TiO2/Co/TiO2 trilayer films…………………………………….32 4-2 Co/TiO2/Co trilayer films…………………………………….34 Chapter 5 Conclusion………………………………………………71 References……………………………………………………………..7