臺灣博碩士論文加值系統

摘要
本論文在探討Fe3O4-Ag的合成薄膜系統中異常正磁阻的現象，並進一步研究此現象的成因。
樣品的製備是在高真空下、用Fe3O4-Ag混合靶、以直流磁控式濺鍍法將樣品製作在Si (100)基材上，以X-ray繞射法、EPMA、SEM等實驗檢測樣品的晶格結構、組成、型態，並且以四點量測法，外加磁場0.8 Tesla，外加磁場平行膜面及電流，並且作外加磁場平行膜面垂直電流的檢測，在350K、300K、225K的溫度下測量磁阻，並且以PPMS檢測大磁場(9T)下磁阻的變化情形，磁阻的定義：MR＝{R(H)－R(0)}/R(0)，並以SQUID在150K~350K的溫度範圍內作磁性的量測。
結果顯示，在室溫(300K)時，Fe3O4薄膜的負磁阻值約0.3%，但是，混合少量Ag的樣品，卻突然變成正磁阻。在此Agx(Fe3O4)1-x系統中(x為樣品中Ag的體積分率)，當x=0.007時，正磁阻值為最大，約為0.5%，隨著Ag的增加，在x=0.015時，正磁阻的效應消失了，當x≧0.02，磁阻值變為負的，而且其值隨著x增加而減少。扣除此一範圍內Fe3O4的負磁阻與Ag的正磁阻(OMR)所合成的磁阻，則可看出異常的正磁阻效應是發生在0.001＜x＜0.02的範圍內，而在x=0.02時，Ag的顆粒大約為10nm。此異常正磁阻的現象是由於自旋電流由Fe3O4入射Ag顆粒而在Ag顆粒內產生自旋電子累積所造成的，因此由量測磁阻的方法可決定電子於室溫下在Ag顆粒內之自旋擴散距離為9nm，而在低溫(T=225K)時，在Ag顆粒內之自旋擴散距離增加至10nm。

Abstract
The anomalous positive magnetoresistance effect of Agx-(Fe3O4)1-x composite films have been investigated. Samples were prepared by dc magnetron sputtering with composite targets and the Si(100) wafer was used as the substrate.
We have studied the composition, structure, and morphology of Agx- (Fe3O4)1-x composite films by EPMA, X-ray diffraction and SEM respectively. The resistivity of Agx-(Fe3O4)1-x films were measured by four-probe method. The MR was in general obtained with a maximum applied magnetic field of 0.8 Tesla at 350K, 300K and 225K. The value of MR ratio is defined to be MR={R(H)-R(0)}/R(0). The magnetic properties of the films were determined with SQUID between 150K and 350K.
It has been found that a negative magnetoresistance of about 0.3% is observed for pure Fe3O4 films. With the incorporation of a small amount of Ag nanoparticles into Fe3O4 matrix, the MR value suddenly becomes positive. The MR ratio reaches a maximum value of about +0.5% at x=0.007. The positive MR effect disappears at x≒0.015. The MR value re-entries into negative region again. Furthermore, its magnitude is decreasing with increasing x. If the contribution from the negative MR effect of Fe3O4 and the OMR effect of Ag are subtracted, the anomalous positive magnetoresistance is present only in a very narrow region between 0.001＜x＜0.02. At x=0.02, the average granular size of Ag is about 10nm.
This phenomenon of extra anomalous MR can be explained from spin current injected into Ag granules, inducing spin accumulation inside Ag granules. Therefore, using MR measurement, the spin diffusion length at room temperature in Ag particle was determined to be 9nm. And as the temperature decreases to 225K, the spin diffusion length increases to 10nm.

目錄
誌謝
中文摘要
英文摘要
目錄……………………………………………………………………Ⅰ
表目錄…………………………………………………………………Ⅳ
圖目錄…………………………………………………………………Ⅴ
第一章前言…………………………………………………………1
1-1 緒論…………………………………………………………….1
1-2 磁阻的簡介…………………………………………………….2
1-3 研究動機與目的……………………………………………….15
第二章 理論背景…………………………………………………….22
2-1自旋電子入射之簡介(spin injection)…………………22
2-2 自旋電子入射造成之磁阻…………………………………….27
第三章 實驗設備及步驟…………………………………………….32
3-1 實驗流程……………………………………………………….32
3-2 薄膜的製備…………………………………………………….33
3-3 薄膜的鑑定…………………………………………………….35
3-3-1 薄膜結構的鑑定…………………………………………..35
3-3-1-1 X-ray繞射……………………………………………35
3-3-1-2 電子束繞射…………………………………………..41
3-3-2 薄膜型態之鑑定…………………………………………..42
3-3-2-1 掃描式電子顯微鏡(SEM)之原理與應用…………...42
3-3-2-2 穿透式電子顯微鏡(TEM)之原理與應用…………...43
3-3-3 薄膜成分的鑑定…………………………………………..45
3-3-4 薄膜厚度的鑑定…………………………………………..45
3-4 薄膜電阻的量測……………………………………………….47
3-5 薄膜磁阻的量測……………………………………………….49
3-6 薄膜磁性的量測……………………………………………….51
第四章 實驗結果與分析……………………………………………..53
4-1 Agx-(Fe3O4)1-x薄膜成分之分析……………………………….53
4-2 Agx-(Fe3O4)1-x薄膜結構之分析………………………………..56
4-2-1 X-ray繞射圖形之分析……………………………………56
4-3 Agx-(Fe3O4)1-x薄膜型態之分析………………………………..70
4-3-1 SEM圖形之分析………………………………………….70
4-4 Agx-(Fe3O4)1-x薄膜電阻之分析………………………………..77
4-4-1 Agx-(Fe3O4)1-x薄膜的電阻與體積分率(x)之關係………77
4-4-2 Agx-(Fe3O4)1-x薄膜的電阻與溫度之關係 ………………79
4-5 Agx-(Fe3O4)1-x薄膜磁阻之分析………………………………..81
4-5-1磁阻之方向性……………………………………………….82
4-5-2 以傳統電磁鐵系統及超導磁鐵(PPMS)系統量測
Agx- (Fe3O4)1-x薄膜磁阻結果之比較...…………………...82
4-5-3 Agx-(Fe3O4)1-x薄膜的磁阻與體積分率(x)之關係………..90
4-5-4 Agx-(Fe3O4)1-x薄膜的磁阻與溫度之關係………………..98
4-6 Agx-(Fe3O4)1-x薄膜磁性之分析………………………………109
4-6-1 Agx-(Fe3O4)1-x薄膜磁滯曲線之分析………………………110
4-6-2 Agx-(Fe3O4)1-x薄膜頑磁力之分析………………………....112
4-6-3 不同體積分率(x)之Agx-(Fe3O4)1-x薄膜磁滯曲線之比較.115
第五章 結論…………………………………………………………121
參考資料………………………………………………………………124

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