臺灣博碩士論文加值系統

利用分子束磊晶(MBE)在MgO(001)、MgO(011)及STO(001)基板上成長Fe3O4/Mn3O4/Fe3O4三層膜，這些薄膜的厚度分別為(5nm / 80nm / 5nm)、(10nm / 80nm / 10nm)及(25nm / 50nm / 25nm)。並利用X光繞射儀(XRD)分析晶格結構，使用電子顯微鏡(SEM)觀察薄膜表面形貌，利用物性量測系統(PPMS)量測薄膜磁性。
由XRD分析得知這些薄膜的晶體結構會受層與層間的交互作用影響，造成晶格常數改變，顯示沿著Z軸的方向，薄膜有形變而Mn3O4與Fe3O4呈現不同的形變，在較薄的系統(5nm / 80nm / 5nm)，Mn3O4的形變約為0.42%，在另外兩個系統中，Mn3O4的形變約為-1.5%，而Fe3O4的形變則在三個薄膜中均約為4% ~ 5%。
利用SEM觀察Fe3O4/Mn3O4/Fe3O4三層膜之表面形貌在(5nm / 80nm / 5nm)上有發現到類似Mn3O4皺化(faceting)的現象，但隨著Fe3O4層厚度的增加在(10nm / 80nm / 10nm)及(25nm / 50nm / 25nm)三層膜系統中觀察到皺化逐漸改變，並且有部分區域呈現較為平坦的形貌，顯示出表面能與薄膜-基板介面能之相互競爭的形勢。而在MgO(011)基板上之表面形貌為長條型島狀，但當Fe3O4層厚度增厚，發現逐漸有塊狀皺化單元出現。
在磁化強度對溫度的曲線(M-T曲線)及磁滯曲線(M-H曲線)分析中發現到補償點與負磁化強度，顯示了Fe3O4與Mn3O4介面間的反平行耦合。在Mn3O4層厚度為Fe3O4層厚度的3倍以上厚的系統下已觀察到補償的現象，然而在Fe3O4層厚度為Mn3O4層厚度的2倍厚的系統依然發現到十分明顯的補償現象，說明一種類似磁疇的螺旋態自旋結構在介面處形成。我們也在升降溫的M-T曲線觀察到了隨外加磁場改變的熱滯現象。本論文研究探討關於鐵氧磁性尖晶石氧化物的介面耦合在薄膜大尺度下磁性耦合機制之詳細特性，鐵氧體薄膜在小尺度下與超晶格系統之比較，在此也將討論。

Fe3O4 and Mn3O4 are the most typical spinel ferrimagnetic oxides. The magnetic coupling between Fe3O4 and Mn3O4 has been studied through superlattices but with relatively smaller length scale of each constituent layer. In this thesis, a Fe3O4/Mn3O4/Fe3O4 trilayered structure on MgO(001), MgO(011) and STO(001) substrates, fabricated by molecular beam epitaxy(MBE), provides a simple system to test the magnetic response to a much larger length scale. These trilayered structure have been also characterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM) for both crystalline quality and surface morphology.
X-ray diffraction along the perpendicular direction reveals that the lattice parameters along the z-direction are off the bulk value -1.3% for Mn3O4 and over 4 % for Fe3O4, respectively indicating that both constituent layers are heavily under strain. This modification of the lattice parameters along the z-axis results from the in-plane matching which is due to probably similar oxygen sublattices. The in-plane matching is further confirmed by a model calculation and suggests a strong coupling between two constituent layers.
The remanence magnetization vs. temperature (Mr-T) shows a magnetic compensation point at ~22K. Although this observation indicates antiparallel coupling at the interface and consistent with the previous superlattice results, this is unexpected for the present configuration. The compensation state is thus attributed to a domain wall structure formed in Fe3O4 along the perpendicular direction due to the strong anisotropic energy of the tetragonal Mn3O4 relative to cubic Fe3O4. Magnetic hysteresis measurements at various temperatures support this domain wall configuration and also map out an H-T diagram which includes a low-field compensation and a high-field spin flop phase.

第1章. 前言 1
1-1 前言 1
第2章. 理論背景 4
2-1 結構及磁性 4
2-1-1. 陶鐵磁性物質 4
2-1-2. 尖晶石(Spinel)結構 5
2-1-3. 尖晶石結構的磁性 7
2-2 Compensation point and twisted state 10
2-3 Domain wall structure 12
2-4 磁相關能量 14
2-4-1. Exchange Energy 14
2-4-2. Zeeman Energy 16
第3章. 實驗儀器與步驟 18
3-1 分子束磊晶系統（molecular beam epitaxy） 18
3-2 實驗流程與步驟 19
第4章. 實驗結果與討論 22
4-1 利用XRD進行晶體結構分析與討論 22
4-2 SEM表面形貌分析與討論 30
4-3 磁性量測結果與討論 39
第5章. 結論 67
References 68

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