Atypical Magnetic, Electrical, and Magnetotransport Behaviors of Bismuth Films

本實驗研究了單晶鉍膜之取向性對其性質的影響，以及在鐵鉍、鈷鉍合成膜中磁性及磁傳輸性質的自然時效效應。首先我們利用直流磁控濺鍍法製作鉍膜於不同取向之矽基版上，並加以後續退火，可得到不同取向之單晶鉍膜。其常磁阻的大小會隨著不同結晶取向而有所不同。而於極低溫(0.3K)及強磁場的作用下，因載子迴旋軌道產生藍道能階量子化，以及隨著磁場強度而改變之能態密度，可觀察到Shubnikov-de Haas振盪的現象，並隨著不同的鉍膜取向而產生不同之振盪週期。 而在鐵鉍合成膜中，因濺鍍過程形成了暫穩態合金，造成樣品之磁性質及磁傳輸性質隨時間而發生改變，其變化速度也與鐵含量的多寡有關。於初鍍狀態下，鐵鉍樣品顯示很大的垂直異向性特徵，以及包含鉍之常磁阻及異向性磁阻的混合模式。隨著時間經過，因樣品發生時效析出作用，使的鐵微粒不斷析出於鉍之基地相，這將導致垂直異向性消失，並使的常磁阻消失，漸漸演變為負磁阻效應。再者，因析出之鐵微粒尺寸過小造成表面形成磁失效層，將導致樣品之磁化量也隨時效作用而降低。相較於鐵鉍合金，鈷鉍系統則未發現明顯之時效作用。In this thesis, we investigated the influence of crystalline orientation of Bi films, and the time dependant magnetic and magnetotransport behaviors of Fe-Bi films. First, the different crystalline orientations of Bi films can be obtained from growing on selected substrates such as Si(100), Si(110), and Si(111), which are prepared with dc magnetron sputtering followed by post-annealing. It was found that the ordinary magnetoresistance (OMR) effect to be oriented dependent. Furthermore, at very low temperature (0.3K), the Bi films exhibit Shubnikov-de Haas (SdH) oscillation; it was due to the Landau quantization of the cyclotron orbits of the carriers. Because the Fermi surface of Bi is highly anisotropic, the SdH oscillations exhibit different periods when the magnetic field is applied along different orientation. For Fe-Bi films, we found that the magnetic and magnetotransport properties of the samples will change with time. This is because that the Fe-Bi formed a metastable solid solution phase during the sputtering process, and different concentrations of Fe yield different rates of magnetic variation. Furthermore, the as-prepared films exhibit a large out-of-plane magnetic anisotropic property and the OMR effect. Due to aging precipitation, the magnetic anisotropy and the OMR decrease and eventually vanish as the duration increases. In addition, with the precipitation process, a magnetic dead layer will form on the surface of Fe nano-grains, it will severely reducing the magnetization and increasing the coercivity. On the contrary, Co-Bi films do not exhibit evidently time-dependent behavior.摘要 目錄..................................................i 圖目錄................................................v 表目錄................................................xii 第一章 前言...........................................1 1-1 緒論..............................................1 1-2 研究動機..........................................2 第二章 相關理論及研究.................................3 2-1 磁阻的簡介........................................3 2-2 鉍（Bismuth）之材料特性...........................14 2-3 相關鉍膜之理論....................................17 2-3-1 鉍之磁阻效應....................................17 2-3-2 低溫下的Shubnikov-de Haas（SdH）振盪現象........18 2-3 相關鉍膜的研究....................................20 第三章 實驗方法與步驟.................................27 3-1 實驗流程圖........................................27 3-1-1 鉍膜的製作及研究流程圖..........................27 3-1-2 鐵鉍、鈷鉍薄膜的製作及研究流程圖................28 3-2 樣品的製備........................................29 3-2-1 鉍膜的製備......................................29 3-2-2 鐵鉍、鈷鉍膜的製備..............................30 3-3 樣品鑑定..........................................31 3-3-1 晶體結構鑑定....................................31 3-3-2 α-step膜厚量測..................................32 3-3-3 TEM微結構觀測...................................33 3-3-4 EPMA成分分析....................................33 3-3-5 Auger縱深分析...................................34 3-3-6 EDAX定性分析....................................34 3-4 樣品物性量測......................................35 3-4-1 電阻量測........................................35 3-4-2 磁阻量測........................................36 3-4-3 磁性量測........................................37 第四章 結果與討論.....................................38 4-1 不同單晶取向之鉍膜對其性質的影響..................38 4-1-1 晶體結構........................................38 4-1-1-1 X光繞射分析...................................38 4-1-1-2 鉍晶體以及矽晶體之晶面結構....................44 4-1-2 鉍膜之晶粒大小..................................48 4-1-3 鉍膜的磁阻變化..................................50 4-1-3-1 不同方向量測的磁阻變化........................50 4-1-3-2 不同溫度的磁阻變化............................52 4-1-3-3 不同取向鉍膜在室溫的磁阻變化..................54 4-1-3-4 不同取向鉍膜在低溫的磁阻變化..................57 4-1-4 鉍膜於低溫下的Shubnikov-de Haas振盪.............61 4-2 鐵鉍膜的磁性及磁傳輸特性之時間效應研究............65 4-2-1 晶體結構........................................66 4-2-1-1 X光繞射分析...................................66 4-2-1-2 XRD繞射峰偏移現象（peak shift）...............72 4-2-2 樣品微結構觀察..................................76 4-2-3 磁性質的量測....................................83 4-2-3-1 鐵鉍樣品磁滯曲線的時效性......................84 4-2-3-2 鐵鉍樣品Squareness的變化......................89 4-2-3-3 鈷鉍樣品磁滯曲線的時效性......................91 4-2-4 氧化對樣品之影響................................94 4-2-5 鐵鉍樣品電阻與時效之關係........................98 4-2-5-1 室溫下的電阻隨時間之變化......................98 4-2-5-2 低溫下的電阻隨時間之變化......................101 4-2-6 鐵鉍樣品電阻與溫度之關係........................102 4-2-7 磁阻隨時效作用的演變............................104 4-2-7-1 鐵鉍樣品之磁阻隨時效作用的演變................105 4-2-7-2 鈷鉍樣品之磁阻隨時效作用的演變................115 第五章 結論...........................................120 附錄..................................................123 參考資料..............................................13