The microstructural and magnetic characterization in NiFe/CoO/Co trilayers and ion-beam bombarded NiFe/Fe-oxide bilayers

本研究利用雙離子束濺鍍系統製備(1)鎳鐵/氧化鈷/鈷三層薄膜，探討不同氧化鈷厚度對交換偏壓效應之影響；(2)鎳鐵/氧化鐵雙層薄膜，探討離子束轟擊氧化鐵表面對鎳鐵/氧化鐵雙層薄膜之磁性質影響。 由X光繞射儀及電子顯微鏡分析顯示：鎳鐵/氧化鈷/鈷三層薄膜分別由面心立方之鎳鐵，鹽岩結構之氧化鈷及六方最密堆積之鈷所組成。磁性質研究結果顯示：在室溫下，鎳鐵/氧化鈷/鈷三層薄膜中氧化鈷厚度為5奈米，其磁滯曲線呈現台階形狀。經由場冷至10 K之鎳鐵/氧化鈷/鈷三層薄膜中，氧化鈷厚度為5奈米之樣品具有最大之矯頑磁力（Ｈc ~ 286 Oe），而氧化鈷厚度為12奈米之樣品具有最大之交換偏壓場（Ｈex ~ -143 Oe）。磁化量對溫度之相依性（零場冷及場冷）研究顯示：鎳鐵/氧化鈷/鈷三層薄膜均具有類似自旋玻璃之行為，其不同之阻隔溫度及不可逆溫度與氧化鈷厚度造成之鐵磁/反鐵磁耦合有關。此外，極化中子反射儀研究之初步結果(如：自旋非對稱與自旋翻轉機制)顯示：磁化翻轉機制隨著外加磁場改變而不同。 在離子束轟擊氧化鐵表面之鎳鐵/氧化鐵雙層薄膜結構分析(X光繞射儀、電子顯微鏡)結果顯示：鎳鐵/氧化鐵雙層薄膜均含有面心立方結構隻鎳鐵、剛玉型六方最密堆積結構之氧化鐵，並由電子能譜化學分析儀證實鎳鐵/氧化鐵雙層薄膜界面無擴散現象。磁性質分析結果顯示：室溫下，鎳鐵/氧化鐵雙層薄膜無明顯之交換偏壓。鎳鐵/氧化鐵雙層薄膜由室溫場冷到160 K時，經70 V轟擊後之樣品磁異向性降低，使矯頑磁力(Hc ~ 18 Oe)較未轟擊之樣品小。場冷至10 K下，未經離子束轟擊之鎳鐵/氧化鐵雙層膜具有最大之矯頑磁力約204 Oe，而經70 V轟擊之樣品具有最大之交換偏壓(Hex ~ -261 Oe)。最後，由磁化量對溫度之相依性結果顯示：經70 V轟擊後之鎳鐵/氧化鐵雙層薄膜具有最高之阻隔溫度(TB ~ 100 K)，而未經離子束轟擊之樣品則具有最高之不可逆溫度(Tirr. ~ 250 K)。Exchange bias (EB) is the term used to describe the unidirectional anisotropy found in a ferromagnet (FM) exchanged-coupled to an antiferromagnet (AFM). In this research, a dual ion-beam deposition technique was used to prepare the NiFe/CoO/Co trilayers with different CoO thicknesss and NiFe/Fe-oxide bilayers for ion-beam bombardment. Results have shown that the trilayer consisted of f.c.c. NiFe, roak-salt CoO, and h.c.p. Co structures. At room temperature, the NiFe/CoO/Co trilayers exhibited soft magnetic properties with coercivity close to those of reference single NiFe and Co layer, However, a step with enhanced Hc was observed in a NiFe/CoO(5 nm)/Co trilayer. The exchange bias coupling interaction between NiFe/CoO and CoO/Co was set in when field-cooling the trilayers below the Neel temperature of CoO down to 10 K. A largest exchange bias field (Hex ~ -143 Oe) resulting from competition between top and bottom FM/AFM interfaces was found in a NiFe/CoO(12 nm)/Co trilayer. In the ZFC/FC results, the NiFe/CoO/Co trilayers exhibited spin-glass-like behavior, while the different block temperature and irreversible temperature were caused by ferromagnetic/ antiferromagnetic coupling with different cobalt oxide thickness. Moreover, the preliminary PNR data were consistent with the hysteresis loop. The exchange coupling effects in NiFe/Fe-oxide bilayers consisted of f.c.c. NiFe and corundum h.c.p. α-Fe2O3 structures, there were no intermixing at ferromagnetic/ antiferromagnetic surface as characterized by Electron Spectroscopy for Chemical Analysis (ESCA). Without the exchange bias at 298 K, and after 70 V Ar ion bombarded observed decrease in Hc after field cooling (FC) to 160 K is mainly attributed to a reduced effective magnetic anisotropy. The hysteresis loops were measured parallel to the film surface after being field cooled from room temperature down to 10 K at 20 kOe. The enhanced coercivity (Hc ~ 204 Oe) for not bombarded NiFe/Fe-oxide bilayer and a maximum exchange bias (Hex ~ -261 Oe) was discovered at after 70 V bombarded. The results of ZFC/FC were discovered a largest block temperature (TB ~ 110 K) with 70 V bombarded, whereas the largest irreversible temperature (Tirr.) about 250 K observed at the NiFe/Fe-oxide bilayer at sample of not bombarded.目 錄 致 謝 I 摘 要 II Abstract III 目 錄 IV 表 索 引 VI 圖 索 引 VIII 第一章、緒論 1 1-1、前言 1 1-2、應用 2 1-2-1、磁阻式隨機記憶體(MRAM) 2 1-2-2、巨磁阻(GMR) 3 1-2-3、穿隧磁阻(TMR) 4 1-2-4、自旋閥(Spin-valve)結構元件 5 1-3、研究動機與目的 6 1-4、第一章參考文獻 9 第二章、基礎理論與文獻回顧 10 2-1、雙離子束濺鍍系統之原理及應用 10 2-1-1、Kaufman離子源 10 2-1-2、End-Hall離子源 12 2-2、磁性材料 14 2-2-1、磁性物質種類與簡介 14 2-2-2、磁異向性之介紹 19 2-2-3、磁電阻 21 2-2-4、異向性磁阻 22 2-3、交換偏壓 23 2-3-1、交換耦合機制 23 2-3-2、零場冷與場冷之磁滯曲線比較 24 2-3-3、理論模型 25 2-4、文獻回顧 30 2-4-1、反鐵磁層厚度效應 30 2-4-2、鐵磁層厚度與溫度之交換耦合效應 34 2-4-3、單層及三層膜磁電阻變化 36 2-4-4、磁化量(M)與溫度(T)之關係 37 2-4-5、離子束轟擊效應 38 2-4-6、X光繞射分析 39 2-4-7、電子能譜化學分析 40 2-4-8、極性中子反射量測 42 2-5、第二章參考文獻 46 第三章、實驗 48 3-1、實驗架構 48 3-2、材料選用 50 3-3、薄膜製備 55 3-3-1、基板前處理 55 3-3-2、鍍膜參數 55 3-4、分析儀器原理之簡介 57 3-4-1、X光繞射儀(XRD) 57 3-4-2、穿透式電子顯微鏡(TEM) 60 3-4-3、震動樣品磁力計(VSM) 64 3-4-4、超導量子干涉儀(SQUID) 66 3-4-5、磁力顯微鏡(MFM) 68 3-4-6、磁電阻量測(MR) 70 3-4-7、電子能譜化學分析儀(ESCA) 72 3-4-8、極化中子反射儀(PNR) 74 3-5、第三章參考文獻 77 第四章、結果與討論 78 4-1、改變反鐵磁層厚度之研究 78 4-1-1、X光繞射分析(XRD) 78 4-1-2、穿透式電子顯微鏡(TEM)微結構分析 80 4-1-3、震動樣品磁力計量測(VSM) 87 4-1-4、超導量子干涉儀(SQUID)磁性量測 92 4-1-5、原子力顯微鏡(AFM)及磁力顯微鏡(MFM)分析 99 4-1-6、磁電阻(MR)量測 101 4-1-7、極化中子反射儀(PNR)量測 106 4-2、離子束轟擊反鐵磁層之研究 109 4-2-1、X光繞射分析(XRD) 109 4-2-2、穿透式電子顯微鏡(TEM)微結構分析 111 4-2-3、電子能譜化學分析儀(ESCA) 114 4-2-4、震動樣品磁力計量測(VSM) 117 4-2-5、超導量子干涉儀(SQUID)磁性量測 119 4-2-6、原子力顯微鏡(AFM)及磁力顯微鏡(MFM)分析 123 4-3、第四章參考文獻 124 第五章、結論 126 i、鎳鐵/氧化鈷/鈷三層薄膜 126 ii、鎳鐵/氧化鐵雙層膜 12