臺灣博碩士論文加值系統

於(La0.7Ba0.3)MnO3中加入Co將使得化合物的晶格結構、電性及磁性等性質發生改變。由X光吸收光譜研究發現在此系列化合物中，鈷離子以+2及+3價鍵存在，而錳離子則呈顯+3及+4價的混合價鍵，如此複雜的混合價鍵使此系列的磁性結構有一複雜的變化。
在加入鈷取代錳後，因Mn, Co離子半徑不同，且因兩者可隨意分佈，不具規則排列，造成原本為立方體結構的(La0.7Ba0.3)MnO3結構在鈷含量比例達約為0.4時，變成對稱性較低的正交晶格結構。而後Co含量繼續增加至x = 0.9時，晶格結構轉變成對稱性較高的立方晶格結構。在x = 0 ~ 0.2時具有金屬絕緣相變遷，且其變遷溫度點隨鈷含量的增加而降低，磁阻性質亦隨之變大。當鈷含量大於0.3以上，在300 K以下的所有溫度範圍金屬態不再出現，而呈現絕緣或半導體的電阻態，且其磁阻率大幅變小。
在磁性分析方面，在純錳化合物中觀察到的長程作用力程的鐵磁性隨著鈷含量的增加而自旋逐漸被削弱，形成玻璃團式(glass-cluster)的短程鐵磁性質。此現象得證於場冷卻及零場冷卻的磁化強度隨溫度的變化依循不同的路徑，零場冷在低溫時因磁矩呈自旋玻璃態以致所得磁矩較場冷卻為小。而場冷卻降溫量測與升溫量測亦無重合，具有熱滯現象。而飽和磁場隨鈷含量x的增加逐漸降低至x = 0.7後又些微增加。與晶格結構隨x的變化趨勢相近。當改以交流磁場量測，c'(T)幾乎與頻率無關，而c"(T)隨頻率增加而增加。c'(T)及c"(T)的最大值溫度點趨勢亦與晶格結構相近。當變化直流磁場時，c'(T)及c"(T)的峰值逐漸分成兩個較平緩的峰值，隨磁場增加，分別向低溫及高溫處移動。即鈷的含量影響(La0.7Ba0.3)(Mn1-xCox)O3系列的物性甚巨。

The effect of substituting Co in the Mn site of (La0.7Ba0.3)(Mn1-xCox)O3 perovskite system with x = 0 - 1.0 has been extensively investigated based on the analyses of x-ray crystal structure, dc magnetoresistance, dc magnetization and ac susceptibility data. This replacement of Mn by Co strongly affects on the magnetic and transport properties in this mixed system. The metallic state and the long range ferromagnetic order in low temperature range observed in the pure manganite (with x = 0) are suppressed with increasing Co content. The metal-insulator transition and magnetoresistance effect is almost destroyed as x > 0.3. At first, the order of magnitude of resistivity increases, but magnetization decreases with increasing x from 0 up to 0.7. While the resistance reduces to aprroximately that of the pure manganite and ferromagnetic signal slightly increases as x > 0.7. The highest resistivity, the weakest ferromagnetism, and the smallest unit cell are observed at x ~ 0.6 - 0.7. This result is possibly contributed to the disorder effect of the mixed valences of the transition metal ions.

誌 謝......................................................................................................................i
摘 要.....................................................................................................................ii
目 錄....................................................................................................................iii
表 目 錄................................................................................................................iv
圖說目錄...............................................................................................................v
I 序論....................................................................................................................1
II 實驗步驟
II-1樣品製備.........................................................................................................3
II-2 樣品量測........................................................................................................6
III 實驗結果與討論
III-1 結構分析.......................................................................................................9
III-2 X光吸收光譜量測.....................................................................................13
III-3 電阻量測與磁阻物質.................................................................................15
III-4 直流磁場磁性性質量測.............................................................................21
III-5 交流磁化率量測.........................................................................................25
IV 結論................................................................................................................28
參考資料..............................................................................................................84

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