以XAS吸收光譜研究過渡金屬磁性材料La1-xSrxFe0.67Mo0.33O3與Fe1-xCoxGa3之特性

Abstract

過渡金屬之材料經常在磁性上有豐富的變化，本實驗與俄羅斯教授合作，分別研究La1-xSrxFe0.67Mo0.33O3與Fe1-xCoxGa3兩系列樣品之特性。在本實驗中利用同步輻射分別對兩系列樣品做價數上之測定，並與磁性結論做比對，觀察其是否相吻合。 La1-xSrxFe0.67Mo0.33O3由光譜分析之結果，Fe的價數並沒有明顯的變化，幾乎為3+；而Mo的價數隨著Sr之摻雜量增加( x = 0→1 )，Mo之價數逐漸3+增加至6+。而在俄羅斯的量測中發現隨著摻雜量的增加，磁性特性從LaFe0.67Mo0.33O3呈現反鐵磁特性轉變為鐵磁性，這是因為鐵磁交互作用的增加，而當x>0.7後鐵磁性又漸漸降低，這是因為Mo6+為non-magnetic使的鐵磁交互作用下降。由上述兩點皆吻合隨著Sr摻雜量增加Mo價數亦會上升之結論。 本論文亦在Fe1-xCoxGa3此系列材料進行了價數上的判定，並與相關文獻做比對。由Fe與Co之光譜觀察得到摻雜Co時，Co將取代Fe的位置，形成了T-T bond (T=Fe,Co)，此T-T bond 由一對spin up與spin down的離子組成，且因為Co比Fe多一顆電子，因此在Co完全取代Fe的情形下，我們期望隨著Co的摻雜量逐漸上升，當0<x<0.5時，此材料磁性會上升；而當0.5<x<1時，磁性則會下降。由[10]文獻中的理論計算，發現其有磁性變化，且大部分磁性變化行為符合上述之期待。

The transition metal materials have various magnetic properties that have been studied in recent years. La1-xSrxFe2/3Mo1/3O3 and Fe1-xCoxGa3 are this kind of materials. From the cooperatively relation, we use the data which were taken in NSRRC and compare them to the recent studies of magnetic properties. The La1-xSrxFe2/3Mo1/3O3 XAS show that the valence of Fe are 3+ for all Sr doping levels. However, the valence of Mo increases from 3+ to 6+ gradually. With increasing Sr doping, La1-xSrxFe2/3Mo1/3O3 changes from being antiferromagnetic to ferromagnetic. This is due to the ferromagnetic superexchange. When x>0.7, the magnetization decreases due non-magnetic Mo6+. Fe1-xCoxGa3 XAS shows that, with Co doping, Co substitutes Fe. Moreover, T-T bond of Fe-Co forms, which consists of a pair of spin up-spin down ions. Co has one more electron than Fe, therefore, the magnetization increase for the Co doping level 0<x<0.5, and will have the maximum magnetization at x=0.5. The magnetization decrease for 0.5<x<1, due to the formation of Co-Co T-T bond. The calculations of magnetic properties in the literature are consistent with our results.