以時間解析光譜研究鑭鍶錳氧/鐵酸鉍異質結構之各向異性超快動力學

Abstract

在本論文中，我們用光學的方法探討由BiFeO3所操控之La0.67Sr0.33MnO3 的各向異性。此樣品的兩軸向([001]、[1-10])的電阻值有很大差異，並且有不同的金屬─絕緣轉變溫度。從紅外光譜我們觀察到小極化子吸收峰，證明BiFeO3使La0.67Sr0.33MnO3電子的雙交換機制增強。並且從變溫紅外光譜可知，載子濃度隨溫度降低會逐漸增加，在160 K以下達到飽和，與[1-10]的金屬─絕緣轉變溫度相符。我們也利用時間解析的反射光譜研究各向異性，在低溫下，光譜有一個明顯的負值訊號，我們推論來源是La0.67Sr0.33MnO3鐵磁的訊號。並且我們得到電子自旋─晶格的弛緩時間(~100 ps)和磁有序的回復時間(~ns)對溫度的關係。從趨勢上可以知道兩軸向的電子自旋─晶格的弛緩時間在其金屬─絕緣相變溫度會有斜率的轉折。而磁有序的回復時間也在此溫度點以上發散以致無法量測。綜合以上各點，我們觀察到兩軸磁的各向異性。

In this thesis, we discuss the anisotropy of La0.67Sr0.33MnO3 manipulated by BiFeO3 by optical method. There is a large difference in resistance along two axises([001],[1-10]), and two metal-insulator transition temperatures(TMI). By IR reflectance spectrum, We observe small polaron absorption in La0.67Sr0.33MnO3 which is upon BiFeO3. It proves that BiFeO3 enhances the double-exchange mechanism between electrons of La0.67Sr0.33MnO3. We also observe carrier concentration increase with lowering temperature, and saturate below 160 K. It matches TMI along [1-10]. We also use time-resolved reflectance spectroscopy to investigate anisotropy. At low temperature, it appears a negative component, and we demonstrate this is from ferromagnetism of La0.67Sr0.33MnO3. By fitting, we obtain temperature dependent spin-lattice relaxation time(~100 ps) and magnetic reordering time(~ ns). For spin-lattice relaxation time, there exists sharp change at TMI in slope. For magnetic reordering time, it also diverges at TMI. In conclusion, it presents the magnetic anisotropy along two axises.