鑭鍶錳氧薄膜之鐵磁共振及磁區結構研究

Abstract

在鈣鈦礦結構的錳氧化物La_(1-x) Sr_x MnO_3 (LSMO)中被證實由電荷、自旋、軌道以及晶格自由度的強關聯性而具有許多重要的物理特性，包括龐磁阻、相分離等等的現象。最近新的文章指出，利用磁力顯微鏡觀察成長在SrLaAlO_4(001)的La_0.7 Sr_0.3 MnO_3的薄膜，當外加水平磁場時呈現出兩種磁區的特殊現象，同時在使用振動樣品磁力計的量測中發現，當外加磁場垂直膜面時，磁滯曲線會呈現一個兩階段式的飽和現象，文章中提出一個理論模型解釋這種特殊的磁區結構，最後透過計算並預測當薄膜在去磁狀態時磁矩易軸和薄膜法線夾角為37°。 本實驗主要分成兩個部分，首先使用脈衝雷射沉積法成長出La_0.7 Sr_0.3 MnO_3薄膜，試圖從相同條件中重現文獻中的特殊磁區結構，透過超導量子干涉儀(SQUID)、振動樣品磁力計(VSM)以及鐵磁共振(FMR)進行比較，發現並未出現預期中的特殊磁區結構，最後由實驗數據去分析失敗的原因。第二部分是將文獻中的La_0.7 Sr_0.3 MnO_3薄膜利用轉角的振動樣品磁力計以及鐵磁共振，驗證磁矩易軸與理論計算中的傾斜角37°是否相符，之後將樣品降入低溫並利用鐵磁共振觀察其磁矩隨溫度的變化，而同時將薄膜內產生的自旋波共振，利用共振場對波向量的色散關係獲得了薄膜內自旋波及磁壁剛性隨溫度的變化，幫助我們從另一個方面對La_0.7 Sr_0.3 MnO_3的雙磁區結構有更多的認識。

Perovskite La_(1-x) Sr_x MnO_3 (LSMO) has been demonstrated to exhibit rich emergent physics, such as colossal magnetoresistance and phase separation phenomena, owing to the strong correlations among the charge, spin, orbital, and lattice degree of freedoms. The latest article pointed out that the magnetic domain observed from magnetic force microscopy (MFM), which showed two different magnetic domain when applied an in-plane magnetic field. The hysteresis loop also appeared a two-step saturation when applied an out of plane magnetic field by vibrating sample magnetometer (VSM) measurement. In this article, there is a theoretical model to explain the special magnetic domain structure. Finally, they predict the angle between the easy axis and the plane normal is 37° through the theoretical calculation. In our research, the experiments are divided into two parts. First, circular LSMO films (120 nm) grown on SrLaAlO_3(001) (SLAO) substrates by the pulsed laser deposition (PLD) method were used to reappear the special magnetic domain structure. By comparing the results from SQUID, VSM and FMR measurements, the expected special magnetic domain structure did not appear. We will discuss the possible reasons that caused the observed discrepancy. Second, we used VSM and FMR by varying the angle between the easy axis and the plane normal to verified the prediction in the theoretical calculation. Then we did the low temperature FMR measurement to observe the magnetic moment behavior. And the spin wave caused by microwave can also provide the spin-wave stiffness and magnetic domain wall stiffness from the energy dispersion relation, which can help us to understand more about LSMO from the other side.