不同厚度與成長於不同基板上鑭鍶錳氧薄膜的磁區微結構與光譜研究

Abstract

摘要 我們研究成長於LaAlO3 基板上不同厚度(10~60 nm) La0.7Sr0.3MnO3薄膜的磁區微結構，及相同厚度(300 nm)但卻成長於不同基板(LaAlO3與SrTiO3)上的La0.7Sr0.3MnO3薄膜之磁區微結構與全頻光譜響應。 La0.7Sr0.3MnO3和SrTiO3的晶格不匹配 (伸長應變相差~ + 0.74 %)遠小於La0.7Sr0.3MnO3和LaAlO3晶格不匹配 (壓縮應變相差~ - 2.5 %)。由磁力顯微術的研究發現，因為晶格不匹配，成長在SrTiO3基板的La0.7Sr0.3MnO3薄膜有團簇狀的磁區結構，而成長在LaAlO3基板的La0.7Sr0.3MnO3薄膜有迷宮狀的磁區結構。 由全頻光譜研究，因為成長在SrTiO3基板的La0.7Sr0.3MnO3薄膜晶格扭曲較輕微，導致其在遠紅外光區，隨著溫度下降，彎曲振動模產生較輕微的分裂；且其在20 K時的居德權重較成長在LaAlO3基板的La0.7Sr0.3MnO3薄膜在10 K時的居德權重為大；而在中紅外光區， 成長在SrTiO3基板的La0.7Sr0.3MnO3薄膜的 電子躍遷能量及 的電子躍遷能量，皆較成長在LaAlO3基板的La0.7Sr0.3MnO3薄膜小。

Abstract We have studied the magnetic microstructures and optical properties of La0.7Sr0.3MnO3 (LSMO) thin films. Magnetic images are critically dependent on the film thickness and structural strain induced by the SrTiO3 (STO) and LaAlO3 (LAO) substrates. The ferromagnetic domains of the LSMO film on LAO substrate become increasingly stronger with increasing film thickness from 10 nm to 60 nm. The 300-nm-thick LSMO film on STO dominated by tensile stress effect displays a cluster-like pattern, whereas the LSMO film on LAO under compressive stress show a maze-type domain pattern. Moreover, the optical conductivity spectra of the LSMO thin film on STO reveal (i) the reduced numbers of phonon fine structures, (ii) the enhancement of the Drude spectral weight at 20 K, and (iii) the decreasing energy positions of the and transitions in comparison with that on LAO substrate, indicating the local lattice distortion induced by the strain effect weakens upon the STO substrate.