鑭鍶錳氧化物薄膜之光譜研究

Optical studies of Lasub[0.7]Srsub[0.3]MnOsub[3] films

郭明憲

指導教授 ： 劉祥麟

國立臺灣師範大學/理學院/物理學系/碩士(2002年)

摘要

鑭鍶錳氧化物(La0.7Sr0.3MnO3)薄膜受到與基板間晶格不匹配的影響，其結構、磁性及光譜響應呈現與塊材大相逕庭的行為。
首先，我們發現薄膜厚度愈薄，其c軸晶格常數愈長，這是歸因於薄膜的ab平面受到基板壓縮應變作用。其次，我們觀察到薄膜的居禮溫度隨著厚度減小而降低。
從光譜實驗數據分析而言，我們發現La0.7Sr0.3MnO3薄膜之Mn-O-Mn鍵角彎曲振動頻率為一定值，但是Mn-O鍵長伸縮振動頻率在厚度愈薄的樣品中會有紅位移現象，此結果與X-ray繞射數據分析(c軸晶格常數變長)相符。有趣地是，我們觀察到La0.7Sr0.3MnO3薄膜在中紅外光區的吸收峰之機制與小極化子的光譜響應有關，隨著薄膜厚度的減少，小極化子的束縛能量有增強的趨勢。最後，在紫外光區的Mn 3d及O 2p軌域上電荷躍遷吸收峰值也隨著薄膜厚度增加而有紅位移現象。

關鍵字

龐磁阻 ； 紅外線光譜 ； 應變作用

並列摘要

We report on a systematic study of strain effects on the structural, magnetic, and optical properties of high-quality La0.7Sr0.3MnO3 thin films epitaxially grown on LaAlO3 substrate by the off-axis magnetron sputtering technique.The X-ray powder diffraction data indicate that the c-axis lattice constant is enhanced with decreasing the film thickness due to the compressive strain produced by lattice mismatch.Magnetization curves show a decrease of the Curie temperature for decreasing thickness of films.Optical reflectance and transmittance measurements provide evidence that the position of Mn-O stretching mode shifts toward low frequency and the charge -transfer gap between O 2p and Mn 3d states increases with the decrease of film thickness.Most importantly, an analysis of small polaron absorption in the midinfrared region shows the polaron binding energy is increasing with decreasing the film thickness,signifying the dominant role of strain-induced electron-phonon coupling in the physical properties of these doped manganites.

並列關鍵字

CMR ； optical ； strain

參考文獻

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