應力對於鉍鐵氧薄膜電子結構的影響之X光吸收能譜學研究

Strain effects on the electronic structure of BiFeO3 thin films studied by X-ray absorption spectroscopy

陳致穎(Zhi-Ying Chen)

指導教授 ： 黃迪靖

國立清華大學/理學院/物理學系/碩士(2014年)

摘要

鉍鐵氧(BiFeO3)為同時擁有反鐵磁性和鐵電性的多鐵材料。許多研究指出藉由基板應力的調控，我們可以改變薄膜的電子和磁性結構。本論文研究鉍鐵氧薄膜沉積於不同的基板，受到不同的基板應力，其電子結構的變化。我們的重點在於比較擠壓應力的鉍鐵氧/鑭鋁氧、拉伸應力的鉍鐵氧/釹鈧氧和幾乎無應力的鉍鐵氧/鏑鈧氧其電子結構的變化。
利用鐵的L-edge和氧的K-edge的X光吸收能譜，我們能研究鐵離子的晶格場對稱性和反鐵磁軸的方向。我們也利用線性二向性研究晶格場對稱性、磁性和軌道的異相性。研究顯示當鉍鐵氧薄膜成長於不同基板時，其鐵3d軌域的電子結構會有明顯的變化。此外，共振非彈性X光散射也被用來研究鐵的電子結構。我們選擇鉍鐵氧/鍶鈦氧來做研究。因為鐵的價數為正三價，其d-d激發可以由多重理論解釋。最後，我們利用電荷轉移多重理論的軟體CTM4XAS和Missing來計算X光吸收能譜和共振非彈性X光散射能譜。模擬的結果和實驗結果定性上大致相同。

關鍵字

鉍鐵氧 ； X光吸收能譜 ； 應力

並列摘要

BiFeO3 is a multiferroic material in which the antiferromagnetism and ferroelectricity simultaneously occur and possess the magnetoelectric coupling. Many experimental results indicate that the electronic and magnetic properties of BiFeO3 thin films can be controlled by strain engineering. The purpose of this work is to investigate the strain effects of BiFeO3 thin films grown on different substrates with varied lattice constants. The comparison between the electronic states of compressive-strained BiFeO3/LaAlO3, tensile-strained BiFeO3/NdScO3 and nearly strain-free BiFeO3/DyScO3 is particularly emphasized.

We used X-ray absorption spectroscopy (XAS) at Fe L-edge and O K-edge to explore crystal field symmetry as well as orientation of antiferromagnetic spin axis. Linear dichroism (LD) is also presented to obtain their orbital and magnetic anisotropy. The measurements suggest that Fe 3d electronic states change when BiFeO3 thin film experiences different strains. In addition, we used resonant inelastic X-ray scattering (RIXS) to further explore the electronic structure of BiFeO3 thin films. BiFeO3/SrTiO3 is chosen as the test case to measure RIXS. Since the valence of Fe is only 3+, the d-d excitations exhibit clear features which can be explained by multiplet calculations. Finally, simulated XAS and RIXS spectrum were obtained by softwares based on charge transfer multiplet scheme such as CTM4XAS and Missing. The calculated results can qualitatively reproduce the experimental features.

並列關鍵字

BiFeO3 ； X-ray absorption spectroscopy ； strain

參考文獻

[3] Journal of Physics: Condensed Matter, 20(43):434220

[6] M. Huijben. PhD thesis, University of Twente, Enschede, The Netherlands

[7] Frank de Groot and Akio Kotani. Core Level Spectroscopy of Solids. CRC Press, 2008.

[8] Performance of the dragon soft x-ray beamline (invited). Review of Scientic Instruments

[9] Phys. Rev. B, 71:060401