應變對於鐵酸鉍薄膜多鐵性質之影響

Abstract

鐵酸鉍(BiFeO3)為一室溫多鐵材料，許多文獻的理論計算出misfit應變會改變此材料晶體結構，並影響其多鐵性質。本實驗係以實際量測方法驗證其理論預測是否正確。 試片以脈衝雷射沉積法先將鑭鎳氧(LaNiO3)薄膜鍍製在LaAlO3、NdGaO3、(La,Sr)(Al,Ta)O3、SrTiO3、DyScO3基板之上當底電極使用，約5nm厚。再鍍製BFO薄膜約20nm，NdScO3惟無適當之底電極，故直接鍍製BFO於基板之上。 我們對於不同試片量測θ-2θ掃描與倒晶格空間分析，判定在LAO與NSO上為Tetragonal與Orthorhombic晶體結構；以壓電力顯微鏡得知鐵電極化方向會隨著壓應力增加而漸漸垂直於膜面；K-edge可探討其離子環境與配位數，T相中鐵離子的配體為氧的五角錐，也造成了正負電荷中心的偏離；L-edge分析我們利用Fe2O3 磁軸旋轉的特性，可單純探討XMLD的貢獻，以此判定BFO的磁矩方向，將以上實驗數據整理可得知隨著相變化，鐵電、反鐵磁的排列如何改變。 以上皆探討應變對於多鐵性質的影響，最後我們用壓電力顯微鏡改變BFO的極化方向，成功的將R相BFO”反轉”成T相，表示應變與極化這兩個物理量是可逆的。

Magnetoelectric multiferroic materials which have coupled electric, and magnetic, that result ferroelectricity and ferromagnetism . These compounds are explored in transition-metal-oxide that present great opportunities for applications in information storage, sensors and green materials. BiFeO3 (BFO)is a room-temperature , single- phase magnetoelectric multiferroic which present ferroelectric polarizations along <111> directions and G-type anti-ferromagnetism . Our study suggests an isostructural change can be induced by epitaxial strain, which is usually driven by temperature or high pressure in solids. Such a transition can induce large displacement of ions that causes high polarizations and show great potential for green piezoelectrics. By using substrates with different lattice parameters for the growth of BFO thin films ,we can get different strain states. In order to fully understand how these strains affect the structure, polarization rotation, ferroelectric domains and environment of ions, we have several techniques to build the framework. X-ray analysis ,such as reciprocal space mapping has been used to understand the structure correlation with strain states. Piezoresponce microscopy has been used to probe the ferroelectric domains .We can also find how the local environment of iron and oxygen ions changed by using X-ray absorption near edge structure (XANES).