SrMn0.5Fe0.5O3 與Sr1-x-yCaxBayMn0.5Fe0.5O3 樣品其電子結構與磁特性在 應力效性下的影響之研究

Abstract

本論文使用基板應力與化學應力兩種方式，來改變SrMn0.5Fe0.5O3材料的晶格結構，並研究應力效應其電子結構與磁特性的影響。在基板應力的研究方面，我們對SrMn0.5Fe0.5O3粉末與其利用脈衝雷射鍍膜技術成長於(100)-SrTiO3 (STO)和(100)-LaAlO3 (LAO)基板的薄膜樣品，進行變溫磁性量測與變溫軟X光吸收光譜量測。實驗的結果顯示所有樣品中的鐵在室溫下的電子結構同時存在有3d5 Fe3+與3d5L Fe4+兩種不同價數的離子，而錳離子則為單一價數的3d3 + 3d4L Mn4+混合態。然而，其中SMFO/LAO樣品的3d5 Fe3+和3d3 Mn4+的特性在低溫35 K時出現很明顯的增長，這同時也表示樣品中磁性部分出現反鐵磁與鐵磁性競爭的溫度遠高於純SrFeO3樣品的尼爾溫度(TN~134 K)。我們認為發生此變化的原因是由於發生樣品發生在低溫時發生電荷歧化，此現象是由配體電洞的侷限化所造成。而在壓縮的應力效應下，發生配體電洞的侷限化的機會也隨之提高。而在化學應力的實驗部分，我們成功的合成一系列Sr1-x-yCaxBayMn0.5Fe0.5O3樣品。而軟X光吸收光譜的結果顯示，不論是利用化學應力法還是基板應力法，兩系列的樣品在應力效應下皆出現一樣的改變。並且我們在變溫磁性量測中發現，不論是參雜了Ca還是Ba的樣品，其自旋凍結溫度皆出現提升，但其應是由不同的原因所導致。在Ba參雜的樣品中，因為樣品的晶格結構出現了嚴重的扭曲，導致其反鐵磁交互作用減弱；而在Ca參雜的樣品中，壓縮的應力導致其反鐵磁與鐵磁交互作用的競爭增加。兩者皆在造成了自旋難以在低溫下形成長程有序排列的狀態。

This thesis use substrate-induced and chemically-induced strains to change the lattice structure of SrMn0.5Fe0.5O3 sample. In substrate-induced strain part, the electronic structure and magnetic properties of SrMn0.5Fe0.5O3 powder and films grown on (100)-SrTiO3 (STO) and (100)-LaAlO3 (LAO) substrates by pulsed laser deposition were investigated by temperature dependent magnetization and soft X-ray absorption. The results exhibit characteristics of 3d5 Fe3+, 3d5L Fe4+, and 3d3 + 3d4L Mn4+ at room temperature in all samples. However, the features of 3d5 Fe3+ and 3d3 Mn4+ increased significantly for SMFO/LAO at 35 K, which also displayed substantial competition between antiferromagnetic and ferromagnetic order well-above the Néel temperature of SrFeO3 (TN~134 K). We attributed this to being caused by charge disproportionation resulting from ligand-hole localization, which is more favorable to take place when sample is under compressive strain. On the other hand, in chemically-induced strain part, we synthesized a series of Sr1-x-yCaxBayMn0.5Fe0.5O3 sample. The results of soft X-ray absorption measurement are similar with to substrate-induced strain sample. Moreover, we found that the freezing temperature of both Ca/Ba doping samples shift to higher temperature. For Ba doping samples, the distortion in crystal structure weakens the AFM interaction. For Ca doping samples, the compressive strain enhances the competition between FM and AFM interactions. Both of which resulted in that the spin long-range order difficult to formed at low temperatures.