Preparation and characterization of manganese and cobalt doped yttrium oxide magnetic nanoparticles

過渡金屬(鈷,錳)攙雜之氧化釔磁性奈米粒子

吳泰興(Tai-Sing Wu)

指導教授 ： 蘇雲良

國立清華大學/理學院/物理學系/碩士(2010年)

摘要

本研究主要針對以高介電值(high-k material)材料為主體的稀磁性氧化物(DMOs, diluted magnetic oxides )系統，研究其氧空缺與磁性的關聯。透過熱分解法(thermal decomposition)於化學液相環境(chemical bath)中合成粒徑尺度為奈米等級的氧化釔顆粒，並嘗試將過渡金屬中具有3d軌域電子的鈷、錳原子攙雜於其中；再通過氣氛退火方式，控制樣品中氧空缺(oxygen vacancy)的含量。
透過X光粉末繞射及高解析穿隧式電子顯微鏡(HRTEM)分析，確認粉末樣品主體為氧化釔方錳鐵礦(cubic bixbyite)晶型，並無鈷、錳氧化物的相(phase)混雜，且奈米顆粒皆為單晶，粒徑大小約為9奈米。使用近邊X光精密吸收結構(XANES)排除雜質原子形成氧化物或叢聚(cluster)的可能，並以延伸X光精密吸收結構(EXAFS)分析確認鈷、錳原子皆鑲嵌(interstitial)於氧化釔主體結構中，同時確認鈷、錳原子的價態(valence stata)以及鈷、錳、釔原子的氧配位數(coordination number)皆會因氣氛退火而有規律的改變。磁性量測則以超導量子干涉儀(SQUID)進行，於鈷、錳攙雜樣品中皆量測到順磁性(paramagnetism)以及鐵磁性(ferromagnetism)訊號的疊加，於錳攙雜的樣品中發現室溫鐵磁特性。在使用相同方式製備無任何攙雜的氧化釔樣品中，則量測到反磁 (diamagnetism)特性，確認經由鈷、錳原子攙雜產生順磁與鐵磁特性。
為了找出磁性與結構缺陷的關連，引進BMPs(bounded magnetic polarons)理論進行定性的分析，成功的解釋磁性行為在不同退火條件下以及不同樣品間的變化，確認在此一系統中產生鐵磁性的機制應該是BMPs理論。

關鍵字

XAFS ； DMOs ； BMPs ； Y2O3

並列摘要

In this thesis, we have studied the correlation between magnetism and oxygen vacancies in yttrium oxide based diluted magnetic oxide (DMO) systems. Yttrium oxide nanoparticles (NPs) doped with 3d transition metals (TM) cobalt and manganese are synthesized by thermal decomposition of yttrium acetylacetonate (Y(acac)3) in oleylamine. Thermal annealing in either oxygen or forming gas was employed to adjust the concentration of oxygen vacancies. The nanoparticles' size and structures of the Y2O3 host were investigated by x-ray powder diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). Cobalt and manganese K-edge x-ray absorption near-edge structure (XANES) spectra were used to determine the oxidation state of the dopant atoms. Local environments surrounding Co and Mn were probed by using extended x-ray absorption fine structure (EXAFS) techniques. Both Co and Mn impurity atoms were found to locate on interstitial sites with O nearest neighbors and shorter TM-O bond as compared to the Y-O bond in the Y2O3 host. Forming gas (5%H2/95%N2) annealing has significantly increased the concentration of the oxygen vacancies while oxygen gas annealing has decreased the oxygen vacancies in all samples. As revealed by SQUID measurements performed at temperatures 10K and 300K, the Co- and Mn-doped samples show combinations of ferromagnetism and paramagnetism while an undoped sample is diamagnetic. This indicates that the observed ferromagnetism is indeed due to the doping of transition-metal atoms in the samples. Our experimental results on the correlation between oxygen vacancies and magnetism appear to be rather consistent with the bound magnetic polaron model.

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