奈米氧化鐵粒子其磁性質對粒徑和細微結構之相依性研究

Abstract

即便已被研究了數十載，氧化鐵現在仍是凝態物理中廣泛受人研究的材料。因奈米氧化鐵的獨特性質，它被應用在許多尖端科技，像是: 高密度的磁記憶元件、自旋電子學、磁熱治療、標的藥物釋放、核磁共振顯影…等；這些應用為近幾年熱門的研究項目。本論文研究對象為熱裂解法合成之奈米氧化鐵粒子，其直徑範圍為6-25nm；研究目標旨在了解改變粒徑大小和合成條件後，其局部結構和磁性狀態的改變對整體磁性行為之影響，使用儀器為光源是同步加速器的X光吸收光譜(XAS)和震動樣品磁量儀(VSM)。使用同步輻射光源去偵測原子尺度排列之訊號，再配合元素選定之微觀磁性量測連結宏觀磁性之展現；由以上三者，詮釋了磁性行為與抽象微妙的電子世界的連結性。 自X光近吸收邊緣結構(XANES)和X光吸收邊緣延續光譜細微結構(EXAFS)，分析發現奈米氧化鐵粒子均為Fe3O4，除了粒徑最小的樣品有α-Fe摻雜其中，而第一鄰近原子層之配位數隨著粒徑變小而下降。基本上，平均配位數下降表示表面原子斷鍵增加，而這導致表面磁矩的偏折、無序狀態，因此飽和磁化量會隨之下降；但粒徑最小卻有α-Fe摻雜其中的樣品，磁化量相比其它樣品有明顯上升之跡象。此外，因為在α-Fe和Fe3O4的介面上，有這兩相的磁交互作用，導致異向性常數K的大幅上升，故含有α-Fe的樣品雖然粒徑最小，但在室溫下卻不是超順磁狀態，有明顯的矯頑力和殘磁存在。

Despite being studied for many decades, iron oxide still remains at the forefront of condensed matter research. Iron oxide has found its way into numerous technology applications. For example, scaling iron oxide down to nano-scale to meet the needs in magnetic recording media and targeted drug delivery has become the subject of intense research in recent years. This work is aimed at understanding the influence of local magnetism/structure change of the cation site on the magneto-structural properties, probing the nature of ferromagnetic (FM) behaviors for iron oxide nanoparticles (NPs, ranging from 6 to 25 nm in diameter) prepared by thermal decomposition syntheses, and their evolution with the NP size, using synchrotron-based x-ray absorption spectroscopy (XAS) and a vibrating sample magnetometer (VSM). The ability of synchrotron radiation to penetrate matter, to probe the atomic scale-length, to couple the magnetism with the selected atomic species provides a critical link between a material’s macroscopic magnetic responses and otherwise a world of invisible electronic interactions. From x-ray absorption near edge structure (XANES) and extend x-ray absorption fine structure (XAFS) analyses the iron oxide samples appear to display Fe3O4 as the major phase. The coordination number of the first neighboring shell decreases with decreasing NP size, and an α-Fe phase was observed in the sample with the smallest size (~ 6 nm). In general a decreased coordination number suggests the increased broken iron bonds at surface, which should result in a magnetically disordered state with reduced saturation magnetization, yet the sample featuring the α-Fe phase violates such tendency but exhibits an enhanced saturation magnetization compared to the other two with larger NP sizes. Moreover, due to the magnetic exchange interactions between the α-Fe and Fe3O4, the sample featuring the α-Fe phase doesn’t exhibit a superparamagentic (SPM) state because of fairly notable anisotropy constant and coercive field at room temperature. The study uncovered new information about the coupling between magnetic and structural properties of the venerable magnet.