以中子繞射與磁性量測探討奈米金顆粒表面與內核之自發磁矩;Intrinsic surface and core magnetic moments in Au nanoparticles - neutron diffraction and magnetization studies

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题名:	以中子繞射與磁性量測探討奈米金顆粒表面與內核之自發磁矩;Intrinsic surface and core magnetic moments in Au nanoparticles - neutron diffraction and magnetization studies
作者:	李祈延;Li,Chi-Yen
贡献者:	物理研究所
关键词:	中子散射;磁性;奈米顆粒;Neutron scattering;Magnetism;Nanoparticle
日期:	2012-08-30
上传时间:	2012-09-11 18:41:25 (UTC+8)
出版者:	國立中央大學
摘要:	我們以氣化冷凝法製備一系列粒徑各異、且表面無附有其他化學分子之奈米金顆粒粉末。利用X-射線繞射譜圖分析粒徑，判定所研製不同顆粒大小之粉末樣品的平均粒徑為3奈米至8.4奈米。從磁化強度量測的實驗結果，觀察到奈米金顆粒的自發磁性，以及當量子侷域效應顯現後由磁場所引發之感應磁矩。顆粒磁矩隨著溫度升高而增大，此可由自旋波模型(Spin-wave model)來解釋熱磁激發之現象。而量子奈米顆粒自發磁性的內涵來源，與顆粒表面電子轉移(charge transfer)有關。從中子粉末繞射實驗的結果，可探測奈米粉體的微觀磁性。由5 K與300 K的磁繞射峰計算出4.3奈米金顆粒的磁矩大小為0.14個波爾磁子，指向[111]晶面方向。且除了表面金原子外，內層金原子對自發磁矩也有貢獻。我們並發現不同粒徑之奈米金顆粒，其各異的s-d 混成軌域之能量，直接反應了自發磁性的強弱。利用壓合模具進一步縮短顆粒間的聚合密度，發現顆粒間交互作用的增強，會造成顆粒表面導電電子密度的下降，進而使表面金原子與內層金原子間電子重新分布(charge redistribution)，同時影響顆粒磁矩的大小。Five sets of capping-free bare Au nanoparticle samples are fabricated by gas condensation method. X-ray diffraction, energy dispersive x-ray spectroscopy, and transmission electron microscopy are used to characterize the samples. We determine the mean particle diameter by fitting the diffraction peaks of finite-sized nanoparticles, assuming the same size distribution obtained from the TEM images, ranging from 3 to 8.4 nm. Neutron powder diffraction detects ferromagnetic moment of μz = 0.14 μB that points in the [111] direction. The magnetization of 3 nm Au nanoparticle assembly reveals a Langevin profile and a Brillouin profile which can be understood as the condensation of quantum-confined electrons into the Zeeman spin-polarized states. The M(Ha) characteristic signifies the existence of spin correlations in the Au nanoparticle assemblies. Spin arrangements in loosely packed 3 nm Au particles assembly are found to be ferrimagnetic-like, where the core and surface moments point in opposite directions. The magnetic transition in interacting nanoparticles will be clearly seen. Reductions in magnetization and density of conduction electrons upon closing up the interparticle separation are also observed. Size dependence of magnetic entropy change at Ha ~ 10 kOe obtained from the isothermal M(Ha) curves, indicating the interplay between surface and core magnetic moment. The entropy change increases for 3 nm Au particle assembly below 50 K, whereas the weak surface contribution is observed as particle diameter increasing to 8.4 nm
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