磁能譜學對電鍍鎳鐵奈米線之磁性質研究

Abstract

在這份研究工作中，以鎳鐵合金為研究對象，此合金系列已經成功被運用在自旋閥和磁遮蔽材料上；近來，鎳鐵合金的奈米結構已經是一個被密集研究的主題。本論文將製作鎳鐵奈米線陣列，並研究其巨(微)觀磁性、材料結構、電子結構三者之間的關係。 利用陽極氧化鋁(AAO)模板和脈衝式定電流電鍍法製作鎳鐵奈米線陣列，其中改變摻雜鎳的成分：NixFe1-x(X=0、0.3、0.5、1)，以及施加快速退火(RTA)製程和剛電鍍完的試片進行各種分析比較。透過振動樣品磁量儀(VSM)可測得飽合磁化量，利用繞射分析儀(XRD)可分析材料結構；為了進一步了解鎳鐵奈米線中單一元素的貢獻，透過同步輻射光源技術中的吸收光譜(XAS)及磁圓偏振圖譜(XMCD)可觀測鎳和鐵分別在費米能階附近的電子結構和自旋極化情況；最後，藉由總合法則(Sum rule)的仔細計算，可以從微觀磁性的角度來分析奈米線隨濃度和RTA製程後的變化。 從VSM結果，我們發現飽和磁化量會隨著鎳增加而減少，加上磁能譜學和總合法則計算結果，都表示由鎳扮演磁軟化的角色；磁軟化同時也伴隨結構變化，從BCC到BCC+FCC再到FCC。在RTA退火效應中，雖然奈米線(X=0.3、X=0.5)的飽合磁化量在退火前後幾乎保持不變，但從吸收光譜結果卻發現電荷從鐵傳輸至鎳的現象，而也看到X=0.3的電荷傳輸(Charge Transfer)效應會較X=0.5明顯。此電荷傳輸現象會導致鐵的氧化情形增加而氧化鎳則會還原成鎳金屬，進而使得鐵的原子磁矩下降而鎳的原子磁矩上升。總而言之，鐵和鎳的原子磁性變化並無法從RTA前後飽和磁化量(巨觀磁性)不變的結果中觀察到，所以本實驗所採用磁能譜學的微觀方法仍有其重要性。

Nickel-iron(Ni-Fe) alloys have been extensively used in magnetic-shielding and spin-valve systems, and their nanostructures have become a subject of intense of research in recent days. In this thesis, Ni-Fe alloys were fabricated into nano-wire structures and the wires’ coupled magnetic, electronic and structural degrees of freedoms which determine their macroscopic magnetic behaviors, were investigated. Pulse-electrodeposition method combined with an anodic aluminum oxide template were used to fabricate highly aligned NixFe1-x nanowires (x= 0、0.3、0.5、1). To improve nanowires’ magnetic properties, rapid thermal annealing (RTA) was applied, and the annealed samples were subjected to the comparison with the as-plated ones in terms of all analyses. The samples’ crystallographic structures and magnetic properties were identified using an x-ray diffraction (XRD) facility and a vibrating sample magento-meter (VSM), respectively. To further understand the wires’ magnetism with elemental specificity, x-ray absorption spectrum (XAS) and x-ray magnetic circular dichrosim (XMCD) were employed to probe the electronic state and spin-polarization in the vicinities of Fermi-levels of Ni and Fe, at BL11A, National Synchrotron Radiation Research Center (NSRRC). By operating XAS/XMCD over the Ni and Fe L2 and L3 absorption edges and followed by a careful sum-rule estimation, we were able to explore the wires’ composition-dependent behaviors, from the viewpoints of micro-magnetism. From VSM results, we found that the magnetization was reduced upon the increase of Ni. This suggests that the role of Ni was to magnetically soften the wires. Such magnetically softening was accompanied by a structural transition starting from a BCC, then a mixture of BCC and FCC, and finally to a FCC structure, with the increase of Ni. For the annealing effect, despite the total magnetizations of the wires (x = 0.3 and 0.5) remaining almost unaltered before and after the RTA, a charge transfer from Fe to Ni was observed upon the heat treatment, as probed by XAS/XMCD, and the transferring effect of x = 0.3 was more pronounced than that of x = 0.5. For x = 0.3, this has resulted in a more oxidized state for Fe, but a more reduced state for Ni, leading to a decrease and increase in Fe and Ni local moments, respectively. The compensation between the Ni and Fe local moments that were invisible to a macroscopic approach therefore explained the nearly invariable magnetization upon the RTA, which emphasized the importance of the microscopic approach adopted in this work.