陽極氧化鋁模板輔助生長奈米柱陣列磁性質研究

Abstract

本研究以不同陽極氧化處理條件在矽基板上有效控制陽極氧化鋁形 貌，並成功在矽基板上製作出大孔徑的AAO模板。利用無電鍍鎳磷反 應，在矽基板上生長不同孔徑的奈米柱陣列以及薄膜。透過掃描式電 子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)觀察試片之表面形貌以及微 觀結構；使用能量散佈分析儀(EDS)探討化學成分之變化，震動樣品 磁量測儀(VSM)則用來比較各試片的微結構以及磁學特性。我們發現 無電鍍鎳磷為2-3nm鎳奈米晶的超順磁結構，而透過快速退火處理可 以誘發晶粒沿長軸成長提升飽和磁化率以及矯頑磁場，產生超順磁- 鐵磁相轉換；增加退火時間會致使晶粒成長，故造成不同條件之試片 有相異磁性質，暗示磁性的改變是由更微觀的結構改變所驅動。經由 XMCD與sum rule計算出微觀磁矩的磁性貢獻程度(spin moment、 orbital moment)發現無電鍍鎳磷的鐵磁性主要來源為自旋磁矩 (spin moment)的貢獻，而經退火處理後自旋磁矩提升亦可驗證增加 退火時間的確可以提升鐵磁性。

In this study, we demonstrate a successful control of the morphology and pore size of anodic aluminum oxide (AAO) template on silicon substrates with proper anodizing conditions. These AAO templates enabled the fabrications of electroless-deposited Ni thin films, and Ni nano-arrays with tunable diameter. Samples’ surface morphologies were probed using a scanning electron microscope (SEM, JSM 6500F) operated at 15 kV. A high resolution transmission electron microscope (HRTEM, JEM-2100F, operated at 200 keV) was used to probe the atomic-scale images. Compositions were identified by an energy-dispersive x-ray spectrometry (EDX). Magnetic properties were analyzed by a vibrating sample magnetometer (VersaLab, quantum design manufacture). XAS spectra were collected over Ni L2 and L3 edges with a total electron yield mode (TEY) at BL11A of National Synchrotron Radiation Research Center (NSRRC). XMCD signals were collected along with XAS by reversing the x-ray helicity to probe the spin-polarization state of the arrays. XMCD signals were taken under an applied field of 1 Tesla. The results show that the magnetic properties were highly dependent on microstructure and crystallinity. The as-plated sample exhibited a superparamagnetic behavior and it transformed into ferromagnetic upon post-annealing process.TEM and VSM results confirmed the grain growth upon heat treatment, which led to the enhancements of saturation magnetization and coercivity. Finally, XMCD sum rules were operated to estimate spin and orbital moments of the samples. The results show that spin moment dominated the magnetic properties.