超薄膜與奈米結構之磁區顯影

Magnetic Domain Imaging from Ultrathin Film to Nanostructure

吳啟彬(Chii-Bin Wu)

指導教授 ： 林敏聰

國立臺灣大學/理學院/物理研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.10493

摘要

磁區顯影工具是研究磁性奈米結構時很重要的設備，而自旋極化掃瞄式穿隧電子顯微鏡（ＳＰ－ＳＴＭ）和自旋解析掃瞄式電子顯微鏡（ＳＥＭＰＡ）是其中的兩種。磁性超薄膜的磁性很微弱，可是他們都具備有該樣品的磁區顯影能力。自旋極化掃瞄式穿隧電子顯微鏡已經發展超過十年，但是仍舊有許多問題還未解決，其中，探針磁性的控制是很重要的一個。在本文裡，磁性探針是用環狀的鎢絲、在其上蒸鍍鐵薄膜所製備。磁光柯爾效應的量測顯示這種探針末端的磁化方向可以由外加的飽和磁場來控制。如此探針的磁化方向便可控制在特定平行樣品膜面的方向。這樣的磁性探針被用來觀察錳/鐵（００１）樣品表面的形貌與穿隧能譜，而不同自旋方向的表面可以成功的被解析出來。另外，為了在超高真空環境下製備室溫下可以穩定存在，且是次微米尺度的磁區結構，我們將鐵超薄膜蒸鍍在部分氧化的鎳鋁（００１）表面。當沒有氧化的時候，鐵在鎳鋁上展現出磁易軸轉向的現象，體心立方鐵（００１）表面磁異向能可能是造成此現象的主因。同時，當磁易軸在膜面上時，鐵薄膜具有單軸異向能的存在。當鍍在部分氧化的鎳鋁表面，自旋解析掃瞄式電子顯微鏡的研究顯示，鐵的磁區變得分開來的長方形，長方形的大小不到一微米，由鎳鋁表面氧化的程度所控制。雖然鐵薄膜的厚度只有幾個原子層，磁區之間的磁偶極交互作用力仍然對相鄰磁區的指向起著關鍵性的作用。由這個結果可見磁區顯影工具對磁性奈米結構研究的重要性。

關鍵字

自旋極化掃瞄式穿隧電子顯微鏡 ； 自旋解析掃瞄式電子顯微鏡 ； 磁性奈米結構 ； 層狀反鐵磁結構 ； 鎳鋁(001) ； 鐵超薄膜 ； 單軸磁異向能

並列摘要

Magnetic domain imaging tools are important in studying the magnetic nanostructures. Spin-polarized scanning tunneling microscopy (SP-STM) and scanning electron microscopy with polarization analysis (SEMPA) are two of them. They both are capable of revealing domains of magnetic ultrathin film, which have weak net moment. SP-STM has been developed for more than one decade, but there are still some issues remained. The control of probe magnetization is an important one. The magnetic probe is fabricated by using ring-shaped and iron-coated tungsten wire. Magneto-optic Kerr effect measurement on the probe front end shows that by controlling the saturation field direction, we can fix the probe magnetization in the specific in-plane direction. Such ring is applied to the scanning tunneling microscopy and spectroscopy experiment on Mn/Fe(001), and spin contrast in the in-plane direction is demonstrated. Besides, to in situ prepare a magnetic ultrathin film with domain structure of sub-micron size and still stable at room temperature, ultrathin Fe is deposited on partially oxidized NiAl(001) surface. Without oxidation, Fe/NiAl(001) exhibits spin-reorientation transition (SRT) phenomenon, and has large domain structure and in-plane uni-axial anisotropy. The surface anisotropy is verified to be the origin of the SRT phenomenon. Deposited onto partially oxidized surface, the domain pattern of Fe, as revealed by SEMPA, turns out to be separated rectangles, with size controlled by the degree of oxidation of the NiAl(001) surface. Although the Fe thickness is only of several monolayers, the dipolar interaction between these rectangles is found to play an important role in determining the orientation of neighboring domains.

並列關鍵字

Spin-polarized scanning tunneling microscopy ； scanning electron microscopy with polarization analysis ； magnetic nanostructure ； layer-wised anti-ferromagnetism ； NiAl(001) ； Fe ultrathin film ； uni-axial anisotropy

參考文獻

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