氧化鋅摻雜鈷奈米線磁疇之研究

Abstract

為了發展可應用自旋極化電流的電子元件，在半導體中摻雜少量磁性原子的稀磁性半導體近年來已引起科學家廣泛的研究。由於其結合半導體電性及過渡金屬磁性的物理性質，因此我們選擇具有室溫鐵磁性且磁性來源不受合金團簇影響的氧化鋅摻雜鈷奈米線作為研究材料。 在本論文中，我們探討磁疇反轉的現象並且利用磁力顯微鏡量測單根奈米線的殘留磁化量。首先對我們取得的奈米線磁力影像依照亮暗紋分佈作定性的描述並且分成三類，即沒有磁力訊號、亮暗紋不均勻以及亮暗紋均勻的奈米線。奈米線的磁力影像呈現出不同的亮暗紋區段，表示鐵磁性的磁疇存在於稀磁性半導體奈米線中。在我們的數據中，亮暗紋不均勻的奈米線具有較複雜的磁疇結構以及較高的居禮溫度，並且矯頑力大於探針的游離場，因此此類奈米線無法觀察到磁疇反轉的現象。相較之下，亮暗紋均勻的奈米線則是由多個單磁疇結構所組成並具有較小的居禮溫度，並且此類奈米線觀察到會隨著探針的反覆掃描而隨機發生磁疇反轉現象。根據分析以及理論擬合的結果，我們可推論此磁疇反轉的現象是磁力探針的游離場所造成的。

Owing to a speedy development of the spintronic technology, diluted magnetic semiconductor (DMS) have attracted much attention in recent years. Among all the DMS materials, Zn1-xCoxO is one of potential candidates, thus we study its room-temperature ferromagnetic property by using magnetic force microscopy (MFM). In this work, we will discuss magnetic domain reversal in individual Zn1-xCoxO nanowire and remanent magnetization obtained by using MFM at room temperature. As a first step, according to MFM phase images, we qualitatively categorized Zn1-xCoxO nanowires into “nonmagnetic”, “inhomogeneous-grain” and “homogeneous-grain” nanowires. The MFM phase images show bright and dark areas, implying ferromagnetic domains in our DMS Zn1-xCoxO nanowires. We found that inhomogeneous-grain nanowires exhibit a complicated magnetic domain arrangement and a high Curie temperature. Furthermore, its coercivity is greater than the stray field of our MFM tip, so the effect of magnetic domain reversal cannot be detected on these nanowires. In contrast, homogeneous-grain nanowires reveal multiple single magnetic domains and a low Curie temperature. Magnetic domain reversal has been observed during each scanning. Basing on analysis and fitting to theoretical models, we propose that the magnetic domain reversal is induced by the stray field of the MFM tip.