高定向性熱解石墨表面缺陷誘發鈷及鐵薄膜斜向磁化行為

Abstract

本論文利用氬離子濺射高定向性熱解石墨基板，探討基板表面缺陷對鈷薄膜的成長與磁性的影響。利用掃描式穿隧顯微鏡觀察鈷原子在較平坦基板上小區域的表面形貌，鈷原子有向台階邊緣聚集、生成薄膜的傾向，而鈷薄膜的厚度隨著距臺階邊緣的距離減少而增加。歐傑電子能譜儀定量分析的結果，間接顯示出基板的缺陷會使鈷原子在平台上更均勻的成核、形成較均勻分佈的鈷顆粒薄膜。在磁性方面，我們利用垂直以及平行方向的磁光科爾效應來觀察缺陷對其的影響。在較平整的基板表面，鈷薄膜的易軸為平行磁化方向；然而，基板缺陷上生成的鈷薄膜在垂直及平行方向皆可測得柯爾訊號。經測試發現這個易軸為斜向的磁化方向，在厚度達到60 ML時仍可測得。代表基板表面的缺陷不只影響介面附近的成核行為，更影響之後薄膜成長的行為。為了更進一步探討，我們將鍍源換成鐵，觀察基板表面缺陷對鐵薄膜的磁性影響。在較平整的基板表面，鐵薄膜的易軸為平行磁化方向；基板缺陷上生成的鐵薄膜易軸方向呈現斜向磁化行為，其矯頑場有隨著鐵薄膜厚度增加而增加的趨勢。但鐵薄膜厚度為26 ML時，磁化方向會倒下、躺在平行磁化方向。基板表面缺陷除了誘發斜向磁化行為的發生之外，也影響了測得磁滯曲線的初始厚度。其生成的鈷及鐵薄膜所測得具磁性的初始厚度皆較平整的基板表面的薄。We have studied the effect of interface defects of Co thin films deposited on highly oriented pyrolytic graphite (HOPG). We used Ar+ ions sputtering to produce the interface defects, and studied the defecteffect on both of growth and magnetic properties. Scanning tunneling microscope (STM) and Auger electron spectroscopy (AES) were used to investigate the local and global growth conditions, respectively. The STM images of Co atoms deposited on planar HOPG showed the Co granular films preferring the regions of step edges. And the thickness of Co films increased with closing the step edge. Comparison the AES ratios of Co on smooth and sputtered HOPG, the Co films on sputtered surface formed more uniformly distribution. In addition, the Co concentration of LT(200 K)-grown was lower than RT-grown, indicating LT-grown Co atoms were more uniformly distribution. However, the LT-grown Co atoms aggregated during annealed process from 200 K to RT. Also, we measured the corresponding magnetic properties of Co films and Fe films deposited on planar and sputtered HOPG by in situ magneto-optical Kerr effect (MOKE) in the polar and longitudinal geometries. The onset thickness of sputtered HOPG was earlier than planar one. And the canted perpendicular magnetization was induced by surface defects, which can be observed even at the Co thickness of 60 ML. However, the magnetization of Fe films was transiting to in-plane at the thickness of 26 ML.