在玻璃基板上成長之CoPt及CoPt/Ag薄膜的微結構與磁性質研究

Abstract

本研究以直流磁控濺鍍的方式，於室溫下在玻璃基板上鍍製CoPt奈米薄膜，以及之後在玻璃基板與CoPt膜層之間引入Ag底層，形成CoPt/Ag奈米複合薄膜，並探討當CoPt膜厚小於20 nm，Ag底層厚度對CoPt薄膜之顯微結構、磁性質及其易磁化軸方向的影響。 我們發現CoPt單層薄膜之起始序化溫度介於500∼600℃之間。由TEM平面明視野影像可以看到，隨著CoPt厚度的增加，其表面型態由不連續島狀結構逐漸轉變為連續膜，從MFM圖可以發現於700℃退火30分鐘之CoPt(7.5 nm)為單磁區狀態，在此單磁區狀態其矯頑磁力隨顆粒直徑的變化符合單磁區理論。當CoPt厚度為1 nm時，由於大部分的顆粒尺寸小於臨界直徑Dp，因此產生超順磁效應，造成整體薄膜之矯頑磁力的下降，大致上，於700℃退火30分鐘之CoPt單層薄膜的易磁化軸方向呈現random orientation。 在CoPt/Ag薄膜方面，當CoPt厚度較厚時，Ag底層的加入對CoPt之序化度以及垂直膜面磁性質才有所幫助。經由VSM的量測，我們可以大略看出隨著CoPt及Ag底層厚度的增加，其磁性質的變化如下：對於600℃退火30分鐘之CoPt/Ag薄膜而言，其矯頑磁場隨著CoPt和Ag底層厚度的增加呈現先升後降的趨勢，易磁化軸方向隨著CoPt厚度的增加先由random orientation逐漸轉向垂直膜面方向，再逐漸轉回random orientation；對於700℃退火30分鐘之CoPt/Ag薄膜，其矯頑磁場隨著CoPt和Ag底層厚度的增加而增加，易磁化軸方向隨著CoPt厚度的增加由random orientation逐漸轉向垂直膜面方向，顯示加入Ag底層後，CoPt/Ag薄膜的垂直膜面磁性質得到提升。

In this study, the CoPt films were deposited on glass substrates at room temperature by dc magnetron sputtering and introduced Ag underlayer to form CoPt/Ag nanocomposite films. The effects of Ag underlayer thickness on the microstructure, magnetic properties, and magnetic easy axis orientation of CoPt films were investigated as the thickness of CoPt films is less than 20 nm. The starting ordering temperature of the CoPt thin films is between 500 to 600 °C. From TEM bright field image, it can be seen that the surface image of CoPt formed discrete islands, and gradually change to continuous film as the CoPt thickness increase. After annealed at 700℃ for 30 minutes, the single domain state is found by the observation of MFM images at 7.5 nm CoPt. At single domain state, the variation of coercivity with particle size agree with the theory of single domain particles. Due to the most of particle size are smaller than critical particle diameter Dp at 1 nm CoPt, the effect of superparamagnetism causes the decrease of coercivity. Approximately, the orientation of magnetic easy axis of CoPt thin films is random. For CoPt/Ag thin films, The insert of Ag underlayer can help the property of ordering degree and perpendicular anisotropy as CoPt is thicker. By the measurement of VSM, we find the changes of magnetic properties with the increase of CoPt and Ag underlayer thickness are as following : After annealing at 600℃, firstly the coercivity of CoPt thin films increase with CoPt and Ag underlayer thickness, and then decrease. The orientation of magnetic easy axis firstly from random orientation turn to out-of-plane orientation, and then turn back to random orientation as the CoPt thickness is increased; After annealing at 700℃, the coercivity of CoPt thin films increases with CoPt and Ag underlayer thickness. The orientation of magnetic easy axis from random orientation turn to out-of-plane orientation gradually as the CoPt thickness is increased. The phenomenon reveal that the property of perpendicular anisotropy were promote As the Ag underlayer added.