Co/Ni/Pt(111)的磁性研究

Abstract

我以實驗室自製的磁光柯爾效應儀（SMOKE），進行不同層數的Co原子層在up layer，以1 ML Ni當buffer layer 在Pt(111)上的研究。就實驗室先前的研究，我們知道在平坦的Pt (111)表面鍍上Co超薄膜時，當Co薄膜的厚度小於4 ML時，具有垂直磁異向性（PMA），即磁化易軸的方向為垂直於樣品表面的方向(out-of-plane)，Co厚度繼續增加時，磁化易軸的方向會轉為平行於樣品表面的方向(in plane)，spin reorientation transition發生在4～5 ML間。我所研究的dCo Co/1 ML Ni/Pt(111)系統，室溫與450K下準備樣品，0～3 ML Co/1 ML Ni/Pt(111)進行SMOKE量測，在polar和longitudinal方向皆無訊號，而4＆5 ML Co/1 ML Ni/Pt(111)，我們量測到longitudinal的柯爾訊號；而室溫準備樣品再升高到450K量測，發現1 ML Co/1 ML Ni/Pt(111)的磁化易軸在out-of-plane方向，而2＆3 ML Co/1 ML Ni/Pt(111)無柯爾訊號，4＆5 ML Co/1 ML Ni/Pt(111)，我們量測到longitudinal的柯爾訊號；並對dCo Co/1 ML Ni/Pt(111)系統做升降溫實驗，瞭解Co、Ni與Pt合金之後的磁性狀態。 另外，我也研究了dNi Ni/1 ML Co/Pt(111)系統，和先前實驗室將Ni超薄膜鍍在Pt(111)上，dNi Ni/Pt(111)的磁化易軸與樣品面夾一角度，經分析得 ，做個比較。學生所研究dCo Co/1 ML Ni/Pt(111)的系統，一開始1 ML Co/Pt(111)的磁化易軸為out-of-plane方向，蒸鍍上Ni原子層，直到24 ML Ni/1 ML Co/Pt(111)，磁化易軸仍為out-of-plane方向。並對2、12＆24 ML Co/1 ML Ni/Pt(111)做升降溫實驗，瞭解Co、Ni與Pt合金之後的磁性狀態。 我的研究結果顯示，當僅有1 ML Ni當buffer layer，Ni在白金上為磁性的dead layer，所以當Co原子層鍍上去，沒有out-of-plane方向的磁化易軸，當Co原子層數大於3層，磁化易軸為in-plane方向，與Co塊材的磁性狀態相似。而1 ML Co當buffer layer，Co在白金上造成，磁化易軸即為out-of-plane方向，繼續鍍上鐵磁性的Ni原子層，造成磁化易軸的強度增強；所以Ni-Pt與Co-Pt造成不同的鐵磁性質。Experiments were performed in situ in a stainless ultrahigh vacuum (UHV) chamber. In situ magneto-optical Kerr effect (MOKE) was used to study the magnetic properties of the system. I study the magnetic properties of ultrathin Co films on Pt(111) with Ni buffer layers. We prepare dCo Co/1 ML Ni/Pt(111), and the Kerr signals measured at room temperature. The polar and longitudinal Kerr signal was not exist for 0 ~ 3 ML Co/1 ML Ni/Pt(111). The easy axis of the magnetization for 4＆5 ML Co/ Pt(111) is in-plane. We find the same result, when dCo Co/1 ML Ni/Pt(111) was prepare at 450K, and the Kerr signals measured at 450K or at room temperature. If we prepare dCo Co/1 ML Ni/Pt(111) at RT, and the Kerr signals measured at 450K. The easy axis of the magnetization for 1 ML Co/ Pt(111) is out-of-plane. The polar and longitudinal Kerr signal was not exist for 2＆3 ML Co/1 ML Ni/Pt(111). The easy axis of the magnetization for 4＆5 ML Co/ Pt(111) is in-plane. The variation of AES of 1 ML Co/1 ML Ni/Pt(111) as a function of sample temperature. We find the mixing of Ni and Co layers occurs at 420K. The formation of Co-Pt alloy causes the perpendicular magnetic anisotropy. I also study the magnetic properties of ultrathin Ni films on Pt(111) with Co buffer layers. The easy axis of the magnetization for 1 ML Co/ Pt(111) is out of plane. We were surprised that only the polar Kerr signals were observed when dNi 24 ML. Studies of the magnetic properties showed that the easy axis of the magnetization changed from the cant to the out-of-plane direction when the Co buffer layer was inserted on Ni/Pt(111). After different temperature annealing, cause the different competition alloy. The competition in the alloy formations between Co-Pt and Ni-Pt in the Co/Ni/Pt(111) and Ni/Co/ Pt(111) system is also interesting, cause the different magnetic properties. I also try to understand that.