鈷鉑表面合金與鐵超薄膜介面的磁性研究

Abstract

我們利用歐傑電子能譜儀(AES)、低能量電子繞射儀(LEED)及表面磁光柯爾效應儀(SMOKE)探測鐵超薄膜成長在鈷鉑表面合金系統中表面成分、結構及磁性的變化；並且搭配升溫退火效應探討系統隨著退火溫度上升而造成表面成分、結構及磁性的變化。當鈷鉑形成表面合金的溫度為710 K時，鐵薄膜在成長的過程中會發生結構上的轉變，導致極向的柯爾旋轉角均會有最大值，且矯頑力仍維持原鈷鉑合金之大小，顯示其可以產生有效的垂直磁交換耦合彈簧效應。在較低的合金形成溫度550 K時，鐵薄膜在成長的過程中亦會發生結構上的轉變且極向的柯爾旋轉角亦會有最大值，但其柯爾旋轉角與矯頑場均小於710 K的鈷鉑表面合金系統，推測是由於退火溫度較低時，形成的鈷鉑表面合金結構不完美所導致。將鐵超薄膜成長在不同鈷薄膜厚度下所形成的表面合金，發現當鈷薄膜厚度僅有0.7 ML時，較不利於產生垂直的磁交換耦合彈簧效應；故我們可以推論出鐵薄膜成長在鈷鉑表面合金上要具有有效的垂直磁交換耦合彈簧效應時，增加鈷鉑表面合金的退火溫度與增加鈷薄膜的厚度均有利於增強有效的垂直磁交換耦合彈簧效應，藉此可以有效提高儲存密度。將鐵成長在鈷鉑表面合金上並對其進行加熱退火效應，量測磁性及表面原子的變化，由歐傑電子能譜儀觀測表面原子組成，可發現鐵原子隨著退火溫度上升會往基底擴散，而鈷原子則是會先往表面擴散後再繼續往基底擴散。而觀察其矯頑場的變化，可以發現在退火溫度升高至700 K以上時，矯頑場會大幅度上升，推測來源為Fe-Pt的形成，比較鐵成長在白金基底上的退火效應，發現相同的退火溫度中，Fe/Pt(111)系統的矯頑場會大於Fe/Co-Pt/Pt(111)，顯示在Fe/Pt(111)系統中鈷的摻雜不利於矯頑場的增加。

關鍵詞：表面合金、鈷鉑、磁性、磁交換耦合彈簧效應

Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and surface magneto-optical Kerr effect (SMOKE) are used to investigate compositions, surface structure and magnetic properties of ultrathin Fe films on the top of Co-Pt surface alloys. On Co-Pt(111) surface alloys formed at 710 K, as Fe films grow to moderate coverage, the polar Kerr rotation increases and the polar coercive force remains the same as Co-Pt alloy. A Fe/CoPt system shows polar exchange-spring behavior. On Co-Pt(111) surface alloys formed at 550 K, the structure of Fe overlayer also changes and the polar Kerr rotation increase. Due to the lower annealing temperature and the imperfect of CoPt layer,both the coercive force and polar Kerr rotation are smaller as compared to that formed at 710 K. For Fe films on 0.7 ML Co-Pt, the polar exchange-spring magnetic behavior is less pronounced. We infer that both higher annealing temperature and thicker Co-Pt are advantageous for polar exchange-spring. As annealing temperature increases below 800 K for a Fe/Co-Pt system, Fe atoms diffuse into substrate and Co atoms diffuse into the surface. At higher temperatures, both Fe and Co atoms intermix with Pt(111) to form alloy phase. The coercive force for 3 ML Fe/1 ML Co-Pt/Pt(111) is small than that of 3 ML Fe/Pt(111). The additive of Co atoms in Fe-Pt is harmful for the performance as a hard magnet.

Keyword：surface alloy, cobalt, platinum, magnetic properties, exchange-spring magnetic.