Ti, Zr, Hf底層效應對鐵鉑薄膜結構、內應力與磁性之影響

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Title:	Ti, Zr, Hf底層效應對鐵鉑薄膜結構、內應力與磁性之影響
Other Titles:	Effect of Ti, Zr and Hf underlayer on the structure, internal stress, and magnetic properties of FePt films
Authors:	申繼陽 C.Y Shen
Contributors:	張晃暐 H. W. Chang 應用物理學系
Keywords:	微結構 microstructure
Date:	2014
Issue Date:	2015-12-22T08:28:28Z (UTC)
Abstract:	本研究採用FePt為主要研究材料，以濺鍍方式製備FePt薄膜試片。首先，探討不同底層 ( Ti, Zr, Hf ) 之效應對150 nm厚之Fe45Pt55膜結構、微結構、磁區結構、殘餘應力與磁性之影響。在初鍍態下，所有的薄膜試片均為非序化結構。在單層薄膜中，大規模的L10序化發生在700 oC的熱處理下，且形成強烈的（111）優選方位，矯頑磁力增加到8.6 kOe。另一方面、在雙層薄膜中，大規模的序化發生在500 oC之熱處理條件，導致了更高的矯頑磁力（9.8-12.5 kOe）。具底層的試片不同於單層薄膜的試片，而呈現等方性結構，並產生顆粒狀的表面形貌。而雙層FePt低溫的序化有效降低了晶粒尺寸和磁區的大小，因而增進了磁性晶粒間之交互作用力。此外，底層的加入也誘發了較高的拉伸殘餘應力，雖降低了序化之熱活化能而增進序化，不過，其對序化的影響卻是次要的。底層的加入造成了明顯磁能積的增進，從單層薄膜的9.5 MGOe到Ti, Zr, Hf底層的18.1到20.2 MGOe，提高了110 %之多。其次，藉由降低Fe45Pt55層厚度至20 nm研究Hf底層對其界面之影響。研究結果顯示當雙層薄膜在退火的時候，若溫度較低在400-500 oC，序化提前發生，其界面的擴散並不明顯。然而，當溫度為600-800 oC時，界面的擴散變得非常重要；在700-800 oC時，界面的擴散已從單純的原子混合演變到一個金屬化合物的形成，這個金屬化合物已經證實為Pt50Hf50。這個金屬化合物的形成導致了FePt層內Pt的消耗，所以產生了過量的Fe，又藉由高溫進而形成了Fe3Pt相，導致了FePt層硬磁性的消失。最後，以提高FePt層中鐵的含量至Fe50Pt50，期望得到更高的磁化量值，進而增進磁能積，再將鐵鉑厚度提高到300 nm，使試片的狀況更為接近實際應用的尺度。研究結果顯示當厚度與成份改變時，Hf底層仍具有提高磁能積的效果，儘管磁能積的增進不如之前研究所發現的數據這麼明顯，但是，這種增進仍然是存在的。本文最佳磁性質出現在300 nm Fe50Pt50/10 nm Hf 經600 oC後退火，可得磁能積為23.6 MGOe及矯頑磁力為13.2 kOe。本研究的結果證實以高熔點Ti, Zr,及Hf底層可增進FePt膜之永磁性質，其中以Hf底層之效果最佳，以此簡單方法可提升FePt膜之永磁性質十分具有實際應用的潛力。 FePt with L10 structure shows strong magnetic anisotropy and corrosion resistance, making it become one of the most studied materials in the past decades. Many efforts have been focused on optimizing hard magnetic properties due to the potential of applications for micro-electro-mechanical systems ( MEMS ) and bio-magnets on dentistry. At first, effect of Ti, Zr, and Hf underlayers on the structure, microstructure, internal strain/stress, and magnetic properties of sputter-prepared 150-nm-thick Fe45Pt55 films were studied. The structural results confirm the extensive ordering of FePt in the bilayer films obtained with annealing at temperature ( Ta ) 200 ˚C lower than that at which the single layer film has similar ordering. Microstructure data indicate an obvious grain refinement in the bilayer films. Magnetic force microscopy shows a reduced interaction domain size with M underlayer. We suggest that the intermixing between the FePt and underlayers during the post annealing may lower ordering temperature by stimulating the nucleation of the L10 phase, causing refined grain and magnetic domain sizes, leading to remarkable enhancement in permanent magnetic properties at lower Ta. The optimized ( BH )max of FePt films is greatly enhanced from 9.5 MGOe for single layer to 18.1, 19.8, 20.2 MGOe for Ti-, Zr-, and Hf-underlayered films, respectively. Furthermore, in order to understand effect of Hf underlayer on the FePt/Hf interface, thickness of FePt film is reduced to 20 nm, and their structure and magnetic properties with various post annealing temperature are studied. For both FePt and FePt/Hf samples, the as deposited films are disordered and L10-ordering is triggered by a 400˚C-annealing. At Ta ≥ 600˚C, Hf-Pt intermetallic compound forms with increasing Ta, which consumes Pt in FePt layer and results in the formation of Fe3Pt phase. The film becomes soft magnetic at Ta = 800˚C. The optimized condition of FePt/Hf film is in the Ta range of 500 to 600˚C where the interdiffusion between Hf and FePt layer is not extensive. The value of Hc is 8.9 kOe and Mr is 650-670 emu/cm3. Unlike FePt films, the Hf-undelayered samples show significantly reduced out-of-plane remanent and coercivity. The values for both are around 50% smaller than that of the FePt films. Additionally, Hf underlayer markedly reduces the FePt grain size and narrows the distribution, which enhances magnetic intergrain coupling. Good in-plane magnetic properties are preferred for the uses like a hard biasing magnet in a spintronic device. Finally, in order to obtain higher magnetization and thus enhanced ( BH )max, Fe content is increased to 50 at%, and besides, thickness is increased to 300 nm for applications in MEMS. The experimental results show that Hf underlayer also promote the ordering and thus enhanced ( BH )max of FePt, even the promotion of enhanced ( BH )max is inferior as compared to Fe45Pt55 alloy film system. In this study, the optimal magnetic properties of ( BH )max = 23.6 MGOe and iHc = 13.2 kOe for 300 nm Fe50Pt50/10 nm Hf films post annealed at 600 oC. The results of this study demonstrate that permanent magnetic properties of FePt films can be remarkably improved with Ti, Zr, and Hf underlayers, especially for Hf underlayer and this present simple method to improve magnetic properties of FePt films has potential applications in MEMS
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