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Relationship between Stress Induced Abnormal Grain Growth and (001) Preferred Orientation in FePt
|作者:||Chen, Li Heng
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Nix, Mechanical properties of thin films vol. 20, 1989.  C. V. Thompson and R. Carel, "Texture development in polycrystalline thin films," Materials Science and Engineering: B, vol. 32, pp. 211-219, 1995.  C. V. Thompson and R. Carel, "Stress and grain growth in thin films," Journal of the Mechanics and Physics of Solids, vol. 44, pp. 657-673, 1996.  D. C. Berry and K. Barmak, "Time-temperature-transformation diagrams for the A1 to L10 phase transformation in FePt and FeCuPt thin films," Journal of Applied Physics, vol. 101, pp. 014905-014905-14, 2007.  R.-H. Robert E, Physical Metallurgy Principles: International Thomoson, P.534 1994.  R.-H. Robert E, Physical Metallurgy Principles: International Thomoson, P.533 1994.  J. K. Mei, F. T. Yuan, W. M. Liao, Y. D. Yao, H. M. Lin, H. Y. Lee, and J. H. Hsu, "Effect of initial stress/strain state on formation of (001) preferred orientation in L10 FePt thin films," Journal of Applied Physics, vol. 109, pp. 07A737-3, 2011.||摘要:||
L10 FePt由於具有高磁晶異向能(Ku~7×107erg/cm3)、高居理溫度(~450 oC)等特性，因而具有成為下一世代硬碟材料的潛力。但由於在室溫下濺鍍之FePt薄膜不具有硬磁性，因此將其作退火處理，必須注意其(001)優選方位以及磁性質的優良與否，而得到工業上所需要之硬磁特性。由於FePt之前有許多相關研究為了解其織構翻轉的主要原因，卻沒有人使用微結構來觀察。由本實驗可以發現藉由TEM影像的拍攝，能清楚了解前所未知的織構翻轉機制。
本實驗在室溫下濺鍍FePt薄膜，膜厚的部分分為10 nm、20 nm、30 nm以及40 nm，退火溫度由450 oC到800 oC，每50 oC取一個參數與室溫濺鍍未經熱處理之FePt薄膜做實驗上的觀察，經由國家同步輻射中心X-ray繞射量測其殘留應變並針對FePt之磁性質、結晶結構以及微結構做分析與討論。其中結晶結構的部分包含序化度、LOF等。
實驗中可發現10 nm FePt薄膜，在退火溫度500 oC發生由於相變化的原因，造成晶粒尺寸由12.0 nm縮小至8.5 nm。接著在550 oC時，藉著經應力誘導與表面能誘導以致(001)晶粒與(111)晶粒同時成長，晶粒尺寸大幅上升至16.0 nm。之後因為誘導機制的轉換，使得新的(001)晶粒生成，並且(111)晶粒開始溶解，以致 10 nm FePt薄膜從退火溫度600 oC到650 oC，其平均晶粒尺寸為從16.0 nm縮小至7.5 nm，此現象使得最後試片之中幾乎都是(001)優選方位之晶粒，並且發生應力誘導 (001)晶粒成長之現象，因而我們也得到了適合工業上所使用於記錄媒體的優良磁性質。
藉由FePt膜厚不同的實驗，針對殘留應變、磁性質以及微結構做分析與探討，發現當膜厚越厚時，將會導致磁性質與(001)優選方位不良的結果。其中，觀察到10 nm FePt薄膜其初始應變為0.7 %拉伸應變，反觀，40 nm FePt薄膜初始應變為-0.18%壓縮應變，由於拉伸應變可明顯幫助相變化造成的壓縮應變釋放，此一結果，明顯影響到之後磁性質與(001)優選方位之結果。而且由應變圖中發現，序化後10 nm FePt薄膜要比40 nm FePt薄膜其應變增加量大的許多，使得(001)晶粒有較強的晶粒成長誘導來源，並且(111)晶粒也有足夠的時間溶解，因而造成較低的膜厚有優良的磁性質與(001)優選方位，以提供工業上使用所需之磁特性。
Due to the remarkable intrinsic properties of L10 FePt including high magnetocrystalline anisotropy (Ku~7×107erg/cm3), high Curie temperature and etc, it has the potential of being the material for the next generation ultrahigh density media. However, as the FePt thin film deposited under room-temperature (RT) is disordered, in order to get industry required magnetically hard property, annealing process is essential for phase transformation as well as monitoring its (001) preferred orientation and magnetism quality. There are many previous studies on FePt texture evolution, but no prior research had been done using microstructure study. In this experiment, by using TEM imagery, we can understand the mechanism of texture evolution more clearly than before.
In this experiment, we fabricated FePt thin film at room temperature. There are four parameters of thickness, including 10 nm, 20 nm, 30 nm and 40 nm. Annealing temperature increase from 450 oC to 800 oC, and each 50 oC is set as a parameter. Then we compared them to the controlled room temperature sample by focusing on analyzing the Residual strain, magnetism, crystallographic structure and microstructure. Sorder and LOF are measured through crystallographic structure where Residual strain by X-ray at NSRRC.
The grain size first decreased from 12.0 nm to 8.5 nm which was caused by phase transformation occurred at annealing temperature of 500 oC. From 550 oC to 600 oC , (001)grains and (111)grains growth occurs simultaneously and grain size increased to 16.0 nm due to the coexistence of strain-induced and surface-energy-induced conditions. When grain growth meet certain condition changes as temperature rises from 600 oC to 650 oC, new (001)grains generate, (111)grains dissolve and grain size decrease from 16.0 nm to 7.5 nm. In the end, this phenomenon caused the sample to have grains of (001) preferred orientation, and showed that stress induce (001)grains growth had occurred. We also obtained a good magnetism for ultrahigh density recording media.
By experimenting on different thicknesses of FePt film and focusing on analysing residual stress, magnetism and macrostructure. We found that we'll get less (001) preferred orientation and magnetism in thicker thin film. The initial strain of 10 nm FePt is 0.7% tensile strain; where 40 nm FePt is -0.18% compressive strain. The tensile stress significantly helped compressive strain that is caused by phase transformation to release, which can positively affect the results of LOF and magnetism. From the strain figure, we found post-ordered 10 nm FePt thin film strain increased faster than 40 nm ones. This fast increase provided stronger source for (001)grains growth and (111)grains sufficient time to dissolve. which created good magnetism and (001) preferred orientation in thinner films, thus conveniently provided special magnetism feature for industrial engineering.
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