鐵鉑薄膜與奈米結構之序化溫度及其晶體方向之控制的研究

Control of ordering temperature and growth orientations of FePt films and nanostructures

吳允中(Yun-Zhong Wu)

指導教授 ： 賴志煌

國立清華大學/工學院/材料科學工程學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00688

摘要

論文摘要

鐵鉑薄膜與奈米結構之序化溫度及其晶體方向的控制與研究

本論文主要著重於鐵鉑薄膜與奈米結構之序化溫度與(001)優選取向之研究。首先在序化溫度方面：混合氣（氫5% +氬95%）退火，氫原子在退火期間，會固溶並填入鐵鉑晶格中之間隙位置，導致區域性之應力變化或晶格擾動，進而加速鐵鉑薄膜之序化。可將鐵鉑薄膜之序化溫度降低至275 ℃
另外在(001)之優選取向方面，我們利用鉻鉬錳合金底層製備一具有(200)優選取向之未序化鐵鉑薄膜。並採用混合氣退火，研究發現，相較於先前的結果，鐵鉑薄膜的序化度較低。這是因為未序化之鐵鉑薄膜與鉻鉬錳合金底層間之晶格匹配度高，為降低系統自由能，未序化之鐵鉑薄膜會區域性的磊晶在鉻鉬錳合金底層上。而退火時，需先克服區域性磊晶的能障，鐵鉑才會開始序化。然而，一旦鐵鉑序化開始，往往會造成(111)相的生成，並導致較差的晶體異向性。
因此，我們另外採用一鐵與鉑交替鍍製之多層膜，並搭配真空快速退火爐來製備具有高序化度與高(001)優選取向之鐵鉑薄膜。此一多層膜之特點在於每一層鐵或鉑的厚度皆趨近於單層原子層之厚度。因此可大幅降低序化時，鐵或鉑原子所需擴散的距離，進而達到降低序化溫度與退火時間之目的。此外，如果我們由[001]方向來觀察序化之鐵鉑晶格，會發現此方向之鐵與薄的晶格排列其實就像一原子級之鐵/鉑多層膜。因此此一多層膜除了能降低序化溫度外，還能有效的促進鐵鉑薄膜(001)優選取向的生長。
最後，我們將超薄之二氧化矽薄膜插入至鐵/鉑多層膜中，來控制鐵鉑薄膜之奈米結構。實驗發現，在350℃退火2秒後，鐵鉑薄膜不但具有極高之序化度與(001)優選取向，而且還生成了顆粒狀的結構。這是因為在退火過程中，二氧化矽會擴散至介面處並將鐵鉑之晶粒包起，這個過程不但會造成granular結構的形成，還會促進鐵與鉑原子的擴散，而達到降低序化溫度的效用。更進一步提高火溫度，我們發現鐵鉑薄膜會形成奈米顆粒。我們發現鐵鉑顆粒形狀很像截角八面體 (鐵鉑微粒的平衡形狀)，但是寬度對厚度比卻遠大於截角八面體。並且，我們也發現在二氧化矽基版上，鐵鉑會傾向以（001）的面成長，而這些都可歸因於鐵鉑與二氧化矽之間的表面（介面）能平衡。

關鍵字

鐵鉑 ； 低溫序化 ； (001)優選取向

並列摘要

Abstract

Control of ordering temperature and crystalline orientation in FePt films and nanostructures

This dissertation focuses on the low-temperature ordering of (001)-oriented L10 FePt films and nanostructures. Three main topics are discussed. The first one is reducing the ordering temperature. FePt films with a coercivity of as high as 7700 Oe were obtained by forming-gas (Ar 95% + H2 5%) annealing at 275 ℃. Hydrogen atoms may occupy the interstitial sites of FePt, inducing a local strain or an agitation which accelerates the ordering of FePt. The coercivity increased with the number of hydrogen atoms dissolved in the FePt lattices.
The second topic is the fabrication of (001)-orientated FePt films at low temperature, by subsequently annealing FePt films deposited on Cr65Mo15Mn20 underlayers in forming-gas atmosphere. Fe and Pt intermediate layers were used to adjust the strain energy and the orientation of FePt. Samples annealed at 275 ℃ with thin Pt layers (< 5 nm) have relatively small coercivity because of the stabilizing effect of the underlayers.
The other means of attaining a (001)-orientated FePt films at low temperature is the annealing of an atomic-scale Fe/Pt multilayers (MLs). A highly ordered L10 FePt phase can be obtained by annealing at 400 ℃ for 2 s. The very short annealing time and relatively low annealing temperature demonstrate the alternate depositions of atomic-scale Fe and Pt MLs can reduce the diffusion length of FePt into the L10 lattice of FePt. In the γ2-FePt phase, the atomic arrangement of Fe and Pt in the [001] direction is like that of atomic-scale Fe/Pt MLs. Therefore, (001)-orientated FePt films can be acquired by annealing atomic-scale Fe/Pt MLs
The last topic in this dissertation is control of the nanostructure of the ordered L10 FePt phase. L10 granular FePt-SiO2 films with a (001) preferred orientation and well-separated grains of 5.14 ± 0.75 nm were also obtained by depositing atomic-scale Fe/Pt/SiO2 MLs on glass substrates and subsequently annealing MLs at a temperature of 350 ℃. Further increasing the annealing temperature resulted in the formation of nano-particles. Two kinetic processes were taking place during the post-annealing of a disordered FePt film. One is the ordering of the L10 FePt phase; the other is agglomeration. Inserting ultra-thin SiO2 layers into atomic-scale [Fe/Pt]n MLs cannot effectively accelerate the ordering process but improve agglomeration.

並列關鍵字

FePt ； low ordering temperture ； (001)-textured

參考文獻

[1] N. H. Yeh, D. Wachenschwanz, and L. Mei, IEEE Trans. Magn. 35, 776 (1999).

[6] Hansen M., Constitution of Binary Alloys, McGraw-Hill, New York (1958).

[7] B. D. Cullity, Introduction to Magnetic Materials, Addison-Wesley Pub. Co. (1972).

[8] R. Jansen, J. Phys. D: Appl. Phys. 36, R289 (2003).

[12] M. R. Visokaya and R. Sinclair, Appl. Phys. Lett. 66, 1692 (1995).