表面結構,磁性以及熱穩定性在皺化系統上的研究

Study of Surface Structure, Magnetism and Thermal Stability of Faceted Systems

邱傑振(Chieh-Chen Chiu)

指導教授 ： 陳正中 ； 宋克嘉

國立清華大學/理學院/物理學系/博士(2014年)

摘要

超薄金屬膜成長於金屬基板呈現出非常特殊的物理及化學性質可應用於像是材料科學、催化以及磁性科技。這些性質依賴於薄膜與基板的原子交換、薄膜成長模式以及基板的表面形貌。近年來有些金屬薄膜(Pd, Pt, Au, Ir, Rh)被發現在退火至溫度高於700 K時可以誘發某些基板(W, Mo)發生皺化的現象。W(111)的表面皺化可以用來製造熱穩定的單原子針，且其發射出的電子束有非常高的同調性。如何製造具有熱穩定性的磁性單原子針在近年來吸引了許多研究團隊的興趣。
在本論文中，研究鐡超薄膜成長於Pd/W{112}皺化表面來探究其是否具備磁性單原子針的潛能。其結構與磁性針對溫度及厚度的變化分別做一系列的探討。對於成長於105 K的鐡超薄膜而言，磁易軸的翻轉(從垂直到平行表面方向)發生在鐡薄膜厚度2 TML。我們發現吸附氫氣後，此系統的磁異向性以及矯頑磁力會大幅增加約7倍左右(對於1.9TML的鐡薄膜)。磁易轉翻轉也會發生在鐡薄膜更高的厚度(2.9 TML)，而殘餘殘化量對溫度的變化也會因為高氫氣分壓(1E-7 torr)而有所不同。此外，多出來的Pd會形成3維島狀結構，並且對鐡薄膜的磁性行為有不良的影響。 當其密度太高時，鐡薄膜的垂直方向磁性會消失。
對於3-11 TML的鈷鐡成長於Pd/W{112}皺化表面上我們也做了一系列的表面形貌、成分分析以及鐡磁性在退火過程中的研究。鈷薄膜會在400-450 K時開始形成3維島狀的結構而且其皺化結構可以維持到900 K以上。鐡也會在300-350 K時開始形成維島狀的結構但其皺化結構並不能維持到900 K。對於3-11 TML的鈷鐡而言，它們的矯頑磁力會分別隨著厚度增加而增加或減少。而鈷薄膜的矯頑磁力會因為溫度升高到400-450 K而增加2-4倍但鐡薄膜不會對溫度的變化有所反應。這兩個不同的鐡磁性薄膜成長於Pd/W{112}皺化表面上顯現出不一樣的成長模式以及磁性行為。

關鍵字

皺化表面 ； 薄膜 ； 磁性

並列摘要

Ultrathin metal films supported on metal substrates have extraordinary physical and chemical properties useful in applications such as material science, catalysis, and
magnetic technology. These properties count on factors such as the inter-diffusion between overlayer-substrate, the overlayer growth mode, and the substrate morphology. In recent years, some metal overlayers (Pd, Pt, Au, Ir, and Rh) are found to induce some substrates such as W and Mo to facet after annealing to temperature above 700 K. Moreover, the faceting of W(111) is used to produce a thermodynamically stable pyramidal structure with a single atom at the apex, applied to generate a highly coherent electron beams. How to obtain a well-defined magnetic tip is of great interest in recent years.
Fe films grown on faceted Pd/W{112} surface is studied to assess its potential to serve as a magnetic single atom tip. Structure and magnetic properties of Fe/faceted Pd/W{112} were characterized as functions of temperature and Fe coverage. For Fe films of various thickness deposited at 105 K, spin reorientation transition (SRT)
(from perpendicular to surface to in-plane direction) were found at Fe coverage of 2 thermal desorption monolayer (TML). We found that the adsorption of hydrogen greatly enhanced the perpendicular magnetic anisotropy and thus coercivity of the Fe films, e.g. 7 times for 1.9 TML thick Fe film. As a result, the critical thickness of the Fe film at which the SRT occurs was shifted from 2 to 2.9 TML, and the temperature dependence of the remnant magnetization was significantly influenced by
a hydrogen partial pressure of 1E-7 torr. Furthermore, we found the 3-dimensional(3D) islands formed by surplus Pd had an adverse effect to the magnetization of the Fe film. When their density was too high, the Fe film lost its perpendicular magnetization, with or without hydrogen.
Morphology, element composition and magnetic property of 3-11 TML Co and Fe films on faceted Pd/W{112} surface was systematically investigated upon thermal annealing. Co films aggregated and formed 3D islands at 400-450 K, while the
{112} facet still sustained after further annealing to 900 K. Fe films aggregated and formed 3D islands at a lower temperature, 300-350 K, and there was no observable
faceted structure after further annealing to 900 K. The magnetic coercivity (HC) of as grown Co and Fe films increased and decreased respectively with the increasing
thickness from 3 to 11 TML. The HC of Co films was significantly enhanced by 2-4 times after annealing to 400-450 K, but the HC of Fe films sustained invariant
upon thermal annealing. These comparative studies on n TML Co and Fe films on faceted Pd/W{112} surface clearly revealed the remarkable difference between the two systems and provides valuable information for future applications.

並列關鍵字

faceted surface ； thin film ； magnetism

參考文獻

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