從超薄膜到自組排列奈米顆粒: 低維度磁性系統之成長、晶格結構與磁性分析

Abstract

在低維磁性系統中，由於有限尺寸效應及表面效應，因此常常在晶格結構與磁性分析上產生許多有趣特殊的物理現象。在此篇論文，我們透過調控合金比例 蒸鍍量多寡及增加覆蓋層來改變甚至操縱低維磁性系統的晶格結構與磁性。 在鈷鎳合金超薄膜中，第一次(平行膜面轉成垂直膜面方向)及第二次(垂直膜面轉成平行膜面方向)磁易軸轉向行為都發生在20原子層厚度以內。隨著鈷合金比例增加，垂直異向性存在的厚度區間隨之縮減，直到鈷合金比例超過12 %，垂直異向性就不再被觀察到。透過現象學模型能成功地模擬磁易軸方向相對於鈷合金比例及薄膜厚度的相變圖。 面心立方結構的錳塊材只在~1400 K高溫下存在，然而透過適當地選擇單晶基板Cu3Au(100)，由於其晶格常數與面心立方結構的錳塊材接近，因此能成功地在Cu3Au(100)單晶基板上磊晶成長出穩定存在於低溫的面心立方結構錳超薄膜。在本系列實驗中，面心立方結構錳超薄膜被印證具有反鐵磁性。室溫(300 K)及低溫(100 K)下成長的面心立方結構錳超薄膜展現出不同的結構變化及磁性行為。此外透過選擇適當厚度的鐵膜(6個原子層)於面心立方結構錳超薄膜上，吾人可以成功地製備“垂直易向性鐵薄膜/反鐵磁面心立方結構錳超薄膜”的雙層膜結構。在垂直磁場下冷卻降溫後，磁光科爾效應量測的結果顯示反鐵磁面心立方結構的錳超薄膜確實能提供垂直方向的磁性偏偶合交互作用，這也間接印證了面心立方結構錳超薄膜的3維磁矩結構。此外鐵錳合金超薄膜也成功地磊晶成長於Cu3Au(100)單晶基板上，其結構及磁性行為也有詳細的分析。 透過蒸鍍在超薄單晶氧化鋁層/NiAl(100)基板，吾人成功地製備線狀排列的鈷奈米顆粒陣列，此一系列線狀排列的鈷奈米顆粒陣列具有均勻顆粒大小、高熱穩定度、有序排列等特性。此種超薄單晶氧化鋁層/NiAl(100)基板對於其他鐵、錳、銅奈米顆粒陣列的製備也具有相同的優勢條件。磁光科爾效應亦用於量測鐵 、鈷奈米顆粒陣列，非磁性金屬(銅)的覆蓋層提供了奈米顆粒間額外的磁性偶合作用，因此增加了磁性奈米顆粒陣列的居禮溫度。

In low-dimensional magnetic systems, due to the finite size, the symmetry breaking and the large ratio of surface to bulk atoms, many interesting physics, including the crystalline structure and magnetic properties etc., can be found or manipulated by tuning the alloy composition or deposition coverage, adding the capping layer, and choosing the proper substrate for small lattice mismatch etc.. As the film thickness increases above 8 ML, the CoxNi1−x/Cu3Au(100) alloy ultrathin films clearly exhibited progressively more relaxed structure. Due to the strain relaxation, both the 1st and the 2nd spin-reorientation transitions (SRT) occurred within 20 ML. The thickness region with perpendicular magnetization was strongly reduced by increasing Co concentration. By combining both alloy and strain relaxation effects, the SRT boundaries in the phase diagram can be described in a phenomenological model on the basis of magnetoelastics. Face-centered cubic (fcc) Mn, which exists at 1400 K in bulk material, can be successfully grown on Cu3Au(100) at 300 K (RT) and 100 K (LT), because of the small lattice mismatch at the interface. Mn films deposited at RT and LT demonstrate very different behaviors in the crystalline structure, morphology and magnetism. Both the RT and LT-Mn films proceed a thickness-dependent structural transition from face-centered cubic (fcc) to face-centered tetragonal (fct) at 12-14 and 8 ML, respectively. Significant exchange bias is observed in Fe/RT-Mn bilayers and monotonously increases with Mn thickness. The exchange bias coupling in Fe/LT-Mn is much weaker as compared with Fe/RT-Mn and drastically varies with Mn film thickness. Both the RT and LT-Mn/Cu3Au(100) films are concluded to be antiferromagnetism. Fe films grown on 15, 9 and 6 ML Mn/Cu3Au(100) revealed a structural transition from face-centered tetragonal (fct) to body-centered tetragonal (bct) during 3.7-6.9 ML, corresponding to the spin reorientation transition (SRT) from polar to longitudinal direction. Therefore we may prepare polar magnetized 6 ML Fe grown on 6 and 9 ML Mn/Cu3Au(100). After the polar field-cooling, a significant enhancement in the coercive field and a small bias field were observed. Thus the AFM-Mn/Cu3Au(100) ultrathin films were proved to have the capability of providing polar exchange bias coupling. The structural and magnetic properties of Fe/FexMn1−x bilayers prepared by epitaxial growth on Cu3Au(100) are investigated. For FexMn1−x with x=54-83%, the periodical oscillations of medium electron diffraction (MEED) persist up to 15 monolayer (ML). After field-cooling, the large exchange bias up to 200-300 Oe is measured at 100 K in 21 ML Fe/15 ML FexMn1−x for x=0-54%, indicating the antiferromagnetic properties of the single crystalline FexMn1−x films and the significant exchange bias coupling in the Fe/FexMn1−x bilayers. Co nanoparticle chains are grown by vapor deposition over a single-crystalline Al2O3 layers on NiAl(100) with such features as self-limiting size distribution with the average size of 2.7 nm, well-ordered alignment, and high thermal stability. We attribute these features to peculiar one-dimensional long stripes with, 4 nm inter-distance on the surface of the ultrathin Al2O3 template. This also provides a natural explanation why several different metals (Fe, Cu, Mn) we tried all show the same kind of spectacular alignment. The ferromagnetism of Fe nanoparticle assembly on Al2O3/NiAl(100) is observed above 150 K with the coverage larger than 5 monolayer (ML). Cu capping layer induces an enhancement of the Curie temperature (TC) in both Fe and Co magnetic nanoparticle assembly. The TC of Fe nanoparticle assembly with 2 ML and 6 ML Cu capping layer is enhanced by 20 K and even higher, indicating the critical effects of metallic capping layer in such magnetic nanostructures as nanoparticle assembly.