多相複雜氧化物材料其奈米結構與物理性質間交互作用之研究

Interplay of Nano-structure and Physical Properties in Multi-phases Complex Oxide Materials

劉恒睿(Heng-Jui Liu)

指導教授 ： 林樹均 ； 朱英豪

國立清華大學/工學院/材料科學工程學系/博士(2012年)

https://doi.org/10.6843/NTHU.2012.00339

摘要

本論文係研究多相系統之材料如多鐵性材料或奈米複合材料其結構與物理性質間之關聯。本論文亦分成兩部份，第一部份探討多鐵性材料鉍鐵氧薄膜之相變化過程與鐵電極性間的響應；第二部份則為觀察具有光致伸縮特性之鈣鈦礦鍶釕氧(SrRuO3)與磁致伸縮特性之尖晶石鈷鐵氧(CoFe2O4)材料所組成之奈米結構薄膜之間的耦合效應。
首先，許多關於多鐵性鉍鐵氧薄膜的研究上指出其電性與磁性有序的耦合現象常與本身豐富的相結構變化有關。尤其是成長在不匹配性過大的單晶基板上如鑭鋁氧(LaAlO3)或是釔鋁氧(YAlO3)時，原本為菱方晶結構的鉍鐵氧會因在平行表面方向上承受的過大的壓應力而變成似長方晶的結構，同時其鐵電的極性也會因此結構的變化而產生改變。本實驗裡，我們採用X光倒置晶格圖與穿透性電子顯微鏡來研究隨著厚度變化的應力發展。在適中厚度的試片裡可發現在應變釋放的過程中存在著似長方晶與菱形晶的結構，即應變誘發的形態相界區域。在更深入的結構分析中顯示為了彌補似長方晶與菱形晶間的晶格不匹配性，還需要多生成兩個傾斜的單斜晶中間相來容忍此極大的側向應力，故可在表面上觀察到週期性的帶狀結構。此外我們將鉍鐵氧成長在釔鋁氧單晶基板上時，其形態相界區域會由於基板在不同方向上所造成應變量的差異，而出現不同週期性的排列帶狀特徵。而這兩個中間相也被發現與溫度變化有強烈的關聯。同時我們也在溫度相關的XRD圖譜中發現約在100oC~200oC附近會發生單斜晶間的相變化，其被觀察到為MC對稱性到MA對稱性的轉變過程。然後我們也用壓電力顯微鏡觀察到在相變化過程中鐵電極化方向的旋轉，並量測其壓電係數，其在相變溫度下呈現出最大的壓電反應。這些研究可使我們更加瞭解鐵電系統在承受應變下其結構改變與極化特性間的關連
另一部份自組裝奈米結構薄膜近年來亦受到廣泛的注目，其優勢為有非常大的界面對體積比率，並且可藉由選擇適當的材料組合來設計新的功能性。雖然現今大部分的研究仍然在強調使用磁場或電場來控制奈米結構，但在此部分實驗裡，我們企圖在此自組裝的系統中使用另一種外在的控制參數：光，來控制奈米結構。我們成功的在鍶鈦氧單晶基板上(SrTiO3)製作出嵌入在鍶釕氧母材裡的鈷鐵氧奈米柱，並且經由超快雷射、導電原子力顯微鏡，超導量子干涉儀等量測來確定各個組成都能保有原來的性質。同時此具有光致伸縮特性的鍶釕氧與磁致伸縮特性的鈷鐵氧結合成的自組裝結構也展現出另一種的交互作用機制：光磁耦合效應，即可利用光來誘發且導致鈷鐵氧奈米柱呈現非常快速的磁性變化。因此，此實驗闡述了一種新的奈米結構設計工程的概念並開啟了另一條不同於傳統方式的新功能性之探索。
最後，我們可發現，不管在單一組成的材料或是多種組成而成的複合材料，多相共存的系統雖然結構上較為複雜，但高度的界面比所帶來豐富及多變的性質卻是單相系統無法比擬的。研究多相共存的系統可提供我們更多值得發掘的新物理現象。

關鍵字

複雜氧化物 ； 自組裝奈米結構 ； 鉍鐵氧 ； 鍶釕氧 ； 鈷鐵氧 ； 單斜晶相變 ； 極化旋轉 ； 光磁耦合效應

並列摘要

In this thesis, we investigated the correlations between structural variation and physical properties of the some multi-phases materials, such as multiferroic or nano-structured composite thin films. It will be discussed in two parts: one is the study of phase transition and corresponding ferroelectric response in multiferroic BiFeO3 thin films; and the other is the study of coupling effect in self-assembled nano-structured thin films composed of photostrictive perovskite SrRuO3 (SRO) and magnetostrictive spinel CoFe2O4 (CFO).
First, recent researches have shown that the multiferroic bismuth ferrite (BiFeO3, BFO) thin films exhibit high correlation between coupled electric and magnetic orders and abundant structure variation. Especially for those grown on the substrate with large lattice misfit such as LaAlO3 (LAO) or YAlO3 (YAO), it is found that BFO becomes tetragonal-like structure instead of original rhombohedral structure due to the large in-plane compressive strain, and its ferroelectric polarization also changes with the structural variation. In this thesis, we studied a series of the strain evolution on the structural nature of BFO thin films by X-ray reciprocal space mapping (RSM) and transmission electron microscopy (TEM). A strain-driven phase boundaries occur at medium thickness with coexistence of tetragonal-like and rhombohedral-like phases because of strain relaxation. The detailed structures reveals that two extra tilted monoclinic phases form to accommodate the large lattice mismatch from tetragonal-like structure to rhombohedral-like structure, resulting in the feature of periodic strips presented in topography. Besides, the BFO thin films grown on YAO substrate even show a discrepancy of arrayed stripe morphology due to the anisotropic strain in different direction of YAO substrate. We then focused on the intermediate phase at the boundaries between the tetragonal-like and rhombohedral-like phases, whose content and structure are strongly dependent on temperature. We also found a monoclinic phase transition at temperature around 100~200℃ in ambient condition. The observed transition is between an MC symmetry and an MA symmetry. Studies of the ferroelectric domains of the MC and MA phases clearly show that their ferroelectric polarizations rotate when the phase transition occurs. Piezoelectric response in the BiFeO3 thin films displays a substantial enhancement at the MC−MA transition temperature. These findings directly unveil the close correlations between structural changes, polarization rotation and high piezoelectricity in ferroelectrics.
On the other hand, self-assembled vertical nanostructures have also attracted extensive attentions recently due to their advantage of high interface-to-volume ratio. They could be used to design new functionalities by choosing proper combination of constituents. While most of the studies up to date have emphasized the functional controllability of the nanostructures using external electric or magnetic fields, we try to demonstrate a new coupling mechanism: the photo-magnetic coupling effect, which describes light (or photons) as the external control parameter in this study. We have successfully synthesized oxide nanostructures with CoFe2O4 (CFO) nanopillars embedded in SrRuO3 (SRO) matrix on (001) SrTiO3 substrate and confirmed that all constitutes still keep their original properties form measurements of ultra-fast laser, conducting atomic force microscopy (CAFM), and Superconducting Quantum Interference Device (SQUID). Combination of the photostrictive SRO and magnetostrictive CFO in the intimately assembled nanostructures can leads to a light-induced, ultrafast change in magnetization of the CFO nanopillars. Our work demonstrates a new concept on oxide nanostructure design and engineering and opens a pathway alternative to the traditional routes for the explorations of new fuctionalities.
At the end of this work, we can simply conclude that whether in the single-constitute materials or composite materials, the multi-phase systems always have more complicated structural characteristic and abundant properties than single-phase systems due to the high interface-to-volume ratio, so studying these kinds of system can provide us more new un-discovered physical phenomena.

並列關鍵字

Complex oxides ； Self-assembled nanostructure ； BiFeO3 ； SrRuO3 ； CoFe2O4 ； Monoclinic phase trasition ； polarization rotation ； Photo-magnetic coupling effect

參考文獻

4. J. F. Nye, Physical Properties of Crystals, Clarendon Press, Oxford, pp. 1-42 (1990).

6. N. A. Hill, J. Phys. Chem. B 104, 6694 (2000).

7. W. Cao, Ferroelectrics 172, 31 (1995).

8. K. Aizu, Phys. Rev. B 2, 754 (1970).

9. I. E. Dzyaloshinskii, Sov. Phys. —JETP 10, 628. (1959).