複雜自適應物質研究---子計畫二---複雜自適應磁電錳氧化物摻雜研究

Abstract

本計畫主要延續本研究群多年來在強關聯繫統之鈣鈦礦結構過 渡金屬氧化物薄膜研究心得，並利用既有的薄膜製程機台與低溫物 理量測系統，探討摻雜鈣鈦礦結構錳氧化物的磁電多鐵性質。尤其 是鑑於現今相關的研究，均仍集中在分辨這些複雜自適應物質中驅 動各種相形成的主要作用力的情況，我們認為適時的研究這些作用 力間有沒有耦合？透過什麼機制耦合?都是非常重要也必須得到解 答的基本問題。舉例而言，直覺上，鐵電性和結構扭曲造成之中心 對稱偏移有關，而磁性則與局部自旋（local spin）相關。這兩種看 似不相關的現象(或相)，為何會共存在某些特定的材料？相的共存是 彼此競爭後的妥協，或是相互作用衍生的結果？他們之間是否可以 通過外加的perturbation 來誘發或改變彼此之間的相互作用？更有趣 的是，這二者在既有的所謂磁電多鐵性材料中，相互間的作用似乎 還不夠大到可以衍生功能性的應用。尋求一可以調變此一作用的機 制，顯然是瞭解這類新穎複雜自適應物質的基本物理以及進一步衍 生新穎功能性元件的重要關鍵。本計畫希望在未來三年內，利用我 們從龐磁阻效應研究所得到的經驗，(即藉由摻雜鑭錳氧反鐵磁相， 引進可以在材料中漫遊的非侷域性載子，並進而衍生雙交換作用 (double exchange)並導致材料從順磁絕緣態轉變為鐵磁金屬態的物 理)，結合雷射鍍膜技術對氧化物薄膜的成長與摻雜的優異性，有系統的經由摻雜磁電多鐵性錳氧化物(如YMnO3， HoMnO3，TbMnO3 等)，調變材料中不同作用力間的相互作用，以瞭解這類新穎複雜自 適應物質的基本物理以及進一步衍生新穎功能性元件。

This proposal is intended to extend our efforts and experiences gained from the extensive investigations on the emergent behaviors exhibited by the complex adaptive matters characterized by strongly-correlated electrons, in particular, in the perovskite oxides-related thin films over the past few years. We will utilize the existent PLD system and low-temperature physical properties measuring system to investigate emergent properties of the doped magneto-electric multiferroic systems. In particular, in regarding to the fact that current researches have been mainly focused on identifying the major interactions that lead to the coexistence of multiferroic phases, we think the following questions should be addressed, at least, equally importantly. The questions are: 「Is there a coupling between these interactions?」「If yes, what is the dominant coupling mechanism?」 These are all the fundamental questions centered to the understanding of these so-called 「complex adaptive matters」that needed to be answered. For instance, intuitively, ferroelectricity often originates from the off-center asymmetry induced by structure distortion of the lattice, whereas, magnetism is intimately related to the local spin structure. Why would these two seemingly irrelevant phases (or phenomena) coexist in the some particular materials? Is the coexistence of multiphases a result of competition or compromise between different interactions among different quasiparticles? Does the interplay between them can be somehow modulated by the external perturbations in a controllable way? Even more interestingly from the application point of view, in most of the current situations, the existent multiferroic materials appears to be incapable of providing enough interplay between the ferroelectricity and ferromagnetism (or anti-ferromagnetism) to put up viable functional devices. It is thus very important to find a way of tuning these effects. It will be a key to not only gain understanding on the physics behind the complex adaptive nature of these materilas but also to give rise devices with novel functionalities. There is a clue that these may all be accomplished. Inspired from the experience in studying the colossal magnetoresistive materials (which are also complex adaptive matters by definition), doping can play a key role in changing the interactions. In that case, the introduction of itinerant carriers by doping triggers the double-exchange interaction between local spin moments and, in turn, leads to a transition from paramagnetic insulator to ferromagnetic metal. Similar physics is expected to prevail in these multiferroics, as well. We intend, in the next three years, to combine the superior advantages of pulsed laser deposition technique in growing stoichiometric oxide films and achieving wide range doping and the experiences we had gained previously, to systematically investigate the physical properties of doped multiferroic systems, such as YMnO3, HoMnO3, TbMnO3. It is hoped that by finding a way of tuning the interactions that lead to various robust pahses existent in the system, the fundamental physics and possibly novel device functionalties can be sucessfuly explored.