基板穩定之六方與正交結構多鐵性稀土元素錳氧化物薄膜的磁與電子特性研究

Abstract

本論文將介紹如何分別製備具有六方與正交結構之多鐵性稀土元素錳氧化物的粉末、靶材、與薄膜樣品。同時針對不同結構的樣品進行電子結構與磁性特徵的量測。 首先我們利用固態燒結法可以製備出具有六方結構的稀土元素錳氧化物（離子半徑較小之稀土元素），接著利用摻雜鈣離子或鍶離子取代稀土元素的方式，試圖直接從材料內部施加應力，以穩定其正交結構。我們發現藉由摻雜鈣超過30%，就可以完成結構轉換。藉由摻雜造成了錳氧的結構從MnO5轉換到MnO6，同時摻雜之後錳離子的價數從+3變成了+3/+4的混合價數。因為混合價數的雙交換機制，原本的反鐵磁特性被鐵磁性行為所取代。但是我們發現摻雜鍶離子並沒有造成類似的結構的轉變，而且造成了磁性特徵的消失。 我們同時利用外部施加應力的方式製備六方與正交結構的薄膜。六方結構的薄膜可以藉由YSZ(111)的基板製備或是其他具有表面三角形結構同時晶格常數又不會差異太大的基板。我們利用X-ray繞射儀探討脈衝雷射蒸鍍法製備的單一軸向六方結構釔錳氧薄膜，我們可以看到薄膜具備六重對稱特性同時基板與薄膜的對稱性亦與我們的估計相符。 在另一方面正交結構的薄膜亦可利用選擇適當的匹配基板製備。經過預先的比較計算在這裡我們分別選擇了三種基板鈦酸鍶(110)、鑭鋁氧(110)、與鈦酸鍶(100)，我們可以在這三種基板上分別製備具有單一軸向的a軸、b軸、與c軸薄膜。藉由X-ray繞射儀的判斷我們可以發現薄膜分別具備二重、二重、與四重對稱性。藉由這些判定我們可以了解薄膜的軸向性。同時我們發現LAO(110)是我們在探討這類薄膜時最佳的利器，因為可以利用此類基板製備出軸向分離的薄膜，對於我們探討這些軸向異性的材料具有相當大的幫助。我們可以在成長於鑭鋁氧基板上的釔錳氧與鈥錳氧薄膜看到反鐵磁相轉變溫度與第二次磁性相轉變。兩種材料在三個軸向上都可以看到反鐵磁的相轉變，但是第二次相轉變只會發生在a軸(只在釔錳氧看到）與c軸(釔錳氧與鈥錳氧都可看到)。雖然他們的結構特徵非常的相近，但是在這一系列的化合物一點點的差異性就會改變錳氧間的鍵角與距離，也因為這些微的差異性造成材料間不同的性質。 最後我們利用線性偏振的同步輻射光譜探討不同結構間的軸向異性行為。在六方晶系與正交結構的釔錳氧材料，錳光譜與氧光譜分別展現了ＭnO5與MnO6正三價電子結構的差異性。在氧光譜中可以清楚看出能階因晶場與姜-泰勒效應所造成的分裂：在六方結構中能階分裂為三個，在正交結構中分裂成四個。我們利用軸向異性的光譜特徵與磁電耦合的理論預測低溫可能造成的光譜變化行為。利用這些方法我們可以製備出一系列稀土元素錳氧化物薄膜，同時利用這些薄膜可以進行相關的量測確認磁電耦合的多鐵機制是否存在，並嘗試找出較明顯的磁電極化行為。

In this dissertation, we present how to prepare hexagonal and orthorhombic structured multiferroic rare earth manganese oxide powders, bulks, and thin films. We also probed the anisotropies in the magnetic behavior and electronic structure of these materials. Firstly, we prepare hexagonal RMnO3 (R: rare earth and Y) compounds with smaller rare earth ions. We tried to stabilize the orthorhombic structure by replacing the R-site ions with Ca2+ or Sr2+. This method could provide inner strain force and the material from the thermodynamically stable hexagonal structure to orthorhombic perovskite structure. We find that the entire structure transforms into orthorhombic by doping Ca2+ up to 30%. In the doping process, the MnO5 structure is no longer stable and starts to convert into MnO6 structure, which in turn substantially modifies the magnetoelectric properties and electronic structures of the material. The trivalent manganese converts into +3/+4 mixed valence and induces the double exchange mechanism. Consequently, the antiferromagnetism is replaced by ferromagnetic interaction and revealed traditional spin glass behaviors. The Sr2+ doped sample remains as hexagonal structure and magnetic characteristic was significantly suppressed. The external strain force originated from the epitaxial relation between the substrate and thin films during deposition can also serve as an excellent method in obtaining samples with desired crystal structures. We calculated the in-plane mismatch and used the YSZ(111) substrate in preparing hexagonal thin films. The sample revealed six-fold symmetry and in-plane arrangement has corresponded nicely to our estimation. On the other hand, we deposited thin film on SrTiO3(110), LaAlO3(110), and SrTiO3(100) substrates and obtained a-, b-, and c-axis thin film of orthorhombic structure. With the X-ray diffraction characterizations, we see the thin films are having 2-fold, 2-fold, and 4-fold symmetry, depending on the substrate chosen. In particular, we find that the a-axis and b-axis films are having distinguishable crystalline axis, while there exists a twin growth behavior in the c-axis film. Owing to the relatively smaller in-plane mismatch, the LaAlO3 (110) is proved to be the best substrate in stabilizing orthorhombic RMnO3 for smaller ionic size of R. The films with specific growth directions allow us to directly probe the anisotropies existent in magnetism, polarization, and bonding relations. We observed the antiferromagnetic ordering and spin reordering transition in both YMnO3 and HoMnO3 thin films grown on LaAlO3 (110) substrates. The antiferromagnetic ordering can be probed with applied field parallel to each crystallographic axis. However, the spin reordering was only observable for field applied parallel to a-axis (only observed in YMnO3) and c-axis (observed in YMnO3 and HoMnO3). Although the crystal structure of these two films is nearly the same, RMnO3 compound is sensitive to the Mn-O bonding distance and Mn-O-Mn bond angle, thus might explain the different behaviors exhibited by the two neighboring compounds. Finally, we used the linear polarized x-ray to probe the anisotropic bonding relation in both of these two crystal structure. In the hexagonal and orthorhombic YMnO3 thin films, Mn L edge and O K edge x-ray absorption spectra exhibited the MnO5 and MnO6 structure with trivalent manganese ions in respective crystal structure. We attribute the energy splitting in the O K edge spectra to the effect of crystal field and Jahn-Teller effect. There are three splitting energy levels in hexagonal structure and 4 splitting energy levels in orthorhombic structure. We also utilize the anisotropic spectra and the magnetoelectric theory in E-type magnetic orders to estimate the possible spectra in the lock-in state. By extending these methods, we should be able to prepare a series of rare earth manganese thin films that are potentially able to realize the magnetoelectric state expected in the E-type magnetic structure for more dramatic magnetism-induced polarizations.