标题: | 基板稳定之六方与正交结构多铁性稀土元素锰氧化物薄膜的磁与电子特性研究 Magnetic and electronic characteristics of substrate-stabilized hexagonal and orthorhombic structured multiferroic rare earth manganese oxide thin films |
作者: | 谢志昌 Chih-Chang Hsieh 庄振益 Jenh-Yih Juang 电子物理系所 |
关键字: | 钇锰氧;多铁;磁电耦合;稀土元素锰氧化物;六方晶系钇锰氧;正交晶系钇锰氧;YMnO3;multiferroics;magnetoelectric;RMnO3;hexagonal YMnO3;orthorhombic YMnO3 |
公开日期: | 2007 |
摘要: | 本论文将介绍如何分别制备具有六方与正交结构之多铁性稀土元素锰氧化物的粉末、靶材、与薄膜样品。同时针对不同结构的样品进行电子结构与磁性特征的量测。 首先我们利用固态烧结法可以制备出具有六方结构的稀土元素锰氧化物(离子半径较小之稀土元素),接着利用掺杂钙离子或锶离子取代稀土元素的方式,试图直接从材料内部施加应力,以稳定其正交结构。我们发现藉由掺杂钙超过30%,就可以完成结构转换。藉由掺杂造成了锰氧的结构从MnO5转换到MnO6,同时掺杂之后锰离子的价数从+3变成了+3/+4的混合价数。因为混合价数的双交换机制,原本的反铁磁特性被铁磁性行为所取代。但是我们发现掺杂锶离子并没有造成类似的结构的转变,而且造成了磁性特征的消失。 我们同时利用外部施加应力的方式制备六方与正交结构的薄膜。六方结构的薄膜可以藉由YSZ(111)的基板制备或是其他具有表面三角形结构同时晶格常数又不会差异太大的基板。我们利用X-ray绕射仪探讨脉冲雷射蒸镀法制备的单一轴向六方结构钇锰氧薄膜,我们可以看到薄膜具备六重对称特性同时基板与薄膜的对称性亦与我们的估计相符。 在另一方面正交结构的薄膜亦可利用选择适当的匹配基板制备。经过预先的比较计算在这里我们分别选择了三种基板钛酸锶(110)、镧铝氧(110)、与钛酸锶(100),我们可以在这三种基板上分别制备具有单一轴向的a轴、b轴、与c轴薄膜。藉由X-ray绕射仪的判断我们可以发现薄膜分别具备二重、二重、与四重对称性。藉由这些判定我们可以了解薄膜的轴向性。同时我们发现LAO(110)是我们在探讨这类薄膜时最佳的利器,因为可以利用此类基板制备出轴向分离的薄膜,对于我们探讨这些轴向异性的材料具有相当大的帮助。我们可以在成长于镧铝氧基板上的钇锰氧与钬锰氧薄膜看到反铁磁相转变温度与第二次磁性相转变。两种材料在三个轴向上都可以看到反铁磁的相转变,但是第二次相转变只会发生在a轴(只在钇锰氧看到)与c轴(钇锰氧与钬锰氧都可看到)。虽然他们的结构特征非常的相近,但是在这一系列的化合物一点点的差异性就会改变锰氧间的键角与距离,也因为这些微的差异性造成材料间不同的性质。 最后我们利用线性偏振的同步辐射光谱探讨不同结构间的轴向异性行为。在六方晶系与正交结构的钇锰氧材料,锰光谱与氧光谱分别展现了MnO5与MnO6正三价电子结构的差异性。在氧光谱中可以清楚看出能阶因晶场与姜-泰勒效应所造成的分裂:在六方结构中能阶分裂为三个,在正交结构中分裂成四个。我们利用轴向异性的光谱特征与磁电耦合的理论预测低温可能造成的光谱变化行为。利用这些方法我们可以制备出一系列稀土元素锰氧化物薄膜,同时利用这些薄膜可以进行相关的量测确认磁电耦合的多铁机制是否存在,并尝试找出较明显的磁电极化行为。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT009021803 http://hdl.handle.net/11536/82269 |
显示于类别: | Thesis |
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