Multiple Stable States with In‐Plane Anisotropy in Ultrathin YMnO3 Films
Multiple Stable States with In‐Plane Anisotropy in Ultrathin YMnO3 Films
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DOI：
10.1002/adma.201002743
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发表时间：
2010-12
影响因子：
29.4
通讯作者：
Z. Sheng;N. Ogawa;Y. Ogimoto;K. Miyano
中科院分区：
文献类型：
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作者：
Z. Sheng;N. Ogawa;Y. Ogimoto;K. Miyano
复合氧化物具有铁电性、磁性、超导性和多铁性等固有的物理功能，为氧化物电子器件的新概念提供了基础。[1-3]将这些材料形成为具有赝晶应变的外延膜可以导致新的功能，这些功能在相应的本体母体化合物中不存在，并且已经在具有钙钛矿结构的铁电体中进行了一些研究。[2-6]这种策略在不适当的铁电体中也应该是有效的。由于其张量性质，在具有分离多个稳定态的多铁性中，应力的作用可以与磁场和电场的作用有质的不同。[7]本文报道了六方晶系YMnO 3（YMO）铁电薄膜中应变诱导的面内三角效应。在应变驱动下，晶体结构由六角对称转变为三角对称，产生了一种新的六重对称面内极化。结合普通的面外铁电极化，可以单独用电子极化定义四种稳定态，这为数据存储器件的设计提供了新的可能性。斜方晶系YMO是目前研究最多的多铁性材料之一，表现出铁电性（TC ~ 900 K）和反铁磁性（TN ~ 70 K）的共存。晶体结构由多层扭曲倾斜的MnO 5双锥构成，并由波纹状的Y层隔开。MnO 5双锥具有一种不寻常的五重配位结构，其中有两个顶氧（O T）和三个面内氧（O P）。[9，15] MnO 5双锥的倾斜导致与Y离子配位的氧离子的位移，具有形式上的d 0-ness状态。证明了YMO中的铁电态与YOT反棱柱配位和沿自发极化方向c轴的YOP键沿着有关。[16]作为铁电材料，YMO具有低介电常数，并且不包含在Pb（Zr 1-xTix）O3和SrBi 2 Ta 2 O 9中发现的挥发性元素如Pb和Bi。[11]对于大块单晶和较厚的薄膜，YMO的反射极化（Pr）分别为5.5 μC/cm 2和1.7 μC/cm 2。[14[17-19]最近，许多努力致力于探索YMO中的多铁性。[10-12]其中，非线性光学方法，特别是二次谐波产生（SHG），由于其在研究铁电和磁有序方面的非破坏性和高灵敏度特性，已被用于研究YMO单晶和厚膜。[20例如，Fiebig等人在YMO单晶中观察到耦合的磁畴和电畴以及通过电场的磁相位控制。[12通过在YMO薄膜中使用SHG方法，我们发现除了通常的c偏振之外，还存在具有六重对称性的面内SH信号，这在体中是被禁止的。我们认为，这是由于应变揭示的三角对称的基本单元（uc）的积木。它的持久性到几个单位细胞的厚度意味着假晶应变效应的作用，作为一个偏置场的基板。
Complex oxides exhibit intrinsic physical functionalities, such as ferroelectricity, magnetism, superconductivity, and multiferroicity, providing the basis for novel concepts of oxide-electronic devices.[1–3] Forming these materials as epitaxial films with pseudomorphic strain can lead to new functionalities, which do not exist in the corresponding bulk parent compounds and some research have been done in ferroelectrics with perovskite structure.[2–6] This strategy should be also effective in improper ferroelectrics. Due to its tensorial nature, the effect of stress can be qualitatively different from those of magnetic and electric fields in the multiferroics with separating multiple stable states.[7] Here, we report a strain-induced in-plane trigonality in hexagonal ferroelectric YMnO 3 (YMO) ultrathin films. A new in-plane polarization with six-fold symmetry was created accompanied with strain driven structural transition from hexagonal to trigonal symmetry. Combined with the ordinary out-of-plane ferroelectric polarization, four stable states can be defined in terms of electronic polarization alone, which can provide new possibilities in the design of data-storage devices. Hexagonal YMO is one of the most intensively studied multiferroic materials, showing the coexistence of ferroelectricity (TC∼ 900 K) and antiferromagnetism (TN∼ 70 K).[8–14] The crystal structure is formed by layers of distorted and tilted MnO 5 bipyramids, which are separated by corrugated Y layers. The MnO 5 bipyramid has an unusual fivefold coordination with two on-top oxygens (O T) and three in-plane oxygens (O P).[9, 15] The tilting of MnO 5 bipyramids leads to the displacement of oxygen ions that coordinate Y ions, having formally d0-ness state. It has been proved that the ferroelectric state in YMO is associated both with layered YO T antiprismatic coordination and YO P bonds along the c-axis, the direction of the spontaneous polarization.[16] As a ferroelectric material, YMO has a low permittivity and contains no volatile elements such as Pb and Bi found in Pb (Zr 1–xTi x) O 3 and SrBi 2Ta 2O 9.[11] For bulk single crystals and thicker films, YMO has a remanent polarization (Pr) of 5.5 μC/cm 2 and∼ 1.7 μC/cm 2, respectively.[14, 17–19]Recently, many efforts have been devoted to explore the multiferroic properties in YMO.[10–12] Among them, nonlinear optical method, and in particular second harmonic generation (SHG), has been used for studying YMO single crystals and thick films due to its nondestructive and high sensitive characteristics in studying ferroelectric and magnetic order.[20, 21] For instance, Fiebig et al. observed coupled magnetic and electric domains and magnetic phase control by an electric field in a YMO single crystal.[12, 13] By employing the SHG method in ultrathin YMO films, we found in-plane SH signals with sixfold symmetry, in addition to the usual c-polarization, which is forbidden in the bulk. We argue that this is due to the strainrevealed underlying trigonal symmetry in the building block of the unit cell (uc). Its persistence into a few unit cells thickness implies the role of the pseudomorphic strain effect by the substrate acting as a biasing field.