正铁氧体和正交锰氧化物单晶膜中反铁畸变晶格的外延应变调控及室温磁电效应探索

After decades of research, transition-metal perovskite oxides, more than ever, seem destined for a great future. The interest in these oxides arises from the fact that they are based on the same simple perovskite structure and can exhibit a wide diversity of behaviors. Large and tunable dielectric constants, piezoelectricity, ferroelectricity, colossal magnetoresistance, charge-orbital ordering, magnetism, and superconductivity are examples of their most common properties, many of which are exploited in various technological applications. And recently, thanks to advances in deposition techniques, the interest in perovskite oxides moves greatly to films, multilayers and superlattices. The functional properties of ABO3 compounds are strongly sensitive to external parameters and it was shown that the engineering of epitaxial strain is a powerful tool that can be used to tune ferroelectricity or induce ferromagnetism in these systems. Going further and stacking different perovskites in epitaxial structures not only allows their properties to be combined but, sometimes, also totally new phenomena to be induced. The examples include the metallic interface found at the boundary between the insulators LaAlO3 and SrTiO3, and the emergence of improper ferroelectricity in ultrashort period PbTiO3/SrTiO3 superlattices. And in all the multilayer systems with emergent phenomena, it was demonstrated that the tilt/rotation of BO6 octahedra play a key role in determining the interface properties. This is also true for multiferroics that combine both ferroelectricity and magnetism. Theoretically it was predicted that for Pbnm perovskites CaMnO3 and orthoferrites, strain control over the antiferrodistortive (AFD) lattice instability is useful for room-temperature magnetoelectric coupling. But, at present no systematic data are available for the impact of epitaxial strain on the antiferrodistortive lattice distirtion in the perovskites..The group of rare earth (R) orthoferrites with the general formula RFeO3 belongs to the transition metal oxides with AFD distortion. The magnetic properties of rare earth orthoferrites are interesting because of the magnetic interaction of the different magnetic ions, leading to high-temperature antiferromagnetic transition, spin reorientation and strong anisotropy of magnetic properties. In this project, we focus on the coherent growth, strain control of the AFD lattice instability and their effect on the magnetic property of these orthoferrites and some orthorhomic manganites. The demonstration of the anisotropic strain control of the lattice distortion and the related magnetoelectric coupling will be significant for the development of oxide electronics.

具有典型Pbnm正交结构的稀土正铁氧体RFeO3是一类有趣的功能材料，显示高温（620-740 K）反铁磁序、强磁晶各向异性和自旋转向转变，且与其共角FeO6八面体的倾斜和旋转所导致的反铁畸变晶格不稳定性密切相关。近期研究表明八面体的倾、转自由度还与铁电序共存，一方面可导致如PbTiO3/SrTiO3界面处非正常铁电性的产生，另一方面在CaMnO3中反铁畸变的凝聚使其本征的正常铁电序淬灭，而若施加外延应变增强MnO6八面体的铁电畸变可诱发体系中显著的磁电耦合效应。因此，选择具有高温磁有序和正交反铁畸变的过渡金属氧化物薄膜进行研究，有望发现室温磁电效应；同时开展低对称性材料的共格生长及外延应变调控也是构筑和研究新奇氧化物界面的必要前提。本项目以正铁氧体及若干正交锰氧化物的外延生长为主干，重点探究应变调控对反铁畸变构型及其异质界面耦合和多铁序的影响规律，为氧化物电子学的进一步发展提供实验基础。

选择具有高温磁有序和正交反铁畸变的过渡金属氧化物薄膜进行研究，开展低对称性材料的共格生长及各向异性外延应变调控是构筑和研究新奇氧化物界面的必要前提。本项目以若干正铁氧体型正交锰氧化物、镍氧化物、钌氧化物及铁氧化物的外延生长为主干，重点探究应变调控对反铁畸变构型及其异质界面耦合和多铁序的影响规律，为氧化物电子学的进一步发展提供实验基础。项目基本按立项时的主题思路，针对一系列具有正交低对称性的氧化物薄膜材料，尤其是外延生长、各向异性应变调控、以及界面新奇效应展开了系统深入的研究，取得了一系列研究成果。..项目瞄准如下科学问题开展：1、过渡金属氧化物ABO3单晶膜中BO6八面体畸变方式的有效检测、界面耦合机制；2、各向异性应变对八面体倾斜、旋转协同畸变的作用机理及其与多铁物性的关联。合成了一系列具有正交低对称性的氧化物薄膜材料，就其外延生长、各向异性应变调控、以及界面新奇效应展开了系统深入的研究。主要研究内容包括：正交铁、锰氧化物外延薄膜和超晶格中氧八面体的畸变与耦合机制；La0.67Ca0.33MnO3/SmFeO3/NdGaO3外延结构中的各向异性应变弛豫；各向异性应变对镍氧化物外延薄膜金属绝缘转变的调控；La0.67Ca0.33MnO3/CaRuO3外延薄膜中界面稳定的TC；锰氧化物相分离的实空间观测；以及超快光激发和各向异性外延应变相结合实现锰氧化物薄膜亚稳态的光调等。.