外延锰/钌氧化物多层膜和超晶格中的界面八面体邻近及其磁性关联

Recently, the interfaces of transition-metal oxides (TMOs) have drawn much attention. On one hand, they can exhibit emergent phenomena, e.g., superconductivity at SrTiO3/LaAlO3 interface, exchange bias in LaNiO3/LaMnO3 superlattices (SLs), and ferromagnetic (FM) proximity effect at YBa2Cu3O7/La2/3Ca1/3MnO3 (LCMO) interface. These interface-mediated new phase or enhanced properties have greatly enriched the physics and devices design of oxide electronics. However, it is also true that due to the so-called "dead layer", arising primarily from the lattice-mismatch strain, charge/orbital ordering, or dimensionality switch, the stabilization of bulklike properties in the films is also very important. In both cases, the interface-involved properties are usually too weak, thus fabricating epitaxial multilayers and SLs with controlled interfaces is indispensible for exploring the interface effects. On the other hand, the heteroepitaxy depends sensitively on the symmetry mismatch in the systems. The orthorhombic CaRuO3 (CRO) can grow coherently on orthorhombic NdGaO3 (NGO) substrates, but poorly on pseudocubic LSAT substrates. In this case, the lattice symmetry does play a crucial role, since both substrates have very similar average lattice constants. The symmetry, orthorhombic or cubic, is determined by the octahedral tilt and rotation patterns, a-a-c+ or a0a0a0 in Glazer notation, in the corner-shearing perovskites, and the tilting mismatch could explain the different growth mode. We found that even for the same Pbnm symmetry, the different octahedral tilt and deformation can lead to strong phase separation in LCMO/NGO(001) films, although the average lattice mismatch is negligible. The ruthenate/manganite SLs are rather interesting. For CRO/CaMnO3, where in bulk they are paramagnetic-metal and antiferromagnetic (AF) insulator respectively, weak ferromagnetism with TC near 100 K has been induced in 1-2 unit cells near the interfaces. For SrRuO3/La0.7Sr0.3MnO3 (LSMO), however, the two FM layers show AF interlayer coupling, giving rise to an exchange bias and the inverted hysteresis. Stabilized FM order near 300 K was observed for the ultrathin LSMO. We have fabricated high-quality all-orthorhombic CRO/LCMO SLs on NGO and found that the LCMO layer in the SLs can keep bulklike Tc even at 1.6 nm. Our preliminary results also imply that the coercive field is tunable via controlling the interfacial octahedral proximity and relaxation depending on the relative component thicknesses. Clearly, the ruthenate/manganite epitaxial systems are excellent playground for investigating the interfacial issues, as we have outlined above. In this project, by incorporating new components like Ca(Ru1-xTix)O3 we will design various epitaxial systems in order to delineate the octahedral tilting coherency and relaxation across the various interfaces, and to uncover their relationships to the transport/magnetic properties that still need to be carefully and fully explored.

近年来，过渡金属氧化物外延体系中的界面结构和界面演生效应引起人们的重视。一方面，即使就最简单的ABO3材料体系而言，BO6八面体子结构单元的存在使其空间结构多变而复杂，由于八面体的转动构型对材料性能起支配作用，它们在外延界面处的共点连接方式和畸变形式急待掌握，并直接关系到实际材料能否真正外延。转动自由度的引入导致外延界面处八面体的结构邻近，它们在多层膜和超晶格中的弛豫规律有待探索。另外，作为强关联体系，界面对称破缺、序竞争、演生效应及其与界面结构的关联更是扑朔迷离。这些无疑要求实验工作者构建更合适的外延体系，在解析界面结构的同时探索更丰富的界面物性，并建立其间的关系。近期，我们发现某些锰/钌氧化物多层膜和超晶格具有"死层消失"和矫顽力可控等新奇特性，本项目在此基础上拟对相关外延体系展开全面深入的研究，力争在界面结构邻近及其磁性关联方面给出清晰的物理图像，为界面科学的发展提供可靠的实验依据。

关联电子氧化物界面处对称性破却，以及电荷、自旋、轨道和晶格自由度间的相互作用是产生界面效应的物理基础。但是氧化物界面新物态的发现仍处于“撞大运”的阶段，由于物理图像不清晰，理论上的缺乏指导，实验上被发现的具有独特界面效应的外延体系仍稀少。本项目开展前期，我们在CaRuO3(CRO)/La2/3Ca1/3MnO3(LCMO)/CRO三明治结构和LCMO/CRO超晶格的研究中，发现了一些新奇结果：1、即使LCMO的厚度低至1.2 nm，体系仍具有255 K的TC，预示着界面死层的“消失”；2、对于相同厚度(3.2nm)的LCMO层其矫顽力随着非磁性层CRO厚度的改变有近4倍的变化，预示着界面八面体邻近和弛豫的关键作用。显然LCMO/CRO及相关外延体系，包括多层膜和超晶格，不仅有难得的界面效应，从研究界面结构的角度更是模型体系，因为它们具有相对简单的正交/正交构型，无疑值得更深入全面的研究。.本项目以正铁氧体型正交锰、钌氧化物及其超晶格的高质量外延生长为主干，重点在于进一步探索该体系可能具有的更丰富的界面效应，进而探究界面相互作用对其异质界面耦合和多铁序的影响规律。项目按立项时的主题思路，针对一系列正交锰/钌氧化物超晶格的界面展开了深入细致的研究，取得了丰富的研究成果。特别是首次制备出基于全氧化物外延体系的人工反铁磁体 [LCMO/CaRu1/2Ti1/2O3]N (N=2-10)，观察到随外加磁场清晰的具有层分辨的分步磁化翻转模式，成果以All-oxide-based synthetic antiferromagnets exhibiting layer-resolved magnetization reversal为题发表在《Science》杂志上【Chen et al., Science 357, 191-194 (2017)】。.项目共发表研究论文19篇，包括Science、Nat. Mater.、Nat. Commun.、Adv. Mater.各1篇，ACS Appl. Mater. Interfaces 2篇，Appl. Phys. Lett. 2篇，Phys. Rev. 系列4篇，及J. Appl. Phys. 4 篇。获授权国家发明专利1项。培养研究生5名，其中陈斌斌获2018年中国科学院优博论文。