探索基于人工超晶格LaFeO3-YMnO3和自然超晶格n-LaFeO3-Bi4Ti3O12薄膜多铁性

基于纳米尺寸下复合铁电材料和反铁磁性材料是一个探索多铁性材料有效的方法. 利用激光脉冲沉积制备出LaFeO₃-YMnO₃人工超晶格和掺入不同层LaFeO₃, BiFeO₃的Bi₄Ti₃O₁₂的外延薄膜. 通过系统的X射线衍射、透射电子显微术、扫描透射电子显微术下的能量损失谱表征证明这些样品具有原子尺寸上清晰的界面和完整的层状结构. 磁性测试证明这些材料具有亚铁磁性. 特别是在0.5和1.5LaFeO₃-Bi₄Ti₃O₁₂中的亚铁磁性甚至能保持到室温. 就铁电性而言, 铁电性测试显示出LaFeO₃-YMnO₃和插入BiFeO₃的Bi₄Ti₃O₁₂样品中存在较大的漏电流, 而在0.5LaFeO₃-Bi₄Ti₃O₁₂样品中存在铁电性. 因此在0.5LaFeO₃-Bi₄Ti₃O₁₂中能够实现亚铁磁和铁电共存. 其次发现当掺入多层的钙钛矿(3层SrTiO₃或2.5层LaFeO₃)后, Bi₄Ti₃O₁₂ 的层状结构将出现结构失稳现象. 这些工作对于利用纳米复合开发新颖多铁性提供一些实例.

Exploring multiferroic materials based on artificial superlattice LaFeO3-YMnO3 and natural superlattice n-LaFeO3-Bi4Ti3O12 thin films

Combining ferroelectric with antiferromagentic materials in nanometer scale is an effective method for exploring multiferroic materials. We preflent two kinds of systems to show the possibility of multiferroic properties in such nanometer composites. One is the artificial superlattice LaFeO₃-YMnO₃, and the other is the natural layered Aurivillius material Bi₄Ti₃O₁₂ doped with different layers of LaFeO₃, BiFeO₃. Both materials were synthesized by pulsed laser deposition method on SrTiO₃ substrates. Microstructural charterizations with XRD, TEM, and EELS in scanning transmission electron microscopy mode substantiate that the samples have atomically sharp interfaces between neighboring layers; this is important for producing possible magneto-electric coupling in multiferroic materials. Magnetic characterization proves that these materials have ferrimagnetic properties, in spite of their anti-ferromagnetic nature before coupling. Magnetic characterization also proves that there is 0.55-0.9 μ_B remanant magnetization generated at LaFeO₃-YMnO₃ interface. And the 0.5 and 1.5LaFeO₃-Bi₄Ti₃O₁₂ samples show ferrimagnetism which can remain even up to room temperature. Ferroelectric tests prove that there is a large leakage current in LaFeO₃-YMnO₃ superlattice and BiFeO₃-inserted Bi₄Ti₃O₁₂, but 0.5LaFeO₃-Bi₄Ti₃O₁₂ shows ferroelectric hysteresis loops. It can be therefore concluded that 0.5LaFeO₃-Bi₄Ti₃O₁₂ is a multiferroic material. If more perovskite layers (3-layer SrTiO₃ or 2.5-layer LaFeO₃) are inserted, the Aurivillius structure of Bi₄Ti₃O₁₂ may appear structural instability that can be observed in our HRTEM measureflent. Our first principles calculations show that the degeneracy of formation enthalpies is the reason why the intergrowth in these materials forms and their structures are not stable. Our work may provide some examples for exploring new multiferroics by means of nano-meter composite.