BiFeO3外延薄膜的相图及其对压电性能的调控研究

Lead-free BiFeO3 (BFO) provides good opportunities for tunable functionalities through its rich misfit strain-temperature related phase diagram because of the inherent coupling among elastic, electric, and magnetic order parameters. The effect of strain in BFO epitaxial thin films has received considerable attention leading to several interesting theoretical predictions and experimental verifications in strained BFO films. However, recent experimental investigations suggested that the previous strain-temperature phase diagrams are not accurate enough. Despite recent phenomenological and first principles calculations in (001)-oriented BFO epitaxial films, the experimental results of varying temperatures indicated the mechanism for the enhanced ferroelectricity and piezoelectricity is still vague. A systematic and more accurate experimental verification of theoretical predictions is necessary. We will study a detailed structure characterization of BFO films with various thicknesses grown on single crystal substrates using PLD and laser-MBE. To determine the role of substrate on the phase transition in the strained BFO films under controlled varying temperatures, high resolution x-ray diffraction measurements will be performed at different synchrotron sources in addition to the regular in-house XRD measurements. The surface morphology and piezoelectric force microscopy investigations will be carried out. Transmission electron microscopy (will be employed to study the phase and symmetry of different variants of BFO thin films. Based on our experimental results, we will use first principles and phenomenological theories to study and simulate the atomic structure and phase transitions and domain evolutions, explore the mechanism of interactions multi-field (depolarization field, temperature and stress fields) in a multi-scale framework.　With all the experimental and theoretical efforts combined, we aim to provide a more accurate strain-temperature phase diagram, shed light on the strain-driven morphotropic phase boundary.

无铅多铁BiFeO3材料同时具有反铁磁和铁电、铁弹等多种序。与块材样品相比，由于失配应变效应的存在，BiFeO3外延薄膜的铁电、压电以及磁性等性能有明显的提高。理解与调控不同温度下的应变效应是提高BiFeO3外延薄膜铁磁，铁电和压电性能的基础和关键。然而，目前BiFeO3外延薄膜的应变-温度相图还不清楚，应变驱动的铁电、压电增强机理也不明确。本项目拟实验与理论相结合，在不同基底和底电极上通过激光脉冲淀积、激光分子束外延等手段生长BiFeO3外延薄膜，应用变温可控的同步辐射, 透射电镜和压电力显微镜等原位方法研究不同应变和温度下薄膜的结构和铁电、压电等性能的变化。同时结合多尺度理论，模拟原子结构、相（畴）变与BiFeO3外延薄膜特性的关系，探讨其多外场（退极化场、温度、应力条件下的）场致响应机制。建立更为精确的BiFeO3外延薄膜的应变-温度相图，阐明应变驱动的类准同型相界。

作为唯一在室温下具有单相多铁性的材料，BiFeO3在压缩应变下的相变和其复杂畴的结构是我们主要研究对象。我们引入一层超薄的介电层进行退极化场的调控，精确控制BiFeO3 的71°和109°周期性条带畴结构；同时证明了CoFe/BiFeO3中交换偏置根源于109°畴壁。我们围绕外延BFO薄膜的物理性质和结构做了进一步的研究，结果发现一种新奇的负热膨胀系数。在BFO/LaAlO3的外延异质结结构中，发现混合相的晶格c随着温度增加而减少，具有明显的负系数特征。通过稀土La掺杂可以有效调控四方相BFO的奈尔温度在270-380K之间，我们发现c轴晶格常数表现明显的反常变化（负的热膨胀）：在自旋有序向自旋无序转变点处，这一个过程依赖于多铁材料中存在着自旋晶格耦合作用。四方相Bi0.9La0.1FeO3外延薄膜的负热膨胀具有非常宽的温度窗口，从室温到673K甚至更高的温度区域，并且在温度跨度370K的窗口区域中伴随着一个巨大负热膨胀系数为α = -12.8×10-6 K-1。此外，在更厚的四方相BFO中，混合相可以极大的促进负热膨胀行为，在50 nm厚的四方相BFO薄膜中，获得了一个巨大的负热膨胀系数22.3×10-6 K-1。这些发现很好的扩展了多铁材料BFO在新型电子器件的应用范围，可以满足一些极端条件下的应用环境。