双钙钛矿La2NiMnO6的B位有序度对其磁-介电效应和铁弹性影响研究

Double perovskite La2NiMnO6 is a rare example of a single-material platform with multiple functions, which is a ferromagnetic-ferroelastic semiconductor with excellent magneto-dielectric coupling property near-room temperature. Due to its excellent multiple functions, it has the potential applications in the fields of spintronics. However, its physical mechanisms of magneto-dielectric effect, ferromagnetic and ferroelastic transitions are still ambiguous up to now. In addition, the B-site ions of double perovskites have significant effects on their physical properties. Thus, building the physical models between B-site degrees and physical properties would be a better way to study the mechanisms of these phase transitions and coupling effects. Thus, we will firstly synthesize La2NiMnO6 double perovskite samples with various B-site degrees through the optimism of prepared technology in this project. Then their information of crystal structures will be characterized and analyzed. On this basis, the techniques of X-ray diffraction and resonant ultrasound spectroscopy etc. will be used to study the ferroelasticity, ferromagnetism and magneto-dielectric effecs during phase transitions with various B-site degrees of La2NiMnO6. Especially, large variations of elastic constants occur at structural and magnetic transitions and, because of the relatively long interaction length provided by strain fields in a crystal, we will provide an effective formal framework for characterizing their form and magnitude. We will also investigate the elastic anomalies related to the phase transitions and dynamic mechanisms of microstructures, such as the ferroelastic twin walls. These insights into the role of B-site degrees in a system will make a great contribution to facilitating the development of the excellent physical properties in double perovskites, and also provide optimism for theoretical and practical applications of novel-type double perovskites.

双钙钛矿La2NiMnO6是一种罕见的在近室温下具有较强磁-介电耦合的铁磁-铁弹半导体多铁性材料，在室温自旋电子器件等领域具有重要的潜在应用。但到目前为止，其磁-介电效应、铁磁和铁弹相变机理尚不明确，普遍认为，双钙钛矿的B位离子对其物理性能具有决定性的影响，建立B位有序度与性能之间的物理模型，将能够更好的研究这些相变和耦合效应的物理机理。因此，本项目拟通过优化制备工艺，获得一系列不同B位有序度的La2NiMnO6双钙钛矿样品，并对其晶体结构进行表征。在此基础上，综合利用变温X射线衍射、超声共振波谱等实验技术，研究不同B位有序度样品相变过程中的铁弹性、铁磁性和磁-介电效应的变化。探究应变与结构和铁磁相变耦合的形式和强度，阐明与相变有关的弹性异常及铁弹畴壁等微观结构的动力学机理。本项目的开展将进一步揭示双钙钛矿的B位有序度与物性之间的内在联系，进而为设计新型双钙钛矿功能器件提供理论和实践基础。

双钙钛矿La2NiMnO6是一种罕见的在近室温下具有较强磁-介电耦合的铁磁-铁弹半导体多铁性材料，在室温自旋电子器件等领域具有重要的潜在应用。但到目前为止，其磁-介电效应、铁磁和铁弹相变机理尚不明确，普遍认为，双钙钛矿的B位离子对其物理性能具有决定性的影响，建立B位有序度与性能之间的物理模型，将能够更好的研究这些相变和耦合效应的物理机理。本项目从应变和弹性角度研究了B位部分无序双钙钛矿La2NiMnO6的耦合行为，发现双钙钛矿La2NiMnO6是具有三个序参量的双二次耦合系统 。分析超声共振波谱（RUS）发现：在~300-500 K的温度区间，多晶试样La2NiMnO6的点缺陷引起了铁弹畴壁的钉扎行为。而在常温以下温度，随着温度的降低剪切模量增加了40%，这主要是由于铁磁序参量与应变的双二次耦合导致。结合变温拉曼图谱和应变分析，验证了双钙钛矿La2NiMnO6的磁-弹性和磁-结构耦合行为。另外，我们结合实验和理论计算探究了晶格应变对钙钛矿La2NiMnO6两相共存现象的影响机制：使用第一性原理DFT计算了钙钛矿La2NiMnO6（完美晶体和有缺陷的晶体）的静态形成焓和不同压力下的晶体结构，结合XRD、SEM、EDS、拉曼、磁学性能测试结果和应变分析，证明了La2NiMnO6的单斜相和菱面体相在一定温度范围内共存的特性与两相形成焓的差异较小有关，另外局域的不均匀也可引起两相共存现象。基于La2NiMnO6的研究经验，本项目还探究了B位完全无序双钙钛矿La2FeMnO6的应变耦合机理，以及与自旋玻璃相变相关的声损耗机制。使用超声共振波谱（RUS）技术探究了双钙钛矿La2FeMnO6的弹性和滞弹性行为。结合磁学和电学测试结果，证明了在大约55 K发生的自旋玻璃相变符合Vogel-Fulcher动力学行为，并且其磁-弹性耦合行为主要是由于局域电子结构的变化促进了局域自旋状态的应变弛豫。而250 K以下另一个相变行为主要由团簇自旋玻璃引起，并且与应变不存在明显的耦合特性。本项目的开展进一步揭示双钙钛矿的B位有序度与物性之间的内在联系，为设计新型双钙钛矿功能器件提供理论和实践基础。