BiFeO3纳米岛中铁电拓扑畴及其纳米多铁性异质结中逆磁电效应的相场模拟

It is significant to explore a low-power and high-density method of information storage for designing novel memory devices. There are abundant physic phenomena existing in BiFeO3 ferroelectric topological domains and BiFeO3-based nanoscale multiferroic heterostructures. Voltages rather than currents are utilized to control ferroic domains in order to accomplish information storage, which is promising for enabling low-power storage. The ferroelectric topological domains in BiFeO3 nano-islands were observed and they were switched by the applied voltage experimentally, but the mechanisms of their spontaneous formation and voltage manipulation of the topological domains remain elusive theoretically. Furthermore, there are several physical mechanisms of converse magnetoelectric effect in the BiFeO3-based nanoscale multiferroic heterostructures, including exchange coupling, strain transfer and charge effect and so on, but the coexistence and complex interactions of interfacial coupling mechanisms in the nanoscale heterostructures are still confusing. In the present proposal, the phase field models of the BiFeO3 nano-island and BiFeO3-based nanoscale multiferroic heterostructures will be constructed to investigate the ferroelectric topological domains and converse magnetoelectric effects. The formation mechanism of topological domains will be revealed in the view point of different boundary conditions (e.g., geometrical constraint, mechanical and electrical boundary conditions) of the BiFeO3 nano-island and the reversible switching behaviors of topological domains manipulated by the voltage will also be studied. Moreover, the quantitative relationship among the converse magnetoelectric effect and the sizes of the heterostructures and ferroelectric domains will be established in the BiFeO3-based nanoscale multiferroic heterostructures. This present proposal aims to deepen the understanding of ferroelectric topological domains and different interfacial coupling mechanisms at the heterostructures, providing theoretical guidance for the experimental studies. This will stimulate the development of high-density and low-power multiferroic magnetoelectric-based storage devices.

探寻低能耗高密度的信息写入方式对于新型存储器件至关重要。BiFeO3铁电拓扑畴及其纳米多铁性异质结具有丰富的物理现象，利用电压而非电流来调控铁性畴以完成信息写入，有望实现低功耗存储。实验上已观察到BiFeO3纳米岛铁电拓扑畴及其在电压下翻转，但拓扑畴自发形成与电压调控机制仍不清晰。基于BiFeO3纳米岛的多铁性异质结中存在几种逆磁电效应物理机制，包括交换耦合、应变传递及电荷效应等，但纳米异质结中界面耦合机制的共存及复杂作用仍使人困扰。本项目提出构建BiFeO3纳米岛拓扑畴及其纳米多铁性异质结逆磁电效应的相场模型，从BiFeO3岛的不同边界条件（几何束缚、力学及电学）角度来揭示拓扑畴形成机制及电压调控其可逆翻转行为；建立BiFeO3基纳米多铁性异质结中逆磁电效应与异质结尺寸及铁电畴之间定量关系，加深对拓扑畴与异质结界面耦合机制的理解并为实验提供理论指导，促进高密度低功耗的多铁性磁电器件发展。

探寻低能耗高密度的信息写入方式对于新型存储器件至关重要。BiFeO3铁电拓扑畴及其纳米多铁性异质结具有丰富的物理现象，利用电压而非电流来调控铁性畴以完成信息写入，有望实现低功耗存储。本项目围绕铁电拓扑畴形成及其纳米多铁性异质结中逆磁电效应的物理机制，构建了BiFeO3自支撑铁电薄膜与其纳米多铁性异质结的相场模型，阐明了电学/力学边界条件下铁电拓扑畴（涡旋畴）形成机制，建立了纳米多铁性异质结中逆磁电效应与铁磁岛的面内尺寸、厚度之间定量关系。本项目取得主要成果为：(1) 借助相场模拟，在由凹面三角形Terfenol-D纳米磁体与Pb(Mg1/3Nb2/3)O3-PbTiO3薄膜组成的纳米尺度多铁性异质结中，实现了一种由电场调控Y型磁态确定性、快速(~2 ns)、可重复的120度翻转。(2) 针对不同（力学及电学）边界条件对铁电畴态的影响，构建了自支撑BiFeO3薄膜的相场模型，畴演化动力学发现了奇异的铁电涡旋畴自发形成，模拟揭示了自支撑BiFeO3铁电薄膜在大的弯曲变形（~5%应变）下超弹性的起源：BiFeO3薄膜的菱方相到四方相的可逆相变和几乎可逆的畴结构演化共同贡献了其超弹性。构建了一个以自支撑的薄膜厚度和弯曲角度为函数的相图和畴结构图，这可方便地预测在弯曲的BiFeO3薄膜中T相和铁电涡旋畴(vortex)的形成。(3) 揭示了在不同边界条件约束下铁电BiFeO3自支撑薄膜中极涡畴的形成机制；涡旋畴的手性（即顺时针或逆时针涡旋）能够通过n型弯曲和u型弯曲来调控。这些研究发现有助于加深对拓扑畴与异质结界面耦合机制的理解，为实验上设计非易失、快速、高密度、低功耗的柔性磁电存储器件提供了理论指导。项目成果发表高水平论文10篇，其中以第一作者或共同一作在Science Advances，Advanced Functional Materials，Acta Materialia，Physical Review Applied，ACS Applied Materials Interface等期刊发表论文8篇。项目培养了1名硕士生。