BiFeO3基多铁性薄膜异质结中尺寸相关逆磁电效应的相场模拟

It has been established that there are abundant physic issues existing in magnet/BiFeO3 heterostructures, of which the converse magneto-electric effect can be used in the design of spintronic devices with low energy consumption and multi functionalities. In such BiFeO3 based multiferroic heterostructures, the mechanisms that induced the converse magneto-electric effect include charge modulation, magnetic exchange interaction, strain transfer, and so on. With the co-existence of these mechanisms at the interface of the heterostructures, the converse magneto-electric coupling strength is determined by the interactions among these mechanisms. The strength of each mechanism depends not only on the chemical composition, but also on the in-plane size and out-of-plane thickness of the heterostructures. Phase field model has been demonstrated to be a powerful tool for studying the meso-scale domain structures of the ferroelectrics, ferromagnets, and multiferroics, as well as their domain evolutions under externally applied electric field, magnetic field, and stress field, respectively. The macroscopic properties can also be evaluated according to the domain structures obtained from phase field simulations. In the present proposal, the phase field model of BiFeO3-based heterostructure will be constructed to study the couplings/interactions among the interfacial mechanisms on the converse magneto-electric effect as well as their dependences on the size of the heterostructures. The present proposal aims to make further understanding of the interfacial mechanisms that inducing the converse magneto-electric effect of the multiferroic heterostructures, so as to instruct the experimental studies and promote the design and development of multiferroics-based devices.

"磁性/BiFeO3"多铁性异质结具有丰富的物理内涵，其逆磁电效应可用于设计新型低功耗、多功能自旋电子器件。对于此类BiFeO3基多铁性异质结，其逆磁电效应的物理机制包含电荷效应、磁性交换作用、应变传递等多种。这些机制可在异质结界面处共存，它们相互之间的相互作用决定了逆磁电效应的大小。每一种机制的强弱不仅依赖于异质结的化学组分，也取决于异质结的面内尺寸和纵向厚度。 相场模型可模拟铁电、铁磁、多铁性材料的介观铁性畴结构在外加电场、磁场、应力场作用下的动态演化，并可据此评估材料的宏观物理特性。本项目提出构建BiFeO3基多铁性异质结逆磁电效应的相场模型，研究异质结多种不同跨界面耦合/作用机制的相互作用及它们对异质结尺寸的依赖，以加深对多铁性异质结中逆磁电效应物理机制的理解并为实验研究提供理论指导，推动多铁性磁电原型器件的设计与发展。

电压驱动的垂直磁化翻转对研发高密度、低功耗的垂直磁记录技术及磁阻变随机存储器有重要意义，但目前报道的相关工作中都存在外加磁场的影响。.本项目首先运用唯像热力学计算、有限元分析及相场模拟的方法，探究了多铁性异质结中电压调控磁极化180o翻转的物理机制。(1) 在设计的方形纳米磁体基的多铁性异质结中，相场模拟和热力学分析表明，通过单向电致应变脉冲能够实现纯电场驱动的磁极化面内均匀180o翻转，且这种翻转是非易失的且可重复。(2) 在椭圆形的Co40Fe40B20纳米磁体与多晶Pb(Zr,Ti)O3薄膜组成的异质结中，通过结合有限元分析与相场模拟揭示了利用磁化强度的进动，单极电压脉冲驱动的磁化强度面内均匀180o翻转，它是非易失的、可重复的、快速的(~2.3 ns)且具有超低的能量消耗（~1.3 fJ），比电流驱动的模式能耗低近三个量级。(3) 针对面外电压调控铁磁/BiFeO3异质结构存在的问题，发展了(110)-BiFeO3基多铁性磁电异质结构的相场模型，研究了在面外电压作用下BiFeO3层的铁电序和反铁磁序参量，以及铁磁(Co90Fe10)层中的铁磁序参量的翻转动力学。(4) 理论上利用相场模拟在BiFeO3方形纳米岛上来设计自发形成的导电畴壁，计算表明，自发形成的头对头畴壁的稳定性主要由三个因素的贡献来决定，即纳米岛的几何束缚，电学边界条件和补偿的屏蔽电荷。.针对器件设计方面，(1) 优化了基于垂直磁极化翻转的存储单元设计，将弹性动力学与微磁相场模拟相耦合，通过结合压电应变动力学与磁极化动力学揭示了应变调控磁极化翻转的四个动态过程，准确评估了电压驱动应变诱导的垂直磁极化翻转速度，并构建了垂直磁极化翻转速度相图；该电压驱动模式也具有快的速度(~2.5 ns)及显著低的能耗(~fJ)。(2) 基于相场模拟，提出了一种新型面外电压驱动的磁涡旋畴基的磁电存储单元；在三角形纳米磁体/铁电层组成的异质结中，该存储单元基于压电应变调控的磁涡旋畴手性确定性的、可逆的翻转。分析了磁畴手性翻转的动力学过程及尺寸效应等。构建了交换偏置场-磁体厚度-临界应变相图，可为实验上设计器件提供指导。