Explore novel ferroelectric properties in BiFeO3 multiferroic mesocrystal

目前，推进对晶格、电荷、轨道和自旋自由度的电场控制是追求下一代低功耗、多功能和绿色纳米电子学的重点途径之一。需要通过外部刺激对新功能材料中的这些自由度进行更复杂的控制。为了控制这些自由度，必须建立一种具有这些自由度之间耦合的介质。铁电和磁性材料的成功结合导致了各种技术的出现。为了进一步增强功能，与传统的信息存储和计算机处理电子设备相比，铁磁性/自旋的电场控制成为一种令人兴奋的新范式，有可能影响数据存储、自旋电子学和高频设备。在磁电多铁体的应用中，有希望的解决方案和丰富的物理领域，其中可以利用电场来改变其磁序。同时支持强铁电序和磁序的多铁体通常是具有反铁磁自旋排列的绝缘体。为了实现对铁磁性的电场控制，人们以铁磁-多铁异质结构的形式使用了多铁材料。在众多正在探索的多铁体系中，BiFeO_3(BFO)是目前研究最多、理解最深入的体系。BFO具有较大的铁电极化和G型反铁磁性，在室温下具有较弱的斜磁矩，因此在非挥发性逻辑和存储器件中具有广泛的应用前景。铁电-反铁磁多铁性BFO的存在为利用电场控制自旋提供了一个令人兴奋的机会，尽管BFO为通过电场控制自旋自由度提供了理想的模板，但在实现新的器件之前，必须解决几个关键问题。主要的控制参数是铁电开关。在实现实际器件之前，必须解决铁电可靠性问题，如压痕、保持和疲劳。例如，保留率可以归因于结构域的热力学不稳定性。远离和朝向衬底的偏振之间的不对称自由能景观导致至少一个不稳定的偏振态。当极化束缚电荷没有被完全屏蔽时，不稳定区域中的去极化场的影响变得显著。尽管相关研究已经给出了通过控制化学环境、打破平面外成分对称性或利用应变梯度来减小极化双势垒能量差的方法，但铁电保持仍然是一个有待解决的关键问题。为了阐明保持问题，我们打算引入弹性能项来改善BFO的铁电保持，因为BFO的铁电转换涉及铁弹性形变。在我们之前的研究中，观察到应变BFO膜的混合相区的保留率有了巨大的改善。T/R混合相界利用周期电势分布的优点，在驰豫过程中作为磁区壁钉扎中心。与其他铁电材料中SPM针尖所写的反转磁畴相比，基于BFO周期应变的对称电势设计提出了一种利用弹性能来提高铁电保持率的可能解决方案。在这个方案中，自组装的BFO介晶将作为一个模型系统。我们认为BFO介晶与周围基质之间的弹性耦合对减小BFO的滞留起着重要作用。该系统保持率的大幅提高将为铁电保持率开辟一条新的途径，并可能在非易失性存储器和自旋电子学中得到应用。

Nowadays, one of the focal approaches to pursue next generation low power consumption, multifunctional, and green nanoelectronics is to advance the electric field control of lattice, charge, orbital, and spin degrees of freedom. More sophisticated control of these degrees of freedom in new functional materials by external stimuli are desired. In order to control these degrees of freedom, a medium possessing the coupling between these degrees has to be established. The successful incorporation of ferroelectric and magnetic materials has led to a variety of technologies. To further enhance functionalities, as compared with conventional information storage and computer processing electronic devices, electric-field control of ferromagnetism/spin becomes an exciting new paradigm with the potential to impact data storage, spintronics and high-frequency devices. Promising solutions and a rich field of physics reside in the use of magnetoelectric multiferroics, in which the electric field can be employed to switch its magnetic order. Multiferroics that support both strong ferroelectric and magnetic orders are typically insulators with an antiferromagnetic spin arrangement. To achieve electric-field control of ferromagnetism, multiferroics have been used in the form of ferromagnet-multiferroic heterostructures. Among numerous multiferroic systems being explored, BiFeO3 (BFO) is currently the most studied and best understood. BFO exhibits large ferroelectric polarization and G-type antiferromagnetism with weak canted magnetic moment at room temperature making it appealing for applications in non-volatile logic and memory devices. The presence of ferroelectric-antiferromagnetic multiferroic BFO has offered an exciting opportunity for controlling spin through the application of an electric field.Although BFO sets an ideal template of manipulating the spin degree of freedom via electric field, before the realization of new devices, several key issues have to be solved. The primary control parameter is the ferroelectric switching. Solving the ferroelectric reliability issues, such as imprint, retention, and fatigue has to be made prior to realizing a practical device. For example, retention can be addressed to thermodynamic instability of the domain. Asymmetric free energy landscapes between polarizations directed away and toward the substrates result in at least one unstable polarization state. Effects of depolarization fields in the unstable domain become significant when the polarization bound charges are not fully screened. Although efforts on related studies have shown their ways to reduce the energy difference of the polarization double-well by controlling chemical environment, breaking the out-of-plane compositional symmetry, or using strain gradient, ferroelectric retention is still a key issue yet to be dealt with. In order to shed light on the retention problem, we intend to induce the elastic energy term to improve ferroelectric retention of BFO, since the ferroelectric switching of BFO involves a ferroelastic deformation. In our previous study, an observation on a giant improvement of retention in the mixed-phase region of a strained BFO film was found. By taking the advantages of periodic potential distribution, the T/R mixed phase boundaries act as pinning centers of domain walls in the relaxation process. Compared to the reversed domains written by SPM tips in other ferroelectrics, the symmetric potential design based on the BFO periodic strain suggests a possible solution to use elastic energy to improve ferroelectric retention. In this proposal, self-assembled BFO mesocrystal will serve as a model system. We expect the elastic coupling between BFO mesocrystal and surrounding matrix plays an important role to diminish the retention of BFO. The achievement of great improvement on the retention in this system will open a new avenue to ferroelectric retention and possible applications in non-volatile memory and spintronics.