Explore novel ferroelectric properties in BiFeO3 multiferroic mesocrystal

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.

The potential outcomes of the proposed work will directly impact high technology industry in the long run. In today's electronic systems, information is stored, transported and processed by devices mostly built from conventional metallic, semiconducting or dielectric materials. Introducing ferroic orders such as ferromagnetism and ferroelectricity in these materials can bring memory into logic, a key goal for beyond CMOS (complementary metal oxide semiconductor) electronics. Progress in controlling different ferroic orders at the nanoscale could offer unprecedented possibilities for electronic applications. Beside memory applications, high-performance nanoferroic devices may also operate as non-volatile switches and find use in programmable logic architectures or new forms of computational schemes. In addition, some magnetic properties that may be modulated by ferroelectricity bring novel possibilities for the non-volatile electrical control of spin-dependent transport. In most of these potential applications, ferroelectricity serves as a primary/control ferroic order, which only requires an electric field instead of electric current to drive giving the merits of no Joule heating and low power consumption. However, ferroelectric retention has been a critical issue of realizing ferroelectric/multiferroic devices. Although in the metal-ferroelectric-metal capacitor, the problem can be solved by carefully controlling the bottom and top electrode contacts to ensure the electrostatic boundary conditions are balanced, in order to reduce the circuit size, the structure of one transistor with ferroelectric/multiferroic is more favorable. In such a structure, one side of ferroelectric/multiferroic materials is in contact with a semiconductor; while, the other side usually contact a metal. The imbalance of electrostatic boundary conditions causes severe retention problems. In this proposal, a possible solution of this reliability issue, ferroelectric retention, will be implemented. The success of this proposal can solve this critical issue in the push of many laboratory ferroelectric/multiferroic devices to practical productions.
The pathway to the economic and technological impact of the developments of the proposed work will be multistage. Researchers in the ferroelectric community can use the same concept to improve the performance of electronic devices built based on ferroelectrics. In turn, this concept will be realized as technologically useful sets of results. It is therefore important to engage workers in advanced device technologies the course of the development work. The PI will attract companies such as Western Digital and Seagate Technology who would benefit from this work to set up collaboration and support to further develop new devices prototype into market. Furthermore, students and staff who participates this project will have the skill and capabilities to move on to a range of UK and worldwide technology based companies (in addition to those in academia) after the training (described below). Their training can be applied, having a direct impact on industrial projects and the UK economy.