Explore novel ferroelectric properties in BiFeO3 multiferroic mesocrystal

Lead Research Organisation: Durham University
Department Name: Physics

Abstract

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.

Planned Impact

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.
 
Description BiFeO3 (BFO) mesocrystal system was successfully grown by depositing BiFeO3 and CoFe2O4 (CFO) together on SrTiO3 substrate to self assemble into vertical nanostructures - BFO forming the nano-pillars and CFO forming the matrix. BFO mesocrystal with the thickness of 80 nm exhibit the everlasting ferroelectric retention, revealing a much improved stability of the switched polarization state. The relaxation behaviours of other BFO mesocrystal of three different thicknesses were also characterized and analysed. The experimental results show that size and thickness of BFO nanocrystals affect the retention behaviour significantly. The effect of thickness shows a competition between the intrinsic depolarization field and the stress imposed by the CFO matrix, resulting in an optimal thickness of 80 nm. The BFO nanocrystals of smaller size suffer a strong clamping from the CFO matrix. Thus, the smaller BFO nanocrystals are much stable than the larger ones at the same thickness. The time constant of the still-remained BFO mesocrystal is infinite since it does not show any degradation, meaning the lifetime of the BFO mesocrystal is over 10 years of data retention. The prediction through the phase field simulation reveals that the ferroelastic deformation of BFO is restricted by the stress originating from the intimate structural interaction between the BFO mesocrystal and CFO matrix. Therefore, the switched polarization in BFO nanocrystals cannot spontaneously reverse back. These results suggest that the approach of improving the ferroelectric retention by clamping the crystal structure is practical. The permanent ferroelectric retention of the strain-confined BFO mesocrystal presents a great leap towards realizing the non-volatile multiferroic devices.
Exploitation Route Ferroelectric retention has been a critical issue of realizing ferroelectric/multiferroic devices. In the metal-ferroelectric-metal capacitor, the problem can be solved by carefully controlling the bottom and top electrodes to ensure the electrostatic boundaries are balanced. However, 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 contacts a metal. The imbalance of electrostatic boundary conditions causes severe retention problems. To solve this problem, additional energy term has to be induced into the system to balance the free energy landscape. The elegant approach proposed here is to use self-assembled BFO mesocrystal[26]. The intimate connection between the mesocrystal and matrix material provides a tunable structure coupling. This elastic energy term may be exploited to improve ferroelectric/multiferroic retention. This work is very crucial on designing magnetoelectric coupling routes and device geometries to realize non- volatile magnetoelectric coupling devices at the nanoscale.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy

 
Title PFM/c-AFM simultaneous imaging 
Description In the conventional atomic force microscopy methods employed in previous studies, the images from different channels (e.g. piezo-response force microscopy (PFM) and conducting atomic force microscopy (c-AFM) channels) can only be taken in the same area in sequence, which results in a delay of about four minutes between images (with the typical setting of 256 scan lines at 1 Hz scan rate). With the new method we've developed, PFM and c-AFM signals (or any two kinds of AFM signals) can be alternately acquired at each scan line, resulting in the time delay of only ~1 second between the requisition of the signals, thus providing better temporal resolution to directly investigate the local conduction and ferroelectric behavior of a metastable system. Furthermore, this technique can also significantly suppress the image drift between the PFM and c-AFM channels. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? No  
Impact This method allows to investigate varies physics properties of microscopic metastable systems by atomic force microscopy with much higher temporal resolution. 
 
Description Chungang Duan 
Organisation East China Normal University (ECNU)
Country China 
Sector Academic/University 
PI Contribution My group contributes ideas in the research and our expertise in scanning probe microscopy and soft x-ray absorption related techniques.
Collaborator Contribution My partner's group contributes their expertise in DFT simulations.
Impact http://dx.doi.org/10.1002/adma.201602281 http://dx.doi.org/10.1039/c7tc00974g http://dx.doi.org/10.1038/am.2016.55
Start Year 2013
 
Description Eddie 
Organisation National Chiao Tung University
Country China 
Sector Academic/University 
PI Contribution My group contributes ideas in the research and our expertise in scanning probe microscopy and soft x-ray absorption related techniques.
Collaborator Contribution My partner's group contributes ideas in the research and their expertise in material growth, hard x-ray diffraction, and man power.
Impact http://dx.doi.org/10.1038/ncomms13199 http://dx.doi.org/10.1002/adma.201602281 http://dx.doi.org/10.1063/1.4954172 http://dx.doi.org/10.1039/c7tc00974g http://dx.doi.org/10.1002/aelm.201600295 http://dx.doi.org/10.1038/am.2016.55
Start Year 2007
 
Description Judith 
Organisation University of Cambridge
Country United Kingdom 
Sector Academic/University 
PI Contribution My group contributes ideas in the research and our expertise in scanning probe microscopy and soft x-ray absorption related techniques.
Collaborator Contribution My partner's group contributes ideas in the research and their expertise in material growth.
Impact We have three papers in preparation towards publication soon.
Start Year 2017
 
Description Keji 
Organisation University of Texas at Austin
Country United States 
Sector Academic/University 
PI Contribution Our group contributes ideas in research and our expertise in piezoresponse force microscopy in high frequency of AC signal.
Collaborator Contribution My partners' group contributes ideas in research and their expertise in piezoresponse force microscopy.
Impact A paper has been published on Nano Letter. http://dx.doi.org/10.1021/acs.nanolett.7b02198
Start Year 2017
 
Description Pu 
Organisation Tsinghua University China
Country China 
Sector Academic/University 
PI Contribution My group contributes ideas in the research and our expertise in scanning probe microscopy and soft x-ray absorption related techniques.
Collaborator Contribution My partner's group contributes ideas in the research and their expertise in material growth, hard x-ray diffraction, and lots of man power.
Impact We have published paper on Nature. A new technique with PFM/c-AFM simultaneous imaging has been developed. Publications: http://dx.doi.org/10.1002/andp.201800130 http://dx.doi.org/10.1002/adma.201806335 http://dx.doi.org/10.1103/PhysRevB.97.235135 http://dx.doi.org/10.1038/nature22389
Start Year 2007