Ferrotoroidic structures: polar flux-closure, vortices and skyrmions

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

The impressive amount of data produced daily by modern society requires more efficient information storage. Current capabilities need not only to be increased to meet demand, but also to be fundamentally changed to offer better density, power consumption, access speed and time stability. Existing data encoding is based on switching of ferroic order parameters such as magnetisation and polarisation that exist in ferromagnetic and ferroelectric materials, respectively. The effect of finite size sets a fundamental limit of the data density and retention that is being rapidly approached by current technologies. Therefore, there is critical need for novel data encoding mechanisms. One alternative is offered by ferrotoroidic structures that show multiple order parameters, related to properties such as chirality and winding number, which can be used to encode extra information. Not only do these additional parameters multiply the achievable information density, but they are predicted to exist exactly at the characteristic length where classical ferroic parameters are no longer effective for data encoding. Thus, ferrotoroidics provide an alternative way to overcome the limits of classical data storage.
Ferrotoroidic structures have been theoretically predicted but only very recently have they been experimentally observed. Besides the enormous application potential, especially in non-volatile memories, these exotic polar entities may require new physics to be fully understood. The present research aims to experimentally elucidate the origin of the ferrotoroidic structures through a comprehensive program of work. By understanding this complex phenomena, we will gain control of and tune the ferrotoroidicity in terms of density, chirality and spatial positioning.
We are especially targeting oxide polar ferrotoroidics in which the reorientation of the spontaneous polarization is a result of atomic displacement. For this reason, transmission electron microscopy is the technique of choice to determine the oxide properties at nanoscopic scale by measuring the displacement of atoms relative to each other. In-situ electron microscopy will provide real time information to investigate the effective interactions between electric fields and polar entities as well as potentially switch the toroidal moment chirality to demonstrate data encoding.

Planned Impact

Ferrotoroidic structures have become a very hot topic of research, having potential technological applications whilst also challenging fundamental theories. We expect to achieve an immediate impact by publishing our results in high impact broad interest and focused reviewed journals. The interplay between structure, strain, electric and magnetic field results in novel ferroic materials showing different ferroic order parameters and topologically protected states. This will lead to new science, developing the field of functional oxide materials in particular.
In addition to the significant scientific impact in the short to medium term, the research program will develop trained research personnel to fill positions available in Universities in the high technology areas such as thin film growth, micro- and nano-fabrication, transmission electron microscopy, etc., all in high demand in the UK. There will also be benefits in terms of this project's impact on the ability of the investigators of obtain funding for further research from European and US funding agencies, as well as to build collaborative links with new partners.
We will organise an international workshop focused on topologically protected state in ferroelectric materials that will ensure engagement with the scientific community and facilitate the establishment of new collaborations in the area. Outreach activities aimed particularly at local school pupils will be organize in collaboration with the Warwick Ogden Trust Teaching Fellow.
Our Regional Development Agency, Advantage West Midlands (and the European Regional Development Fund) funded in the past the Science City Initiative between Universities of Warwick and Birmingham for research into Advanced Materials. The aim of this initiative was to establish the region as an international competitor in materials Physics, undertaking world-class research in the development and characterisation of new materials for applications in a diverse range of industries. The present proposal will enhance the present capabilities in the area of Advanced Materials and stimulate research that will be beneficial to local, UK and European companies.

Publications

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Description We have already shown that in PbTiO3 (PTO)-based multiferroic tunnel junctions (MFTJs) the ferroelectric domain pattern for PTO at a thickness of 9 u.c. is
generally classic antiparallel (180°) with Ising type domain walls decorated with coupled clockwise and anti-clockwise vortices. For 6 u.c., a peculiar domain pattern with curling flux-closure type structures and incommensurate phase was observed. For only 3-u.c.-thick PTO films, domain structure is widely suppressed with polarization pointing out of plane and remanent domain structure.
We have successfully high quality grown BiFeO3-(La,Sr)MnO3 superlattices. They showed that BFO develops a novel lamellar phase that has never been reported before. This phase is still under investigations.
Exploitation Route The polar vortices can in principle be at the basis of novel non-volatilel memory devices in which the information is encoded in the toroidal moment or chirality of the vortices instead of polarisation, is in the classical ferroelectric memories (FeRAMs), or magnetisation in MRAMs.
Sectors Education,Electronics

 
Description SiliNano
Geographic Reach Europe 
Policy Influence Type Participation in a advisory committee
URL https://www.sili-nano.de/advisory-board.html