Addressing Current Issues in Multiferroics

The smooth operation of the modern world depends on our ability to store and access data reliably. Almost everything we need, from the accurate management of bank accounts to the flexibility of digital entertainment, requires the reading and interpretation of strings of binary '1's or '0's. At the heart of data storage, such binary numbers usually exist in the form of either the polarity of electrical charge (in DRAM, Flash or FRAM) or the orientation of magnetisation (in magnetic hard-drives). Charge-storage devices and magnetic storage devices both have negative aspects about their architectures or operation, and so for some years there has been interest in developing a memory element that combines the positive features of each, allowing 'writing' of information to be done electrically, and 'reading' to be done magnetically. Materials that are both ferromagnetic and ferroelectric would be highly desirable for such applications and, as a result, so-called 'multiferroics' have become a topic of great recent research interest.
Unfortunately, there are very few known multiferroic systems and none has been discovered to date which can readily be made and simultaneously displays both large polarisation and magnetisation. The first element of this proposal is therefore to explore two relatively new groups of multiferroics (birelaxors and lattice strained EuTiO3) to see if they can offer properties that are superior to the best known multiferroic currently available (bismuth ferrite).
The use of a multiferroic in a memory element requires the manipulation of magnetic and ferroelectric regions, known as domains. While a great deal is known about domain behaviour in ferromagnets and in ferroelectrics separately, much less is known about the static and dynamic behaviour of multiferroic domains. Exploration of domains in meso and nanoscale objects (dimensions relevant to high density memory) will be performed on small scale single crystals, cut from high purity bulk material using a Focused Ion Beam-based methodology uniquely developed by the applicants. To date this has given extremely clear information on ferroelectrics and should be ideal for fundamental investigations into multiferroic domain properties.
In addition to interest in multiferroic memory, researchers have become increasingly excited by the potential use of multiferroics in more exotic applications - the domain walls in bismuth ferrite have been found to act as planar conductors and large photovoltaic effects have been displayed. To date, such effects have only been probed in thin films grown by pulsed laser deposition. While this is a useful and flexible growth technique it has a tendency to introduce significant levels of defects that can lead to properties which are extrinsic, rather than intrinsic to the material. We wish to examine the properties of single crystal thin films of bismuth ferrite (and later birelaxors) made using the established Focused Ion Beam process mentioned above for such exotic domain wall and photovoltaic effects. Importantly, using this approach should allow a different view, which may corroborate or conflict with information to date only obtained through PLD grown films.

Multiferroics are still primarily in their fundamental research phase; nevertheless, it is important to appreciate that these materials have the potential to revolutionise a number of key technologies in the future:
(i) firstly, roadmaps for data storage development show that there is a finite lifetime for FLASH non-volatile memory. Plans to reduce operational voltages are likely to present insurmountable technical challenges for FLASH and so searching for a new form of solid-state non-volatile storage (apart from hard-drives) has been an issue for a number of years. Some systems have been designed using magnetic materials, while others have used ferroelectrics. Neither is ideal, but combining the best properties of magnetism and ferroelectricity could yield an almost ideal memory device. Magnetoelectric multiferroics could be the key - acting like ferroelectrics in the WRITE phase of data recording, but as magnetics in the READ phase. Because of the potential for multiferroics in data storage, and because of our intention to explore magnetic domain imaging as part of this proposal, Seagate Technologies have significant involvement, donating a Magnetic Force Microscope (~£250k). The PI at QUB and the PDRA will report to Seagate on progress during the period of the grant;
(ii) secondly, several multiferroics possess band-gaps that are in the visible spectrum, such that they absorb light efficiently, leading to the production of electron-hole pairs. The internal fields associated with ferroelectricity immediately split the electron and hole apart, dramatically lowering recombination rates, making for (at least in theory) extremely efficient photovoltaics. Bismuth ferrite has already been shown to act as a strong photovoltaic, and YMnO3 is likely to have similar properties. Thus, multiferroics could be a material of the future in energy production from sunlight;
(iii) thirdly, a great deal of excitement has been generated by the discovery that domain walls in multiferroics can be conductive, and that conductivity depends on the exact form of the domain wall. Using engineered domain wall arrays as nanoscale interconnects is clearly a long way off, but the possibility could have dramatic ramifications in self-assembled nanoelectronics.