Addressing Current Issues in Multiferroics

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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.