Magnetoelectricity and Multiferroic Behaviour in Tungsten Bronze Oxides

The aim of this study was to characterise ferroelectric tungsten bronze oxides with varying amounts of magnetically active species on both A- and B-sites with a view to developing a methodology for designing magnetoelectric and multiferroic tungsten bronzes. The study involved increasing the level of magnetically active species in niobate tungsten bronzes, initially based on Ba6FeNb9O30. Defect chemistry was carefully controlled by co-doping with compensating species and by scrutiny of synthetic conditions (e.g. temperature, partial pressure of oxygen) in order to minimise detrimental electronic and ionic conductivity due to anion vacancies. By judicious choice of dopant species on A- and B-sublattices it was possible to extend solid solution limits and/or induce cation ordering, both of which offer the best chance of successfully producing magnetoelectric coupling in a ferroelectric material.

Magnetoelectrics are a special class of solid-state material which simultaneously possess both magnetic and electrical ordering properties. Currently, many information storage technologies are based on either magnetic or electrical ordering and switchability. If a materials possesses both these properties then, in principle, it should be possible to exploit this advantageously, for example by storing data electrically, but reading it magnetically. There has therefore been a recent resurgence of interest worldwide in such magnetoelectric materials, driven largely by the materials science and physics communities. Although this has led to considerable fundamental understanding of exisiting materials it has also flagged up a serious lack of new materials being discovered and developed. This 'discovery' aspect is the realm of the solid state chemist, and in this project we sought to address this problem from a solid state chemistry perspective. We explored new tungsten bronze materials as potential magnetoelectrics, basing our search on a sound understanding of the structural and compositional chemistry of this family of compounds. We shall characterise our new compounds using a variety of crystallographic and physical (ie. magnetic and electrical) techniques in order to pin down the key structure-property-composition relations of these materials. Ultimately, we aimed to provide a range of new materials exhibiting magnetoelectric effects, on which the materials science and physics communities will be able to base more applied and developmental work.

We discovered a new family of materials which exhibited relaxor behaviour which is closely related to ferroelectricity. Within this family we showed that B-site substitutions could be used to control the crystal structure resulting in stabilisation of the dipoles responsible for the relaxor behaviour to room temperature . In a similar study we used the size variance of the A cation species to achieve the same effect. By combination of both types of general behaviour we were able to "design" materials which had strongly coupled dipoles at room temperature and exhibited ferroelectricity.
The insights gained into composition-structure-properties in these materials offers the exciting prospect of develop novel multiferroics, which are of great academic interest and in turn could enable a host of new device technologies and applications in the consumer electronics industry.