NEW MULTIFERROIC AND MAGNETORESISTIVE MATERIALS FOR INFORMATION STORAGE: EMERGENT BEHAVIOUR IN COMPLEX MOLYBDATE PEROVSKITES

The electronic properties of materials are intimately related with the structural characteristics of the solid. In crystalline materials the atoms are arranged in a periodic fashion that is determined by the chemical preferences of the atoms, and so careful selection of the atoms, and an understanding of their chemistry, can drive the formation of unusual crystal structures that show exotic and useful electronic properties.We have recently shown that molybdenum can drive an unusual structural distortion in complex, mixed metal oxides that is stabilised by a lowering the electronic energy. Most importantly, this subtle distortion overcomes the spherical nature of the atom and so introduces directionality into the electronic interactions. The strength of this distortion is such that it can be overcome by thermal vibrations in the crystal at 140 K. The proposal will stabilise this distortion up to room temperature and incorporate it into related compounds that show metal to insulator transitions.The distortion disrupts the high symmetry that favour the formation of the metallic state and so will provide a delicate way of tuning the switching from metallic to insulating behaviour. This will be further manipulated by the application of a magnetic field that should drive this transition and so give new magnetoresistive materials capable of operating at roomtemperature. Such materials allow the control of electron spin and can provide access to exotic physics as well as being of interest for applications such as the read heads of magnetic hard disks.The behaviour of molybdenum gives a handle on the magnetic properties of these materials and we will combine this effect with Pb2+ cations that are known to give a strong distortion that gives control of the electric polarisation of these crystal structures. The lead(II) cation is strongly non-spherical and tends to displace the surrounding negatively chargedions to give a region of local charge separation. If these charges are ordered throughout a crystal then the material can interact with an electric field. Such ferroelectric behaviour is the basis for high speed memory. We plan to combine this electric polarisation with the magnetic ordering observed in molybdates and so obtain materials which can be switchedbetween different magnetic and electric states by magnetic and electric fields. These multiferroic materials would have multiple states and so hold out the promise of storing information at a much higher density than current binary methods.