Structure and properties of magnetically ordered ferroelectric and relaxor systems

Multiferroic materials have attracted renewed interest because they could induce magnetoelectric coupling, offering additional degrees of freedom in micro electromagnetic device design and applications. In this paper, the structural, ferroelectric and magnetic properties of some multiferroic materials will be presented with emphasis on the solid solution systems between BiFeO3 and PbTiO3 [(1??x)BF??xPT] and Pb(Fe2/3W1/3)O3 and PbTiO3 [(1??x)PFW??xPT]. In the (1??x)BF??xPT system, a morphotropic phase boundary (MPB) region is present with coexisting tetragonal, rhombohedral and orthorhombic phases and a large tetragonality in the tetragonal phase region. The temperature variation of magnetic moment in the MPB compositions shows three anomalies, arising from the antiferromagnetic orderings of the rhombohedral, tetragonal and orthorhombic phases, respectively. A low temperature magnetic ordering is found to arise from the long range superexchange of Fe3+-O-Ti-O-Fe3+ in the chemically ordered microregions. The magnetic transition temperature (TC) of this ordering increases with the increasing PT content and shows a maximum in (Bi0.45Pb0.55)(Fe0.45Ti0.55)O3, which is explained based on the competition between the effect of concentration of the chemically ordered micro-regions and that of coupling distance. The magnetic phase diagram of the (1??x)BF??xPT solid solution system is constructed. In the (1??x)PFW??xPT system, the macroscopic magnetization measured under the a magnetic field demonstrates different kinds of magnetic behavior associated with the weakly ferromagnetic, antiferromagnetic and paramagnetic ordering in the temperature range of 2 ?? 390 K. Interestingly, the low-temperature ferromagnetism is enhanced by the addition of ferroelectric PT up to x=0.27. The unique ferroelectric and magnetic relaxor behavior will also be discussed. Selected results on the BF-PT system are shown below.