Heterovalent A-site doping of multiferroic BiFeO3

Multiferroics, where magnetic order and ferroelectric polarization might be combined in a single phase, have been an object of the growing interest. However, most magnetic ferroelectrics tend to have low magnetic ordering temperature and are often antiferromagnets, in which the magnetoelectric effect is intrinsically small. BiFeO3 perovskite seems to be the most suitable object for multiferroic research in view of their high magnetic and ferroelectric ordering temperatures. On the other hand, spatially modulated G-type antiferromagnetic spin structure of BiFeO3 prevents any net magnetic signal and inhibits the observation of linear magnetoelectric effect. A-site doping of BiFeO3 has been found to be one of the way to suppress the spin modulation, however main regularities of changes of the magnetic and ferroelectric properties of the BiFeO3-based multiferroics under the A-site doping are still unclear. To reveal them, we have carried out complex investigation of crystal structure, magnetic and ferroelectric behavior of A= Ca, Sr, Pb and Ba- substituted multiferroic perovskites. The Bi1-xAxFeOy (x= 0.2, 0.3) solid solutions have been prepared and characterized by x-ray diffraction, magnetic, dielectric, Mossbauer spectroscopy and PFM measurements. It has been found that the heterovalent A2+ substitution in Bi1-xAxFeOy is realized through the formation of the oxygen vacancies. It has been shown that the crystal structure of the compounds is described within the space group R3c, permitting the spontaneous polarization, whose existence is confirmed by the PFM data. Magnetic properties of the solids have been found to be determined by the ionic radius value of the substituting element. Experimental results suggest that the A-site substitution with the biggest ionic radius ions effectively suppress the spiral spin structure of BiFeO3 and result in the net magnetization appearance.