Non-symmetric superparamagnetic clusters in the relaxor manganites Sr2-xBixMnTiO6 (0 <= x <= 0.75)

The structural, microstructural, electrical and magnetic characterization of microcrystalline powders of new Ti manganites Sr2-xBixMnTiO6 (0 <= x <= 0.75) is reported. Compositional characterization by means of EDS and XPS confirms the stoichiometry of all samples to be practically equal to the nominal ones. Neutron and X-ray diffraction data show that all the samples crystallize with cubic symmetry, space group Pm (3) over barm, which is broken at the nanoscale as Raman spectroscopy and HRTEM/ED analysis suggest. The complex magnetic response is interpreted by considering the magnetic frustration derived from the presence of structural and atomic disorder; initial AFM behavior evolves to FM predominant interactions as x increases and superparamagnetic clusters stabilize above T-c. The variation of the permittivity with temperature for x >= 0.50, obtained from impedance measurements, seems to be related to a relaxor-type ferroelectric behaviour. The title materials could be regarded as a potential local multiferroic system, given that both the FE and FM responses seem to be dependent on the formation and development of BiMO3-type clusters, and therefore they are correlated.