Papers by Author: Joanna Polnar

Abstract: Several compounds with Aurivillius-type structure in the Bi₄Ti₃O₁₂-BiFeO₃ system with the general formula Bi_m+1Fe_m–3Ti₃O_3m+3 showing multiferroic properties arouse an increasing interest due to their wide potential applications in electronics. The powders composed of the Aurivillius phase with m=5, 7 and 9 were successfully prepared by the co-precipitation-calcination route. The powders were ground and sintered, obtaining dense polycrystalline materials. The dielectric properties of the sintered materials were studied over a wide range of temperature and frequencies. A strong softening of the magnetic properties took place at 410°C, which stayed in good correlation with the recorded changes of the tan δ asymmetry and fact that it did not induce any anomaly of the Curie temperature at 670°C, where phase transition of the ferroelectric properties was determined.

Abstract: Bismuth layer-structured compounds in the Bi-Ti-Fe-O system known as Aurivillius phases are single phase multiferroics. It was stated that substitution of some rare earth elements for bismuth ions in such structure can modified its magnetic properties. Powders of Bi7Fe3Ti3O21 and Bi6.3Sm0.7Fe3Ti3O21 were prepared by co-precipitation – calcination method and then were sintered to dense polycrystalline materials. Low field DC susceptibility was measured in the zero field cooled (ZFC) and field cooled (FC) modes at 10÷350 K. For selected temperatures magnetisation curves and hysteresis loops were also measured. The FC and ZFC curves of both samples diverge at temperatures below 250 K indicating a spin glass-like behaviour. The compound with samarium exhibits magnetic hysteresis already at room temperature with the coercive field increasing to 870 Oe at 10 K. The low temperature hysteresis loops of the samarium containing compound are shift with respect to zero field which can be attributed to a magneto-electrical coupling of the samarium sublattice "exchange biased" by the iron one, which orders anti-ferromagnetically at a higher temperature than the samarium sublattice.

Abstract: The Aurivillius phases in the Bi-Fe-Ti-O system showing multiferroic properties arouse an increasing interest due to their wide potential applications in electronics. These compounds were usually prepared by high temperature solid-state reaction method using respective oxide powders mixed in required stoichiometry. An excess of bismuth oxide was often added due to its evaporation during heat treatment. The mixture of the oxide powders were calcined in air between 700 and 900°C for several or even for several dozen hours. In the present paper the Bi6Fe2Ti3O18 Aurivillius phase was prepared by direct solid state reaction between respective oxides and by co-precipitation – calcination method. Mixture of the oxides and co-precipitated gels were calcined at different temperatures and X-ray diffraction analysis was used for identification of phase composition of the products.

Abstract: The multiferroic Aurivillius phases in the Bi-Fe-Ti-O system are built from alternate (Bi2O2)2+ and (Bin-1XnO3n+1)2 layers, where X = Fe3+, Ti4+ and “n” refers to the number of perovskite-like layers between Bi2O2 layers. Detailed magnetic studies should be done to understand electromagnetic interactions and multiferroic coupling effects. In the present paper, a powder composed of the Aurivillius phase with n = 5, Bi6Fe2Ti3O18, was successfully prepared by the hydrothermal method. The powder was sintered, obtaining dense polycrystalline materials. It was stated that both powder and sintered bodies were paramagnets with a possible antiferromagnetic ordering or a spin-glass state at the liquid helium temperatures.