Barium-Based Filled Tungsten Bronze Niobate Multiferroic Ceramics

Multiferroics have become one of the hottest topics in recent years for their potential applications in multifunctional devices and interesting physics. In the present thesis, filled tetragonal tungsten bronze (TTB) niobate ceramics with Fe3+ at its B-sites were prepared. The structures, dielectric, ferroelectric and magnetic properties were systematically investigated. The effects on structure and properties caused by introducting magnetic cations into B-sites and by putting different cations in A1/B-sites were especially emphasized. The variation tendencies and structure origins of the ferroelectric transition, dielectric relaxations and magnetic properties were discussed.In the Ba4R2Fe2Nb803o(R= La, Nd, Sm, Eu, Y) system, the stability of the filled tetragonal tungsten bronze structure goes down when the radius of cations in A1-sites decreases. Ba4La2Fe2Nb803o ceramics are paraelectric in the measuring temperature range and antiferromagnetic at room temperature. Both the ferroelectric hysteresis loops and magnetic hysteresis loops were observed at room temperature for ceramics with R= Nd, Sm, Eu.In the Ba4(Y1-xLax)2Fe2Nb8O30(x= 0,0.3,0.579,0.684,1) system, the amount of secondary phases decrease with the increase of x. Ceramics with x= 0,0.3 are ferroelectric relaxors. When x= 0.579 and 0.684, though the average radius of A/ equal to that of Sm3+ and Nd3+ respectly, the dielectric and ferroelectric properties of the ceramics are different from that of Ba4Sm2Fe2Nb8O30 and Ba4Nd2Fe2Nb8O30, suggesting that the kind of cations determines the properties rather than the radiu difference between the A1- and A2-site cations. With the increase of x, the saturation magnetization decreases.In the Ba4Nd2Fe2(Nb1-xTax)803o (x= 0,0.05,0.1,0.3,0.5,1) system, Ba4Nd2Fe2Ta8030 ceramics are paraelectric in the measuring temperature range and paramagnetic at room temperature. When x= 0 and 0.05, ferroelectric phase transitions were detected, and the Tc are 337 K and 327 K respectly. When x= 0.1,0.3 and 0.5, no such phase transition was observed. The saturation magnetization decreases with x from 0 up to 0.5. And then after, the ceramics become paramagnetic.In the filled tetragonal tungsten bronze niobate ceramics, the antiferromagnetic ordering is induced by superexchange interaction between Fe3+-O-Fe3+, and the distortion of TTB structure leads to the change of the angle of Fe3+-O-Fe3+ bond, which results in the net magnetization.