Dielectric Relaxation And Multiferroic Characteristics Of BiFeO₃, YFeO₃ And YMnO₃-Based Ceramics

Multiferroics are a class of materials possessing at least two ferroic propertiesamong ferroelectricity, ferromagnetism (antiferromagnetism), and ferroelasticity.Because of its rich connotation in condense matter physics and attracing potentialapplications, multiferroic materials have become the hot research topic in condensematter physics and material science. In the present work, the microstructure, dielectricrelaxation, and multiferroic properties in BiFeO₃, YFeO₃ and YMnO₃-based ceramicswere systematically investigated, and the physical nature was discussed. Thefollowing conclusions were obtained:(Bi_1-xNa_x)(Fe_1-xNb_x)O₃ (x=0.1, 0.3, and 0.5) ceramics were prepared by a solidstate reaction method, and the dielectric, ferroelectric and magnetic properties wereevaluated. By introducing NaNbO₃ into BiFeO₃, BiFeO₃-based solid solution withrhombohedral perovskite structure and (Bi_1-xNa_x)(Fe_1-xNb_x)O₃ solid solution ofperovskite phase with pseudocubic structure were formed subsequently withincreasing NaNbO₃ content. Due to the significantly reduced leakage, allcompositions of (Bi_1-xNa_x)(Fe_1-xNb_x)O₃ ceramics investigated here showed moreregular P-E hysteresis loops, and P_r was enhanced with increasing NaNbO₃ content.Two dielectric relaxations were observed in the temperature ranges of 550-600 K and650-710 K in (Bi_0.9Na_0.1)(Fe_0.9Nb_0.1)O₃ ceramics. The lower-temperature dielectricrelaxation was related to the point defect, and the higher-temperature dielectricrelaxation was related to the antiferromagnetism transition. Meanwhile, weakferromagnetic characteristic was observed in (Bi_0.9Na_0.1)(Fe_0.9Nb_0.1)O₃ ceramics.YFeO₃ ceramics were prepared by solid state reaction method, and the dielectricand magnetic properties were evaluated. Two dielectric relaxations were observed inthe temperature ranges of 123-350 K and 400-623 K in YFeO₃ ceramics and adielectric constant step was detected between them. The low-temperature dielectricrelaxation was an intrinsic thermal activated process following the Arrhenius law withthe activation energy very close to that for electronic ferroelectrics, while thehigh-temperature dielectric relaxation was related to the point defect. Meanwhile, weak ferromagnetic characteristic was detected in YFeO₃ ceramics at roomtemperature.YFe_1-xMn_xO₃ (x=0.1, 0.2, and 0.4) dense ceramics were successfully prepared bySPS (Spark Plasma Sintering) in a very short time at relatively low temperature, andthe dielectric, ferroelectric and magnetic properties were evaluated. An obviousdielectric relaxation was observed for all compositions of YFe_1-xMn_xO₃ ceramicsinvestigated here. It was a thermal activated process following the Arrhenius law, andthe activated energy decreased with increasing Mn content. Regular ferroelectrichysteresis loops were detected at 123 K and 153 K for different compositions, andweak ferromagnetic characteristic was observed at room temperature for allcompositions of YFe_1-xMn_xO₃ ceramics invstigated here. That is, throughMn-substituting for Fe, good ferroelectric properties could be achieved together withthe good room-temperature ferromagnetism of YFeO₃.YMnO₃ dense ceramics with single phase were successfully prepared in-situ bySPS followed by annealing, and the dielectric, ferroelectric and magnetic propertieswere evaluated. Compared with solid state reation method, the synthesis process wasgreatly simplified. An obvious dielectric relaxation was observed in the lowtemperature range (160-300 K), which is a thermally activated process, and adielectric step was detected in the higher temperature range (300-420 K). Weakferromagnetic characteristic was observed in the temperature lower than theantiferromagnetic transition temperature in YMnO₃ ceramics.In addition, YMn_0.8Fe_0.2O₃ dense ceramics were successfully prepared in-situ bySPS followed by annealing, and the dielectric, ferroelectric and magnetic propertieswere evaluated. The temperature dependence of dielectric properties in YMn_0.8Fe_0.2O₃ceramics was very similar to those in YMnO₃ ceramics. An obvious dielectricrelaxation was observed in the low temperature range (150-390 K) in YMn_0.8Fe_0.2O₃ceramics, which was a thermally activated process, and a dielectric constant step wasdetected in the higher temperature range (300-450 K). Obvious ferroelectric hysteresisloops were detected at 153 K, and a weak ferromagnetic characteristic was observedat room temperature and low tempreture. Through Fe-substituting for Mn in YMnO₃, the enhanced room temperature ferromagnetic properties were achieved together withthe good ferroelectric properties.