Study On Enhancement Of Multiferroic Effect At Room Temperature In Fe Based Oxide Materials

As the few single-phase multiferroic materials at room temperature,the bismuth ferrite and the hexaferrites are attracting more and more attention based on their obvious magnetoelectric coupling effects.However,the physical mechanism of the magnetoelectric coupling effect is not clear yet.To achieve the best overall magnetoelectric performance,there are still many problems need to overcome.For example,the intrinsic weak magnetic properties,high leakage current and poor ferroelectric?or dielectric?performance properties in these two material systems.In this thesis,we present some studies on the synthetic process,microstructure,room temperature multiferroic properties?e.g.ferromagnetism,ferroelectricity,dielectric?of bismuth ferrite and hexaferrite multiferroic materials.The main results are summarized as follows.?1?The influences on the structural,microstructural,magnetic,ferroelectric and dielectric properties of the 0.8BiFeO₃-0.2SrTiO₃ ceramics were systematically studied.An appropriate annealing temperature had a remarkable influence on the enhancement of the room temperature multiferroic properties?e.g.ferromagnetism,ferroelectricity,dielectric?.Moreover,the leakage current and dielectric loss were suppressed as the as the annealing temperature rose.According to the scanning electron microscopy results,it indicated that the appropriate annealing temperature was be beneficial to the improvement of crystallinity and crystal domain distribution.?2?The structural,morphologic,ferromagnetic,ferroelectric and dielectric properties of the?1-x?BiFeO₃-x(SrTiO₃-Bi_0.5Na_0.5TiO₃)?x=0-0.5?solid solutions were investigated.The results showed the room temperature magnetic and ferroelectric performance could be optimized by the coexistence of SrTiO₃ and Bi_0.5Na_0.5TiO₃.Moreover,the addition of appropriate amounts of SrTiO₃-Bi_0.5Na_0.5TiO₃ to BiFeO3 effectively reduced the leakage current and dielectric loss at room temperature?3?We present the structural,morphologic,ferromagnetic,ferroelectric and dielectric properties of the Eu doped BiFeO₃-Bi_0.5Na_0.5TiO₃ ceramics.Compared with the single-phase0.7BiFeO₃-0.3Bi_0.5Na_0.5TiO₃,the magnetic properties of Eu doped samples were significantly enhanced,and the ferroelectric measurements proved that Eu doping enhanced the polarization of the solid solutions.Thus,the enhanced coexistence of ferromagnetism and ferroelectricity were observed in Bi_1-xEu_xFeO₃-Bi_0.5Na_0.5TiO₃ compounds at room temperature.In addition,with the increasing contents of Eu,the dielectric properties would be also improved gradually.?4?The structural,magnetic,electric and magnetoelectric properties of the Y-type hexaferrite BaSrCo_2-xMg_xFe₁₁AlO₂₂2 polycrystalline were comprehensively studied.We fabricated c-axis oriented polycrystalline specimens of as-prepared samples by using an in-situ annealing process with the external magnetic field,which exhibited superior multiferroic performance compared with the non-oriented specimens.With the increasing contents of the nonmagnetic-ion,Mg,the magnetization of BaSrCo_2-xMg_xFe₁₁AlO₂₂2 was diluted,whereas the BaSrCo_1.5Mg_0.5Fe₁₁AlO₂₂ hexaferrites present the superior magnetodielectric properties and larger magnetoelectric current compared with the initial BaSrCo₂Fe₁₁AlO₂₂ hexaferrites.