| Multiferroic materials which have ferromagnetic, ferroelectric, and/or ferroelasticorders in the same phase have been received an increasing interest. Besides, couplingbetween magnetic and electric order parameters can give rise to the magnetoelectric effect,in which the magnetization can be exhibited under an external electric field and vice versa,that is say multiferroic materials may exhibit magnetoelectric effects. Multiferroicmaterials are a new type of functional materials, and play a very important role in thedesign of actuators, transducers, next generation memory devices and multifunctionalapplications. Single-phase multiferroic materials duo to excellent properties, has capturedthe eyes of researchers worldwide. The most important multiferroic materials are the oneswhich simultaneously exhibit ferromagnetic and ferroelectric orders in the same phase.But regrettably we have been told that the single-phase multiferroic materials areextremely rare in naturally occurring oxides. As alternatives, researchers found that atroom temperature bismuth-based Aurivillius phase perovskite structure exhibit a relativelystrong ferromagnetism and ferroelectricity. In our work, Bi5Ti3FeO15and polycrystallineceramics of Bi and Fe sites co-doped Bi5Ti3FeO15are synthesized by a conventionalsolid-state reaction technique. We also investigate and study their physical properties indetails, such as microstructure, ferromagnetism, ferroelectricity and optical properties.In this thesis，we have done the following work:(1) We prepared the Bi5Ti3FeO15ceramics by a conventional solid-state reactiontechnique and investigated the microstructures, dielectric constant versus temperature,ferroelectric, ferromagnetic, and optical performances. The results showed that anAurivillius phase containing four-layered perovskite structure clapped between two Bi–Olayers, having particle size of2-5μm. Two obvious dielectric anomalies around1007and1090K were exhibited by this material, indicating that there are two phase transitions.From the tanδ T curve we cannot see any peak. Polarization versus electric fieldhysteresis loops associated with2Prof6.08μC/cm2and2Ecof59kV/cm were obtained.The result of magnetic measurement indicated the weak ferromagnetic order of Bi5Ti3FeO15ceramics at room temperature, we also analyzed the origin of the obtainedmagnetism. An energy band gap of2.03eV was determined from the UV–vis diffuseabsorption spectrum.(2) We prepared the (Bi4RE)Ti3(Fe0.5Co0.5)O15(RE=Nd、Sm、Gd、Dy) ceramics andinvestigated the microstructures, ferromagnetic, ferroelectric, and optical performances.The results also show the same system, but having particle size of1μm. Substitution withRE and Co improved the ferromagnetism polarization, and when the RE is Nd, theremnant magnetization2Mrof0.53emu/g. Intrinsic reasons might contribute to the largeenhancement:(a) the substitution of half the Fe3+by Co3+may lead to a distortion of theoctahedra in the perovskite blocks, because the smaller ionic radius [r(Co3+)=0.061nm] ofCo3+ion than that [r(Fe3+)=0.0645nm] of Fe3+ion, which lead to the size mismatch. Inaddition, the substitution of Gd3+for Bi3+may also tilt Fe(Co)O6octahedra substantially,which facilitate those spins opposite to the field to align towards the applied field;(b) maybe form a direct coupling over Fe–O–Co clusters;(c) RE ion with SRE3+=7/2spins showsa larger magnetic interaction than other compounds.(3) We studied the microstructures, ferromagnetic, and ferroelectric of(Bi5-xLax)Ti3(Fe0.5Co0.5)O15(x=1、2、3) ceramics. From the XRD pattern we known theceramics do not shown any impurity after sintering, confirming to be the formation ofsingle-phase perovskite structure. When x=2, the ferromagnetism and ferroelectricity isrelatively strong. And the ferromagnetism is stronger than the Bi5Ti3FeO15ceramics. |
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