Preparation And Characterization Of New Magnetic Functional Compounds

Multiferroic materials, in which more than one of ferromagnetic, ferroelectric and ferroelastic properties appear simultaneously, have received renewed interest in recent years due to potential applications in new devices based on the mutual controls of magnetic and electric fields. Magnetism and ferroelectricity are involved with local unpaired spins and local dipoles (originating from off-center structural distortions),respectively.These two seemingly unrelated phenomena can coexist in certain unusual materials, termed multiferroics. Despite the possible coexistence of ferroelectricity and magnetism, a pronounced interplay between these properties has rarely been observed. This has prevented the realization of multiferroic devices offering such functionality. Many efforts have been devoted to finding new multiferroic materials or to investigating multiferroic properties in known oxides,such as BiCrO₃,BiMnO₃,BiFeO₃,BiCoO₃,BiNiO₃,BiScO₃,PbVO₃,double perovskite Bi₂MnNiO₆,etc.Very recently,Hur et al.reported that TbMn2O₅ also shows a profound interplay between electrical polarization and the applied magnetic field.For most multiferroic perovskite oxides,a high-pressure technique is required to get a single phase.Not only the equipment for the synthesis is complicated,but also microstructure of the sample cannot be controlled well.As well known,the properties of the samples strongly depend on their morphologies,sizes,and defect densities.Thus the synthesis method is important for the functional materials.The properties of functional materials strongly depend on their morphology,microstructure,dimension and crystallinity.Controllable microstructure and morphology are important not only for fundamental research but also for technological applications.Synthesis method and its corresponding process have proven critical to the control of the microstructure.Hydrothermal route has been extensively used in recent years due to easy control of chemical homogeneity,purity,morphology,shape,and phase composition of the products under moderate conditions.Many important functional materials have been successfully prepared by hydrothermal route,such as microporous crystals,superionic conductors,chemical sensors,electronically conducting solids,complex oxide ceramics and fluorides, magnetic materials,and luminescence phosphors.Basically,the mechanism of hydrothermal reactions abides by a liquid nucleation model.It is different from that of solid-state reaction,where the reaction is carried out through the diffusion of atoms or ions at the interface of the reactants.1.Polycrystalline Co_1-xFe_xS₂ samples with well-defined compositions have been prepared by a simple solvothermal method. The morphology is strongly dependent on Fe doping level.Nanosize round grains,nanorods, and large rectangle rods are achieved for the series.This strategy of synthesis is also applicable to some other ternary sulfides.Magnetic measurements show a broad ferro/paramagnetic transition and an unsaturated magnetization that are the characteristics of small grains.2.Oxygen-deficient BiFeO_3-δ nanospindles were prepared with a mild hydrothermal route at low temperature. Single-crystal samples without any Fe203 impurity were obtained, which showed a good stability to electron beam. Further sintering in air reduced the oxygen vacancies and hence increased the Fe³⁺ concentration. Saturated hysteresis loops were observed at room temperature due to the residual moment from a canted spin structure.Our work may provide a facile route to fabricate nanostructured multiferroic materials for the purpose of fundamental research and technological applications.3.Single-crystalline and uniform Bi2Fe409 particles with different morphologies were selectively synthesized by tuning hydrothermal conditions.The heating temperature and the alkaline concentration both had influences on the morphology,microstructure,and growth orientation of the final products.The high-quality square nanoplates were obtained at a heating temperature of 270℃with 3.0 mol/L NaOH as a mineralizer in the precursor solution. In addition, the anisotropic growth orientation of the crystals was strongly related to the NaOH concentration. The magnetic measurement indicates that the as-prepared sample exhibits an antiferromagnetic order at low temperature.4.The current work demonstrates the synthesis of well-defined regular tetragonal and cubic BilzFe_0.625O_18.9375 crystals, including controllable size and shape,using a simple hydrothermal reaction.The approach may be generalized to the large-scale preparation of other important metal oxides with controllable size and shape.Future research will focus on the investigation of property measurements,such as optical properties, magnetic properties,as well as mechanistic studies of the formation of Bi₁₂Fe_0.625O_18.9375 for the purposes of synthetic optimization.5.As is well-known,the properties of the samples strongly depend on their morphologies,sizes,and defect densities.For most multiferroic oxides,a high-pressure technique is required to get a single phase.Not only is the equipment for the synthesis complicated, but also the microstructure of the sample cannot be controlled well.Mr.Han developed a facile,mild,and easily controlled hydrothermal route to synthesize multiferroic materials.He synthesized selectively multiferroic TbMn₂O₅ nanorods and TbMnO3 micrometer crystals via a one-pot hydrothermal route.The different productions can be obtained by changing the ratio of reactants MnC1₂·4H₂O and KMn04.6.Single-crystalline submicron Bi₂Mn₁O₁₀ particles with controllable morphologies were successfully prepared with a simple one-step hydrothermal route. The resulting products were carefully characterized by using XRD, SEM, TEM,SAED and XPS.The morphology of Bi₂Mn₄O₁₀ is mainly determined by the precursor and the heating temperature.Magnetic measurement shows that the Neel temperature of Bi₂Mn₄O₁₀ is 44 K and the effective paramagnetic moment is 4.66μB.XPS and the magnetic moment both indicate the coexistence of mixed Mn3+ and Mn⁴⁺ valences.