Synthesis And Magnetic Properties Of Spinel-type Ferrite Hollow/porous Microspheres

In the past decade, significant progress in the synthesis of functional materials has been achieved, and many novel structures, including tubes, rods, octahedral, and self-assemble structures with size from several to hundreds of nanometers, have been developed. Among nanomaterials with different structures, hollow magnetic spheres have received much attention for their peculiar properties and potential applications in high-density magnetic data storage, ferrofluids, spin electronic devices, magnetic resonance imaging, and biomaterials diagnostics. Fe3O4 and Cobalt ferrite are both most important spinel ferrite with high stability. Therefore, it is of great significance to mouse out a relatively simple and environmental preparation of the hollow/porous ferrite microspheres, and to study the route of improving the magnetic properties of the hollow/porous ferrite microspheres. In this paper, a special solvothermal route using Ostwald ripening process was being ready for synthesis CoxFe3-xO4 and Fe3O4 hollow/porous microspheres, which microstructure and magnetic properties were Studied; The proper conditions preparing CoxFe3-xO4 and Fe3O4 hollow/porous microspheres were summarize; The route of improving the magnetic properties of CoxFe3-xO4 and Fe3O4 hollow/porous microspheres were discussed; Remarkable positive exchange bias effect was discovered in CoxFe3-xO4 porous microspheres system. Cobalt ferrites have important application in high-density magnetic data storage, the discovery of exchange bias effect in CoxFe3-xO4 porous microspheres has important academic significant and application value.The main content of this paper are summarized as follows:1. Nano-structure CoxFe3-xO4 porous microspheres were firstly synthesized by solvothermal route. The monodisperse CoxFe3-xO4 porous microspheres with polycrystal Structure are assembled by nanoparticles, and their average diameter is about 300 nm. CoxFe3-xO4 porous microspheres with different Co2+ content posses excellent ferrimagnetism, specially, the exchange bias effect was discovered in CoxFe3-xO4 porous microspheres with Co2+ content being higher. By changing Co2+ content, the CoxFe3-xO4 composite ferrite with different magnetic property could be gained to meet the various application requirements.2. According to the ZFC-FC curves and hysteresis loops at different temperature, CoxFe3-xO4 porous microspheres with x=0.8, x=1 possess remarkable positive exchange bias effect at low temperature. Keeping the Co2+content fixed, the bias field reduces fleetly to zero, and coercivity increases gradually to the maximum value near the blocking temperature (TB), then gradually reduces with the temperature. With the increase of Co2+ content, both bias field and coercivity increase, and the blocking temperature also increases, the exchange bias effect is more remarkable. When the Co2+ content is lower (x≤0.2), the exchange bias effect hasn't been discovered in CoxFe3-xO4 porous microspheres. CoxFe3-xO4 nanoparticals could be considered as a system, in which the surface antiferromagnetic order and interior ferrimagnetic order coexisted. Moreover, the interface exchange was antiferromagnetic between the surface antiferromagnetic layer and interior ferrimagnetic layer, So, in the CoxFe3-xO4 nanoparticals system, the positive exchange bias effect was discovered. With the increase of Co2+ content, the magnetic anisotropy of spin-frozen surface layer increases, the "spin pinning" effect of surface layer on interior ferrimagnetic layer enhances, so, the exchange bias effect is more remarkable.3. Fe3O4 hollow microspheres and leaf-like nanoparticles were synthesized using urea as the starting materials by solvothermal route. A series of parallel experiments show that the reaction time, temperature, and reagents have important influence to the type of final products. The Fe3O4 hollow microspheres can be obtained at 220℃, reacting for 12h. Magnetic hysteresis loops indicate as-prepared Fe3O4 powders exhibit a better ferromagnetic behavior. Fe3O4 hollow microspheres reacting for 12 h possess the highest saturation magnetization (Ms) and the lowest coercivity (Hc)4. Fe3O4 hollow microspheres were synthesized using NH4Ac as the starting materials by solvothermal route. The monodisperse Fe3O4 hollow microspheres with polycrystal Structure are assembled by nanoparticles, and their average diameter is about 400 nm. Compared with the preceding paragraph, through this method Fe3O4 hollow microspheres can be prepared in the wide temperature range (200 to 220℃) and time range (12-24 h). Compared with the preceding products prepared with urea, the magnetization (Ms) of the products prepared with NH4Ac increases near 20%, which benefits for practical application of Fe3O4 hollow microspheres materials. According to the hysteresis loops at different temperature, Fe3O4 hollow microspheres don't possess exchange bias effect. Compared with CoFe2O4 porous microspheres, the hysteresis loops for Fe3O4 hollow microspheres at different temperature are all symmetrical, Fe3O4 hollow microspheres don't exhibit exchange bias effect.