Fabrication, Characterization, And Magnetic Properties Of Cobalt-based Nanoparticles

A considerable amount of research has been devoted to the study of magnetic nanoparticles due to its unique chemical and physical properties,and potential for a variety of important applications,such as high density recording media,biotechnology,ferrofluids, and fabrication of exchange-coupled nanocomposite permanent magnets.Manipulated synthesis of nanoparticles is one of the most important sections of nanoscience and nanotechnology,also the base of investigating distinctive properties and applications of nanoparticles.Recently,chemically synthesized magnetic nanoparticles have drawn much more attention than physical because it can yield highly monodisperse magnetic nanoparticles of pure metals,metal alloys,and so on.Moreover,it is well known that the properties of pure metals nanoparticles lack diversification which is an obstacle to rapid development of the multifunctional material.Today,cobalt-based nanoparticles are emerging as the "hot spot" of many investigations owning to their important technological applications in microwave absorption and magnetic refrigeration,especially,in high-density magnetic recording media.The objective of this dissertation is to explore facile and green chemical solution-based synthesis routes to prepare the cobalt-based magnetic nanoparticles which can prevent from oxidation and conglomeration,and study their magnetic property.More details are summarized below:1.Bimetal Pt and Co nanoparticles were prepared using reduction method in aqueous phase.The core-shell Pt₃Co/Pt nanoparticles were synthesized via reduction method of hydrogen absorption on the surface of Pt₃Co alloy nanoparticles which were prepared by the reduction of the mixture of H₂PtCl₆ and CoCl₂ solutions using NaBH₄ as a reducing agent.Average size and standard deviation of Pt₃Co/Pt nanoparticles were 3.6nm and 0.9 nm,respectively.Heating Pt₃Co/Pt nanoparticles in air at high temperature,Co in Pt₃Co nanoparticles was oxidized,while no oxidation tendency was detected for Pt₃Co/Pt nanoparticles.The crystallize structure changed from fcc to fct after the heating treatment. The coercivity of the heated Pt₃Co/Pt is 276 Oe at room temperature.Magnetic composite PEDOT/PSS—Pt₃Co nanoparticles with were synthesized using a step coreduction in an aqueous solution of conducting polymers PEDOT/PSS.The average size and standard deviation of PEDOT/PSS—Pt₃Co nanoparticles were 9.6 nm and 1.4 nm.The films prepared from magnetic composite Pt₃Co-PEDOT/PSS nanoparticles were conductive with conductivity between 1.6 and 4 S/m.The composite nanoparticles are superparamagnetic above the blocking temperature(T_b=110.5 K) while they are ferromagnetic with H_c(701 Oe) and M_r(4.1emu/g) at 10K(below T_b).Compared with the PVP-Pt₃Co,magnetic composite PEDOT/PSS-Pt₃Co nanoparticles are highly promising application in the magnetic films for recording media.2.Trimetal PtCo/Au nanoparticles were synthesized in aqueous phase and reverse micelle,respectively.PtCo/Au nanoparticles with anaverage size of 2.5 nm in aqueous are stable to congregation and oxidation due to the protected of Au.PtCo/Au nanoparticles display fcc structure,while the annealed PtCo/Au nanoparticles exhibit a ordered fct structure.SQUID studies reveal that the coercivity of PtCo/Au nanoparticles with an irreversible magnetization increases to 510 Oe after heat treatment at high temperature. Magnetic thiol-coated PtCo/Au nanoparticles prepared in a reverse micelle are narrow size distribution.TEM confirmed that the self-assembly of PtCo/Au nanoparticles was realized on the surface of carbon coated copper grids.SQUID studies reveal that the particles are superparamagnetic above the blocking temperature(T_b=69 K) while the particles are ferromagnetic with H_c(628 Oe) and M_r(5.2 emu/g) at 5K(below T_b).3.SmCo₅ nanoparticles are the one of the most important binary transition metal-rare earth systems for magnetic materials.SmCo₅ nanoparticles were fabricated by a four-stage route.At first,the mixed aqueous soluble of SmCl₃ and CoCl₂ was adjusted to pH=9～10 using NaOH at 0℃.Secondly,two soluble metal-salts were forced hydrolysis to oxides under atmospheric refluxing at 80℃.Then,oxide nanoparticles with a ca average size of 20 nm were prepared at 150℃for 9 h through a convenient hydrothermal process.Finally, The SmCo₅ nanoparticles were attained via nanoparticles hydrothermally prepared deoxidized using Na under the hydrogen stream at 400℃.The SmCo₅ nanoparticles are superparamagnetic at 300 K while the particles are ferromagnetic with H_c(268 Oe) at 5K.4.The cobalt-based magnetic nanoparticle of iron-doped shows excellent magnetic properties due to its high anisotropy constant.We prepared dispersed Pt₃Co/Fe₃O₄ and CoFe₂O₄ nanoparticles in aqueous by different synthesis routes.The bimagnetic Pt₃Co/Fe₃O₄ nanocomposite can be successfully synthesized by the coprecipitation of Fe³⁺and Fe²⁺on the Pt₃Co nanoparticles in aqueous solution.Fe₃O₄ leads partial Pt₃Co phase transformation from fcc to fct.The as-synthesized composite nanoparticles exhibit ferromagnetic properties at room temperature due to the exchange coupling between superparamagnetic Pt₃Co and Fe₃O₄.Magnetic properties of Pt₃Co/Fe₃O₄ nanoparticle can be tuned by varying of the molar ratio of iron to platinum.Pt₃Co/Fe₃O₄ nanoparticles exhibit higher saturation magnetization when the molar ratio of iron to platinum is 1 because Pt₃Co and Fe₃O₄ is more intimate direct contact to ensure their stronger exchange coupling.Cubic spinel CoFe₂O₄ magnetic nanocrystal was hydrothermally prepared by coprecipitation technique using ferrous chloride(FeCl₂·4H₂O),cobalt chloride (CoCl₂·6H₂O) and sodium hydroxide(NaOH) as starting materials.SEM shows the shap and size of CoFe₂O₄ nanoparticles were controlled by the hydrothermal time.After 8 h under 150℃,the particle with ca 30 nm is cubic nanocrystal.SQUID studies reveal that the coercivity of CoFe₂O₄ nanocrystal is 10.7×10³ Oe at 5K。...