We present a simple process to prepare the hollow ceramic (CoFe_2O_4/SiO_2) composite nanospheres and hollow alloy (Co33Fe67/SiO_2) composite nanospheres. The hollow CoFe_2O_4/SiO_2 composite nanospheres were prepared by calcining polymer/CoFe_2O_4/SiO_2 core/shell composite spheres which were synthesized by the sol-gel method following the chemical co-precipitation. In a typical process, the monodisperse polymer poly(MMA-co-MAA) latex (450 nm) was used as a core template. To create hollow CoFe_2O_4/SiO_2 spherical structures with various sizes of CoFe_2O_4 nanoparticles, the hybrid PMMA/CoFe_2O_4/SiO_2 core-shell spheres were subsequently calcined in the temperature range from 450 to 900°C for 4h. On the other hand, the hollow Co33Fe67/SiO_2 composite nanospheres were formed by reduction of hollow CoFe_2O_4/SiO_2 nanospheres in a stream of H2/Ar mixed gas at 900°C for 8 hrs. X-ray diffraction pattern shows that the coated phase of the hollow CoFe_2O_4/SiO_2 composite nanospheres has a cubic spinel ferrite structure. Based on the thermogravimetric analysis (TGA), we found that the content of CoFe_2O_4 is 73 wt% in the hollow CoFe_2O_4/SiO_2 composite shell. The scanning electron microscope and transmission electron microscope photographs show that the hollow spheres are uniform. According to the line scanning EDX analysis of the cross section of hollow spheres, the SiO_2 is not only coated on the surface of sphere but also distributed over the shell of hollow sphere. The thickness of shell of hollow spheres is about 40 nm. Magnetic measurements show that the saturation magnetization is clearly decreases as the magnetic particle size decreased. This phenomenon can be interpreted as the effect of surface spin canting when the particle size is reduced. As for the hollow alloy (Co33Fe67/SiO_2) composite nanospheres, the magnetic phase has a body-centered cubic structure and an average crystallite size of 28.7 nm. This alloy nanospheres exhibit a ferromagnetic behavior with saturation magnetization of 170 emu/g, coercivity of 250 Oe, and Curie temperature of 968 °C. Due to metallic and ferromagnetic behavior of Co33Fe67 nanoparticles, these hollow spheres can be used as a lightweight electromagnetic wave absorber.
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