The influence of hollow structure on the magnetic characteristics for Fe₃O₄ submicron spheres

In this paper, nearly monodisperse Fe₃O₄ hollow and solid submicron spheres were synthesized using a simple solvothermal method. The TEM investigation clearly reveals the successful realization of the hollow structure of magnetite spherical particles by the method. The average diameter of the Fe₃O₄ hollow and solid submicron spheres is about 300 and 500 nm, respectively. The submicron spheres are composed of nanometer-sized grains, with grain sizes of 21 and 28 nm for the hollow and solid spheres, respectively. Magnetic hysteresis measurements indicate that the hollow structure has higher coercive force and lower saturation magnetization than the solid submicron spheres. Magnetization versus temperature curve shows a peak at 107 K in the zero-field-cooled (ZFC) runs for the hollow structure, which corresponds to the blocking temperature of the nanograins. The blocking temperature correlates well with the volume of the nanograins according to Stoner–Wohlfarth theory. The hollow structure exhibits the magnetic properties of individual nanograins because of the weak coupling among them. However, the above-described feature is absent in the solid spheres, due to stronger magnetic coupling between the grains.