Fabrication and magnetic transformation from paramagnetic to ferrimagnetic of ZnFe2O4 hollow spheres

ZnFe2O4 hollow spheres (ZFHs) with sizes of 200–302 nm were synthesized by simple impregnating method using the as-prepared phenolic formaldehyde (PF) spheres as templates and subsequent annealing at 500–700 °C. The prepared ZFHs are assembled by a large number of small nanoparticles with sizes of 15–20 nm, and many mesopores exist among these nanoparticles. The samples annealed at 500–550 °C exhibit a single cubic spinel structure, while higher annealing temperature leads to the formation of hexagonal ZnO and rhombohedral α-Fe2O3 secondary phases. The size of the assembled nanoparticles increases with the increase in annealing temperature. Novel magnetic transformation from paramagnetic to ferrimagnetic is induced by the reduction of annealing temperature and the saturation magnetization significantly increases from 2.3 to 13.5 A·m2/kg. The effect of the formation of hollow sphere structure on the redistribution of Fe3+ and Zn2+ in the spinel structure was studied.