Well-defined porous Fe3O4 flower-like nanostructures have been synthesized by decomposition of the iron alkoxide precursors that are prepared by heating up the solution of FeCl3·6H2O, urea, and surfactant in ethylene glycol. Time-dependent SEM studies indicate that the structure evolution of iron alkoxide precursors contains a fast nucleation of primary nanoparticles followed by a subsequent growth. By varying the amount of surfactant in the solution, the morphology and microstructure of the iron alkoxide precursors can be controlled. After calcination, the flower-like nanostructures of the precursors are maintained in the final products Fe3O4, with each petal of the flower being transformed from a dense structure with a smooth surface into a highly porous structure consisting of interconnected nanoparticles due to the removal of organic species in the iron alkoxide by pyrolysis. Compared to traditional ferrites and ferromagnetic alloys, the complex permittivity of the flower-like porous Fe3O4 samples is modified, and the permeability presents natural magnetic resonance at about 3.0 GHz, which is higher than that of usual Fe3O4 nanoparticles and symbolizes a break-through of the Snoek’s limit. A maximum reflection loss of the flower-like porous Fe3O4 can reach −28.31 dB at 13.2 GHz with a thickness of 2 mm due to an improved impedance matching that is associated with complex permittivity, complex permeability, and the structure of the material. We believe the prepared porous Fe3O4 nanostructures can be good candidates for electromagnetic absorbing materials.

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